THE PERSEID METEOR SHOWER
-----------------------
John Pazmino
NYSkies Astronomy Inc
nyskies@nyskies.org
www.nyskies.org
2010 July 19 initial
2013 August 15 current
Introduction
----------
Every year, like an annual holiday, the news media announce the
Perseid (PERR-seh-yidd) meteor shower, or the Perseids (PERR-seh-
yidz). This event is in mid August, chattering on the calendar between
the 11th and 13th each year.
While there is a deep litterature about the Perseids, and meteor
showers generally, much of it is altogether too lavish, with words
like 'light up the sky', 'celestial fireworks', 'rain of shooting
stars', 'summer spectacular'.
These descriptions, some from otherwise reputable astronomy news
sources, can turn off public interest and support for our profession.
This is specially possible if the viewer in under a sky lighted by
luminous graffiti or coated with humidity and haze. He sees only the
brightest of the meteors, a couple per hour, during his all-night
watch. This is what astronomers look at?
Here I give some background and advice for watching the Perseids,
and other showers, for both the home astronomer and the layman.
Meteor showers
------------
A meteor shower is a stream of meteors in parallel paths thru
space, as elaborated later. When Earth passes thru this stream tube,
meteors are captured by gravity. They fall thru the air toward the
ground. They incinerate by air friction. We see the streaks of the
glowing burning bit in the sky.
The meteors themselfs are dirt, grit, pebbles, grains, chips of a
comet. They are spilled out of the comet to circulate in the comet's
orbit after the comet moved along. They are small enough to completely
vaporize in our atmosphere and not reach the ground as meteorites.
While there may be examples of meteorites falling from a meteor
shower, we do not expect any.
The parallel paths of the meteors thru the air cause a perspective
effect of seeming to radiate from a point among the stars, This is
purely an illusion, the very same as the vanishing point of artists
and photographers. It's merely the uprange direction along the stream
tube into the oncoming meteors.
The upstream vanishing point is the radiant and is named for the
constellation it sits in. For the Perseids, the radiant is in Perseus
(PERR-seh-yuss). It happens to be in the far north of the group, near
the Camelopardalis border. It's close to the fabled Pazmino's Cluster.
The figure here illustrates the meteor path over you.
. |\ . .
sight line \ . . <-- incoming invisible
toward radiant--\ . . meteors from comet
. \ . .
- - - - - - - - - - - - - - - - - <-- top of atmosphere
\ \ \
\ \ \ <-- shooting stars
\ \ \
a b c
O <-- observer
# # # # # # # # # # # # # # # # # # # <-- ground
The observer sees meteor a to the left of the radiant. Meteor b is
seen almost headon near the radiant. Meteor c has a path to the right
of the radiant.
It is crucial to understand that meteors, unlike almost all other
celestial phaenomena, are not at remote distance from us. They are
close to you, tens of kilometers, and exhibit strong parallax from
locations even within a single town.
Some history
----------
I offer a brief, not exhaustive, history of the Perseid shower
with clarifications of loose accounts elsewhere. A proper history of
the Perseids must include its sister Leonid shower. The two were the
first showers studied and are today the best known ones for the public
and astronomers.
Meteor studies were essentially nonexistent before 1833. Meteors
were treated as an atmospheric event, observed as isolated local
events not shared widely across the world. Their reports were often
recorded in observatory or general diaries.
The wakeup call was the unexpected and dramatic Leonid storm in
1833. Myriads upon myriads of shooting stars dazzled astronomers and
public. A review of the event by Olmsted in 1834 showed the meteors
traveled in parallel paths from some where in outer space. They were
not a local apparition in the air over certain localities.
Quetelet was the first, in 1835, to confirm that one meteor shower
always occurred in mid August. He proposed that this, the Perseids,
was an annual event, He previously in 1834 confirmed the annual nature
of the Leonids.
Herricks in 1837 seems to be the first to anticipate an annual
meteor shower by going out in early-mid August. He and friends did
watch a downthrow of shooting stars.
Heis in 1839 was the first to record the rate of shooting stars by
counting them. His count was about 160 meteor/hour. He also was first
to segregate the Perseid meteors from background meteors.
For the meteors to come every year, they must be in a continuous
swarm spread over an orbit around the Sun. But there was no thought
yet of cometary origin.
Since the 1840s the Perseids and Leonids were regularly observed.
While the Leonids performed badly since the deluge of 1833, the
Perseids put on a good display pretty much every year. This dependible
display makes the Perseids the best documented of all meteor showers.
By the 1860s newspapers and lecturers announced the two events. In
some towns there were public meteor watches. These were noted to be at
times smothered by unfettered industrial air pollution.
Schiaparelli in 1867 linked the Perseid meteors to comet Swift-
Tuttle, discovered in 1862. He determined that the radiant was the
upstream vanishing point of incoming meteors flowing in the comet's
orbit. He also tied the Leonids to comet Tempel-Tuttle.
The name
------
The name 'Perseids' derives from the concept of offspring or
descendents. In Greek the word for a descendent or child of a person
is the patronym of that person. It is formed by adding '-is' to the
stem of the person's name. It's like 'ben' in Hebrew or '-vich' in
Russian. A child of Perseus is a Perseis {PERR-seh-yiss).
The singular form is virtually never used in astronomy because a
meteor shower has lots of descendents of Perseus. The plural is
employed, Perseides (per-SAY-ih-deez). In English this is Perseids.
At first the shower was named the 'August meteors'. Other showers
when recognized were also named for their months. Even after the
comet-meteor link was found the month scheme continued into the 1890s.
It's not certain how the patronum scheme started but 'Perseiden',
German for Perseids', was casually used since 1873 in Europe. It is
not yet confirmed that Schiaparelli first coined the name 'Perseids'.
When by the 1880s some months had more than one shower, the
patronym began to catch on. We read of the Leonids and Adromedids,
both falling in November. By the late 1890s the patronm system became
the standard nomenclature, in force thru today..
The Perseids also have a cultural name, Tears of St Lawrence. St
Lawrence day is August 10th, honoring his death by Roman persecution
in 258AD. The falling stars for some cultures are the heavenly
remembrance coming a couple days later.
Why the Perseids?
---------------
Of the ten or so major meteor showers each year the dominant news
is for the Perseids. All of the others are neglected or only
occasionally mentioned. There are a few reasons.
One I already explained. The Perseids gives a reasonable number of
meteors per hour to please most observers. Most of the other showers
put on erratic shows, weak in some years and strong in others. It is
risky to send layfolk out with good chance of seeing only a few
shooting stars.
Most of the major showers occur in fall-winter, when in the
northern United States it's glatt brutal outside at night. The
Perseids are in summer when the weather is much more tolerable.
Summer is a vacation time for many people, migrating to second
homes where they sit under skies less infected by luminous graffiti,
humidity, haze. The air may be cooler, specially with elevation.
The radiant for many showers is in an empty part of sky away from
obvious stars. A layman can't be sure he's looking in the right spot.
The Perseid radiant is easily located among bright stars between
downtown Cassiopeia and Capella.
Zenith hourly rate
----------------
The density of a meteor shower is stated by the number of meteors
per hour that an ideal observer should see. This number is the Zenith
(or Zenithal) Hourly Rate, ZHR. It ranges for the major showers from
10ish to 100ish with the Perseids hovering around 70. The cited ZHR
ranges widely among authors, Expect to see different values.
This is the count presented in news media. It is hideously
misleading for the layman and new astronomer. It is an IDEAL number
under often unstated conditions:
* The radiant is in the zenith, not at altitude less than 90
degrees
* It includes meteors over the entire sky, not just in a person's
visual field of view
* It assumes a transparent sky of 6.5 magnitude from horizon to
horizon
* It assumes an open sky with no blocking by skyline or clouds
* It assumes normal vision to see 6th magnitude stars
* It includes meteors of all brightness to the 6th magnitude
* It counts the meteors for hourly intervals, not for spans of
only many minutes
It also presumes the optimum properties of the incoming stream of
meteors, which for most showers is unknown and often unknowable.
In the typical meteor watch few of these conditions are satisfied.
Each defect of condition reduces the realized rate. It is well not to
'expect' the ZHR number of meteors! Under your peculiar circumstances
you will do well to achieve 1/3 of the nominal ZHR.
The shortfall from ZHR can be drastic if the sky is infected with
humidity, haze, moonlight, luminous graffiti. It is common for
observer under such skies to report seeing only a dozen Perseids for a
watch of several hours.
Comets
----
As a comet makes its rounds of the Sun it spills off bits and
pieces that then travel along with the comet in its orbit. In time
these tiny pieces drift away from the comet and spread along the
orbit. They do so from their small extra speed, forward or backward,
relative to the comet itself. They form a streamtube along the orbit,
This tube can spread to millions of kilometers wide.
The particles may be uniformly diffused or gathered into clumps or
threads. Our ability to see the meteors in deep space before they
light up in the air is still crude. We can not foretell with any
confidence the aspect of a future shower except by playing with
astrodynamic models. Some times we get the prediction right but other
times we're badly wrong.
If the orbit of the comet passes close to Earth, within a few
million kilometers, the debris is attracted to Earth to fall as
shooting stars. This constitutes the meteor shower.
The diagram here, a general one, clarifies the concept.
::::\::::
::::\:::: <--meteor streamtube
::::\::::
::::\::::
/-\ ::::\::::
-----------| |-------\---------> Earth motion
\-/ :::;\::::
Earth ::::\::::
::::\::::
|||| \| ||
meteor motion
In the diagram the stream tube is much too narrow. Earth would
pass thru it in, uh, about 10 minutes! In reality it's so wide that it
takes Earth several days to traverse.
The meteors flow downward along the comet's path. They enter
Earth's air first at the upper right. As the Earth moves to the right,
more of the world catches the meteors. Only the forward parts of Earth
are hit by the stream. The rearward parts are shielded by the bulk of
the planet and get no meteors.
The comet orbit crosses the plane of Earth's orbit, the ecliptic
plane, in two points. One is the ascending node, where the comet
crosses south to north. The other is the descending node, the crossing
is north to south. The line connecting the two, also passing thru the
Sun, is the line of nodes.
Usually only one of the two nodes is near enough to Earth to
produce a meteor shower. The other is too remote to capture any
debris. A given comet yields one meteor shower.
- - - - - -
/ \
/--comet orbit \
/ \
/ \
a / ecliptic plane \ b
-----------/-o-----------O-----------+-----------------\--
/ \_Earth \_Sun \_opposite side \
| of Earth orbit \
|
\
\
The comet orbit cuts the ecliptic at a and b. Only a is near
Earth. b is beyond both the streamtube and Earth gravity. If the comet
is running clockwise, a is its ascending node (north up); b,
descending.
The best example of a comet whose BOTH nodes pass near Earth is
Halley's comet. In May at the descending node it yields the eta
Aquarid meteors. In October at the ascending node it produces the
Orionids. As a rule, a radiant north of the ecliptic relates to the
descending node; south, ascending.
Since comet and Earth orbits are stable, at least for several
centuries, Earth intersects the node each year at the same date. In
doing so it captures some of the comet debris and produces the
associated shower on about the same date each year.
comet Swift-Tuttle
----------------
The Perseids are the debris from comet Swift-Tuttle, periodic
comet #109. This was demonstrated by Schiaparelli in 1867. Since then
many other showers were linked to comets or, in a couple cases, to
asteroids.
The proximity of Swift-Tuttle and Earth is at the comet's
descending node. At the ascending node the comet is too far from
Earth's gravity to attract its particles.
Here is a comparison of Swift-Tuttle and Perseids. You'll find
variations for the shower because of the difficulty of obtaining good
orbital data for the meteors.
-----------------------------
parameter | S-T | Per
---------------+--------+------
perihelion T | 1992 | not
| Dec 12 | applicable
perihelion AU | 0.958 | 0.942
semimajor axis | 26.32 | 9.61
orbit period | 130.0y | ~130y
excentricity | 0.964 | 0.902
ascending node | 139.44 | 139.5
node crossing | not | 2010 Aug
| applic | 12, 07h UT
argument perih | 153.00 | 149.2
inclination | 113.43 | 113.2
-------------------------------
The perihelion in 1992 was the first return since the comet's
discovery in 1862. With the comet orbit traced continuously thru
observations of the Perseid meteors along it, the recovery search was
more precise and efficient. This is the first time a meteor shower
helped locate its parent comet.
For an other way to appreciate the period of Swift-Tuttle, a
Perseid meteor you see in 2010 is a pebble that missed Earth in 1880!
One that misses us now gets a second chance in about 2140.
The semimajor axis agrees badly because of its sensitivity to the
excentricity and perihelion distance. The errors in these parameters
compound to generate the smeimajor axis.
The steep inclination of 113 degree keeps the orbit far from other
large planets. With no strong perturbations from these planets the
Persieds are a very stable shower.
There is no time of perihelion for the shower. The meteors are a
continuous stream over the whole orbit, not the single point of the
comet.
There were enhanced Perseid displays in the early 1990s when
Swift-Tuttle was in the inner solar system. This suggests that the
comet is surrounded by an extra thick cloud of meteors besides having
a uniform tube of meteors over the whole orbit.
Meteor orbit
----------
Comets spawning meteor showers have orbits like Apollo and Amor
class of asteroid. The period of these comets is years to many
decades. Swift-Tuttle has one of the longer periods, 135 years. We
know of no Aten or Apehele orbits among shower-producing comets.
Here, and in most meteor litterature, 'meteor' and 'comet' are
equally used for the orbital properties of the meteor. The meteors
flow in the comet's orbit with the its same parameters.
One thing to be wary of is the inconsistent use of terms and
symbols in meteor work. Some authors, not catching the discrepancies,
commingle material from different sources. Their work may be
erroneous. This actually was the bulk of the inquiries about the
initial edition of this article!
In spite of the range of semimajor axis and excentricity of these
comets, it happens that at 1AU from the Sun, comets have about the
same speed of 42km/s relative to the Sun. Earth has an orbital speed
of 30km/s, pretty constant thruout the year.
Meteor speed
----------
The vectors of these two speeds yields the closing, approach,
speed of the meteor relative to Earth. The Earth/Sun vector V(e/s) is
subtracted, not added!, to the meteor/Sun vector V(m/s). Some authors
blithely say the vectors are ADDED, giving wrong results.
This diagram, a general one, shows the interplay of the speed
vectors. S, M, E are the endpoints for Sun, Earth, meteor.
V(m/s)
S+------------------------------->M
\ angle /|
\ /
\ /
\ /
\ / V(m/e)
V(e/s) \ /
\ /
E\|/
As a check, V(m/e) plus V(e/s) yields V(m/s). Examine the diagram
above to verify this.
The vectors for the Perseids are angled at substantially the
comet's inclination of 113 degree. We have, by the Law of Cosines:
V(m/e)^2 = V(m/s)^2 + V(e/s)^2 - (2)*(Vm/s)*(Ve/s)*(cos(angle))
= (42)^2 + (30)^2 - (2)*(42)*(30)*(cos(113))
= (1746) + (900) - (2520)*(-0.3907)
= (3648.64)
V(m/e) = (60.4)
The Perseids are a fast stream because Swift-Tuttle has a
retrograde orbit while Earth's is prograde. Their shooting stars are
the faster ones among showers.
The extreme cases are when the two vectors are parallel and
antiparallel. Then closing speeds are:
V(m/e)^2 = V(m/s)^2 + V(e/s)^2 - (2)*(Vm/s)*(Ve/s)*(cos(angle))
= (42)^2 + (30)^2 - (2)*(42)*(30)*(cos(0, 180))
= (42)^2 + (30)^2 - (2)*(42)*(30)*(+1, -1)
= (1764) + (900) - (2520)*(+1, -1)
= (2664) - (+2520, -2520)
= (144, 5184)
V(m/e) = (12, 72)
The 12km/s case is a meteor approaching Earth from the rearward
side. The angle between the vectors is zero. The 72km/s case is angle
180 degrees. The meteor hits us on the forward side. This is the range
of speeds noted in references of meteor showers.
Earth's gravity
-------------
The relative velocity found above are for the meteor and Earth far
apart, where Earth's gravity is negligible. A distance of one million
kilometers is far enough. As the meteor comes closer to Earth, it
acquires an extra speed component due to gravity.
Because the meteor comes from a remote distance away, the gravity
speed, by the time the meteor becomes a shooting star, is the escape
velocity of Earth, 11km/s. Its path also deflects downward.
The gravity vector points toward Earth from where ever the meteor
happens to be. Its angle from the meteor vector varies over the Earth
from parallel to orthogonal. It is not, as some authors state, a fixed
correction factor for all shooting stars.
The least added speed and greatest path deviation is for the
shooting star moving horizontally. The radiant is at the horizon and
the meteor is skimming the higher atmosphere.
The greatest speed increment, with also no deviation, is for the
meteor coming from the radiant in the zenith. With the extreme closing
speeds above, plus a Perseid, the meteor speed as a shooting star is:
-------------------------------------
closing | minimum gravity | max grav
--------+------------------+---------
12 km/s | 16 km/s, -42.5 d | 23 km/s
60 km/s | 61 km/s, -10.4 d | 71 km/s
72 km/s | 73 km/s, - 8.7 d | 83 km/s
-------------------------------------
The negative angular deviation means the shooting star's path is
more vertical. For meteors between these extremes, the speeds and
deviations have intermediate values. This effect is the zenith
attraction. The book location of the radiant omits the gravity vector
because this vector depends on the observer's hour and location for
each meteor..
Node crossing
-----------
The crossing of Earth thru the comet's orbit plane is a geometric
calculation, a specific date and hour. In the absence of knowing the
pattern of debris around the comet's orbit, this moment is a working
value for our deepest incursion into the streamtube of meteors.
If the streamtube is smoothly populated, with only a radial
thinning from densest at the comet orbit to a diffuse perimeter, this
is a good estimate when to expect a maximum number of shooting stars.
This is not always true. If the meteors are in clumps or threads
or glomera along the comet orbit, the node crossing may not be the
moment of maximum downthrow of shooting stars. If by bad luck we pass
BETWEEN these concentrations, we get few meteors, in spite of our
proximity to the comet's orbit.
There are two times noted for the Perseids and other annual
showers. One is the node crossing; the other, expected peak for the
number of meteors. They may differ by hours or days. For the Perseids,
from the parameters above, the crossing is on 2010 August 12 07h UT.
The peak is expected on 2010 August 12 23:30-August 13 02:00 UT.
This comes from models of the meteor distribution in the streamtube and
will vary among authors. It is a range of dates to recognize the
uncertainty in the streamtube structure.
Mind well that either moment may fall in your daytime or twilight
or when the radiant is down. Please check the location of the Sun and
radiant in your sky BEFORE making plans for your meteor watch..
You must observe the meteors when the radiant is up at night. This
may be many hours away from the maximum or node crossing. For most
showers the trepidation around that moment isn't serious. You'll see
about the proper amount of meteors coming down on you for that night.
Notice how I phrased that. i did NOT say you'll see the promised
or advertised number of meteors! Altho the Perseids are a reliable
shower with a good downthrow of shooting stars, many tens per hour
over you, there were years when it offered up only a modest display.
In other years it put on an extra rich show. We yet can not look out
at the streamtube and judge which part we'll pass thru.
Perseid meteor
------------
A typical Perseid is yellow or yellow-white, flitting rapidly in
the sky. It usually just snuffs out at the end of its path. Few
Perseids burst or explode as bolides. We believe this is from the
small size of comet debris, with few large pieces to form bolides.
Meteors tend to come in bunches, a few within a couple minutes,
then a lull of a few minutes. Under adverse sky this cadence yields
maybe ten bright meteors per hour. If you watch for only a few minutes
and see one meteor, you may be in a lull. The ZHR, or your actual
count, is built over intervals of at least one hour.
Perseids close to the radiant have short paths for coming straight
at you. The paths lengthen with angular distance from the radiant to a
maximum at 90 degrees.
In theory you can see shorter trails for meteors farther away then
90 degree because they are heading away from you. There are few
specmina that remote. Such meteors already did their thing in the
atmosphere and are reduced to ash.
The brighter Perseids, of Jupiter brilliance and greater, may
leave a smoke train behind them. This isn't real smoke but the glowing
air molecules ionized by the meteor's heat. The glow fades quickly as
the gas cool and regain electrons. In 2010 Jupiter is in Pisces, high
in south during your Perseid watch.
Shower behavior
--------------
There is a common misapprehension, prevalent thru the 1980s and
only now being dissipated. Some diagrams and simulation make the
meteors shoot out from the radiant as if they are flat on the
celestial sphere. A picture of a radiant near the horizon shows a fan
of meteors darting upward.
Meteors are events occurring only tens of kilometers away. From
separate locations within a town there is sensible parallax, evidenced
in simultaneous photographs of the meteors. From such pictures awe can
delineate the 3D path of meteors and work up their orbits.
For this reason, the apparition of a meteor shower varies
drasticly with altitude of the radiant. This diagram explains.
__
__
__ ---a--
__ / \
__ b d
parallel paths of __ / \
------------------> __ | |
meteors from comet __ c Earth d
| |
-- \ /
-- b d
-- \---a---/
--
__
The '__' are paths of the shooting stars. They are parallel across
the whole Earth. Observers at 'a' see the radiant near the horizon but
no meteors. They may see a skimmer, a meteor that grazes the upper air
overhead but this is a rare event. The other meteors are too far to
see, well beyond the horizon.
Shooting stars ignite about 100 kilometers up, against the 12,800
kilometer diameter of Earth. Recall the view of Earth from low orbit.
The atmosphere is a thin shell, in the figure grossly exaggerated.
Observers at 'b' have the radiant about half up in their sky and
they see a good display of shooting stars. That's why to see a meteor
shower you have to let the radiant get into high sky. The meteors then
slant downward on you.
The observer at 'c' has the radiant in his zenith, to take in the
headon flow of shooting stars. This is the ideal position of the
radiant for a meteor shower. It may not be possible or practical for a
given shower for your location and hour to have a zenithal radiant.
The zenith pass may occur in twilight or daylight or your latitude-
declination combination prevents a zenith pass.
Observers at 'd' see nothing at all. The radiant, and every meteor
issuing from it, is blocked by the Earth. This happens if by diurnal
rotation the radiant is down or your latitude-declination relation
prevents it from rising. The Perseids can not be seen from far south
latitudes because Perseus is a semperlatent constellation.
The Perseid radiant climbs higher from midnight thru dawn. The
number of meteors should gradually increase over the hours until the
sky brightens from twilight. The most shooting stars may be in the
last hour before dawn breaks.
One early mystery of meteor showers was that they are prevalently
seen in the east quadrant of the sky. Few are in the others. Since the
radiant has to rise to come into view with its meteors, the meterors
are biased into the east quadrant. This is a natural consequence of
their comet origin, as worked out here to fore.
Duration
------
News media stress only the maximum date of the Perseids, as if
this is the one and only night to see the shooting stars. It takes
several days, some say a full month, for Earth to pass thru the
Perseid stream. For practical purposes, once the rate fall to 1/2 or
less of the peak, the apparition is over. In the case of the Perseids,
this is about five days centered on the peak.
This leeway to see at least some Perseids allows astronomers to go
out on the weekend nearest the maximum, the 'Perseid weekend'. They
can sleep late into the next morning as one bonus. By judicious choice
of the weekend, they can avoid an intruding Moon.
Earth's speed in her orbit is quite 100,000km/h. The 5-day passage
makes the stream some 12 million kilometers wide! This is stupendous
compared to the few kilometers for the Swift-Tuttle nucleus. It shows
how far the comet spillage can disperse.
Some observers claim the Perseids are recognizable from late July
thru end August. The number of meteors is then so low that it takes
careful plotting on starcharts to see if they really come from the
Perseid radiant. For casual observers, the viewing window is really
August 9 thru 15.
Radiant drift
-----------
The radiant is the upstream vanishing point of the meteors. The
usual treatment of meteor showers puts the radiant at a fixed point in
the stars. The position given in references is really that only for
the peak date of the shower.
The radiant shifts location because while the Earth passes thru
the stream tube, it is running in its curved orbit. The resultant
vector is altered in direction, and a bit in speed, to aim at a
different point in the stars. This makes the radiant drift during the
passage of Earth thru the stream.
The drift is eastward by quite one degree of ecliptic longitude
per day. The length on the sky of the drift is a cosine function of
the ecliptic latitude. The Draconids, for example, have a very small
drift, because its radiant is near the north ecliptic pole.
For the Perseids of 2010, the drift is:
----------------------
date | Rt Asc | dec | lon | lat
-------+--------+-----+-----+----
Jul 30 | 01h56m | +54 | 048 | +39
Aug 5 | 02h28m | +56 | 055 | +39
Aug 10 | 03h00m | +57 | 061 | +38 ( nominal maximum date;
Aug 12 | 03h12m | +58 | 063 | +38 --( 2 degrees south of
Aug 15 | 03h24m | +58 | 065 | +38 ( Pazmino's Cluster
Aug 20 | 03h48m | +58 | 069 | +37
----------------------------------
If you observe on days earlier than the peak date, the radiant is a
bit west of the nominal position; after, east. Since it takes several
days to pass completely thru the stream tube, being many millions of
kilometers wide. The cumulative drift can be 10 or more degrees.
The actual location in the stars for a given observer and moment
is disturbed by two factors. first is the zenith attraction due to the
deflection of the shooting star's path downward. The meteor seems to
come from a point higher in the sky.
An other effect is diurnal aberration, which is commonly skipped
in meteor orbit work. The observer is carried eastward under the
shooting star as the Earth rotates. This motion displaces the meteor
westward in the sky. The amount is usually small because the ground
speed is at most only 1/2 kilometer per second.
Solar longitude
-------------
As the Sun marches thru the zodiac, he continuously increases its
ecliptic longitude. It starts at zero longitude at the vernal equinox,
near March 21, and completes the 360 degree circuit on the next vernal
equinox. For the Perseids the Sun's longitude is 140 degree, near the
Cancer-Leo frontier, on August 12.
Because our calendar is ganged to the Sun, the longitude and date
are matched one-for-one. Decimal longitudes relate to hours within a
day. The Sun's speed thru the zodiac is pretty steady, taking 365
steps to complete one lap. It is this feature of nature that gives us
the 360-degree division of a circle. 360 is a very manipulable number
close to the actual 365 days or steps of the Sun.
The visibility of a meteor shower is sensitive to the location of
the Sun in the ecliptic relative to the radiant. Some showers do peak
in our daytime because the Sun is too close to allow seeing the
radiant at night. We know of these showers by radio or radar methods,
an area of observing some home astronomers try out.
Altho stating the date gives uniquely the location of the Sun, it
is common to cite the actual ecliptic longitude in the stead. This is
immediately correlated on a starchart with the radiant's location. Some
starcharts dimension the ecliptic with both degrees of longitude and
calendar date. With the leapyear scheme in our calendar the longitude-
date alignment cycles one day every four years, an amount that can be
critical for a shower with a brief maximum of a couple hours.
The location of the radiant is commonly given in ecliptic lat-lon
rather than RA-dec. Some authors merely convert the RA hour-minute
value to degrees and state that, with the degrees of declination.
Please read the text carefully, else any correlation of the radiant
and Sun is erroneous.
There can be a confusion in stating longitude. The ecliptic
longitude (and latitude) of a point in the solar system can be banked
off of either the Sun or Earth. The lat-lon of a point among the
planets are grossly different from the two viewpoints! Specificly, the
longitude of Sun as seen from Earth is 180 degrees opposite that of
Earth as seen from Sun. It is crucial to know which eye is looking.
Latitude
------
The observer's latitude influences the diurnal path of the radiant
across the sky, its rise-set times, the hours of night. The Perseids
from a far north latitude suffers from the White Night season. This is
when the Sun is closely under the north horizon after sunset to leave a
twilight all night long. This kills the meteors.
A too-far south latitude puts the radiant too low in the sky. For
latitudes south of about 30 degree south the Perseid radiant doesn't
rise at all and there is no shower to see. The meteors are falling on
the north half of the world.
A latitude of 35 degree north is about ideal for both duration of
(the short summer) night and altitude of the radiant in owl hours.
This takes in most of the United States, south Europe, North Africa,
north India, south China, Middle East, other places in this band.
Viewing hours
-----------
The Perseids, and any shower, can be viewed only when its radiant
is in the sky, the higher in altitude, the better. Perseus in August
rises about 20h standard time, 21h daylight, with the radiant only
about 10 degree up. These hours are for Algol's rising, to put the
bulk of Perseus above the horizon.
Remember that the meteors travel over the Earth in parallel paths.
With the radiant near the horizon the meteors are falling verticly on
the far side of the Earth. You miss them. Since the air is only a thin
shell of some 100km depth, a shooting star too far away is beyond your
local horizon.
Due to its high declination in the mid northern latitudes, the
radiant is circumpolar. You will not see any meteors when it skirts
the north horizon. The meteors are falling on countries beyond the
horizon.
As the radiant climbs higher, the meteors fall more steeply on
you. By the time it is 30 degrees up, you start seeing meteors. For
our location in the latitude of New York City this occurs at about 23h
standard, 00h (of the next day) daylight. You may start you meteor
watch at or after local midnight.
Most of the major shower have their radiant in high sky in the owl
to dawn hours. Only a couple showers have high radiants in evening to
night. Meteor shower watching means you rouse up from bed or stay up
with late night television.
Meteor activity increases with increase of radiant altitude, which
for the Perseids persists thru dawn. The most meteors are expected in
the hour or so before morning twilight.
In the dawn twilight, when the stars are winking out, there is no
further viewing. Save for the rare really brilliant Perseid, all of
the meteors are veiled in the approaching daylight.
Viewing direction
---------------
The usual advice from the Perseid announcement is to watch the
radiant, described as 1/3 to 1/2 up in northeast. The thought seems to
be that here you'll see the meteors spurting outward in all directions
from a central point. This is true to a degree.
You are looking into the oncoming shooting stars, so they do in
fact spread out from their vanishing point, Precisa mente because you
are looking at them headon, their paths in the sky are short and
easier to miss sight of. It's also harder to tell if a meteor traces
back to the radiant or is only a background meteor that happens to
have s short path.
Experienced meteor watchers look about 45 to 60 degrees away from
the radiant. The meteors have longer trails, being more broadside. It
is also far easier to distinguish Perseids from randoms. In the
Perseid season this means looking in Gemini, Lynx, Polaris, Cepheus,
Pegasus, Pisces, Cetus, Eridanus, Orion, as they may be up.
I do NOT mean to avoid the radiant! Shift gaze from time to time.
Impediments
---------
Haze, humidity, luminous graffiti are almost guaranteed in the sky
over most conurbations in the American northeast and Great Lakes. The
first two slash the transparency to oblitterating the fainter stars
and meteors. The display is a weak one with only the occasional bright
shooting stars.
Luminous graffiti reflects and scatters off of the haze and humid
air to surround you with an artificial twilight. Stars and meteors are
hidden behind a luminous veil, again making for a thin display.
Mind well that the advertised number of expected meteors includes
all brilliances, even the threshold ones lost in imperfect sky. This
is invariably the book value of the zenith hourly rate, a number not
realizable by a single observer.
It's the multitude of the dim meteors that produces the shower
effect. The once in a while bright meteors are not so obviously part
of the shower.
A summer night in some parts of the US can be utterly miserable!
You can be wasted by heat, stagnant air, humidity, insects. You should
have a refuge in an acclimated room, abundant fresh water, fresh-up
facilities. Light music is good, plus an ear up for weather reports.
In many urban zones there is no safe permissible place to view the
shower, which must be observed in owl to dawn hours. Beaches and parks
typicly close by midnight. Other public spaces may be grossly
unappealing for the astronomer during the shower.
For these reasons I and experienced astronomers in the northeast
part of the US do not heavily promote meteor showers as a public
event. The risk of disappointment or underappreciation are too great.
On the other hand, astronomy clubs in the Northeast sometimes
arrange for a suitable viewing site for a Perseid meteor watch and
provide safety and conveniences. These may include restrooms, cooloff
room, snack counter, carpool to a busy transit station.
In addition to viewing the meteors, the club offers general
starviewing, short talks, telescope demonstrations, handouts. If the
meteors come off poorly, there can be other sights to examine in the
sky under the guidance of the club.
Please ask the club about special preparations. Typicly you must
bring your own chair, blanket, drinks, radio, other personal items.
You may leave your scope home if you plan to view thru the club's.
Perseid watches announced early enough are posted in NYC Events
for August of each year at 'www.nyskies.org/nyc.htm'. Late notices are
posted in the NYSkies yahoogroup at 'www,nyskies.org/yahoo.htm'.
The Moon
------
One colossal source of luminous graffiti is the Moon. In owl hours
a Moon in the sky is a large Moon, at large elongation from the Sun.
She is waxing gibbous thru waning crescent. She veils the fainter
meteors and stars.
The Moon indexes thru the zodiac for each year of the Perseids. In
2010 we are lucky that the Moon is a couple days old and sets in early
night.
In 2009 the Moon was in Aries-Taurus, near Perseus. Shed decimated
the number of meteors. In 2011 the Moon is near full in the west,
again to wipe out the fainter shooting stars.
The indexing is comes from the lunar-solar cycles. One solar
cycle, between Perseid maxima, is 365.25 day. This is 12 lunar phase
cycles plus 10.89 days. This is about 1/3 of the next, 13th, cycle.
For a given position of the Moon in one year, on the same date
next year she is about 1/3 of a lap farther downrange in the zodiac;
in the 2nd year, 2/3. The Moon hops around the ecliptic in a rough 3-
year cycle, for one moon-free Perseids between two Moon-trashed ones.
The 3-year rule falls apart after a decade because the numbers
aren't simple ratios. A new round of cycles is set up for the next
decade. A similar analysis applies for other showers and annual
celestial events.
When planning a meteor watch, it is vital to check the Moon. Use a
computer planetarium program set for the viewing hours. Many well-
intended meteor watches are torpedoed by failing to mind the Moon.
Other showers
-----------
There are two other major meteor showers that run with the
Perseids. They are poorly recorded because just about every one
fixates on the Perseids and treats the other meteors as part of the
background of random shooting stars.
The kappa Cygnids radiate from northern Cygnus, in high west
during a Perseid watch. The Capricornids are in the low south. If they
give a good display, they being very erratic year to year, you may see
a cross fire of meteors as if the three constellations are in a
celestial artillery battle.
An other possible source of extraneous meteors is the newly found
Antihelion. This is not a true radiant, not the upstream view along a
comet orbit. It is a 15-degree spot of sky some 12 degrees east of the
instant antihelic point on the ecliptic. With the solar longitude of
140 degrees on August 12, the Antihelion is at longitude 332 degree,
in Capricornus-Aquarius.
The theory of the Antihelion is too vague as at 2010. It appears
that for sporadic shooting stars, there are more per hour from this
region than elsewhere. In August the Antihelion is close to the
Capricornid radiant. In other months it's near other radiants of low
ecliptic latitude. These radiants may have disguised it for so long
before recognition in the early 2000s.
Observations
----------
The Perseids were recognized as a comet product in 1867 but were
noticed occasionally since the 3rd century AD. We figured that these
dispalys were the Perseids by analyzing the local date, hour, and
description.
Once recognized as an annual event they were regularly observed
since the 1840s. They were announced as a public spectacle, with some
of the same vivid language we see today!, in the general newspapers
and outreach presentations since the 1870s.
Other showers attracted public attention but, due to their erratic
performance, they never rose to the status of a annual expectation as
the Perseids. The result is that of all the showers, the Perseids are
the best documented and recorded for over 160 years.
The main mass of knowledge of meteors comes from bare-eye records
of shooting stars made from the ground by home astronomers. We have no
comprehensive data for meteors fainter than the bare-eye threshold.
I muself tried a novel instrument for the 2001 Leonid metero
strom. I viewed the sky thru a night-spy scope. In its field of about
5 degrees were HUNDREDS of very dim meteors beyond bare-eye sight!!
Home astronomy records cover only the meteors when they are
visible as shooting stars, below the elevation of low-Earth-orbit.
Reports from radar, infrared, radio methods are too scarce,
incomplete, irregular, to form a solid knowledge base of meteors.
The behavior of meteors beyond the air is largely guesswork. We
build models that try to reproduce the visual aspect of the shower.
This model shower is compared to the observed one to judge the model's
validity.
It is slow sledding, specially since the Leonids of the turn of
the 21st century. Newer home astronomers no longer have the mind to
dwell outdoors quietly and count or plot meteors. I have to be fair to
note that serious meteor observing demands patience, discipline,
diligence, attention, care for many hours at a time, possibly on
several nights. Today's society no longer promotes or favors such
activity like it used to in bygone generations.
Home astronomers also, to a disheartening degree, commonly stop
watching a shower after seeing a good show on one night. They skip the
rest of the shower's apparition.
This makes the records after the maximum days thinner than before
it. Once the observer gets his good view, he's finished with the
shower until next year.
It is also getting harder to find clear dark skies with the spread
of luminous graffiti in many parts of the world. Locations once bare
of habitation are divided into sprawling suburbs conductive to light-
crazy lifestyles. The remaining good viewing sites are many hours
drive or ride away from home. Such a chore drasticly quenches the
initial enthusiasm to watch a meteor shower.
The global warming trend, even if only for a couple decades, will
close off ever more nights by adding enduring and frequent haze and
moisture into the air. This is specially the case for locations near
large lakes or the ocean.
Spacecraft hazard
---------------
We on the ground are protected from the collision with Perseids by
the atmosphere. All of the Perseid shooting stars are reduced to ash
many tens of kilometers up. Similarly for other showers in that there
probably are only a handful of specimina as meteorites against the
squillions of stones raining on the planet.
Spacecraft lack this protection. They are exposed to the raw
influx of the meteors. The meteors, with speed of tens of kilometers
per second, can seriously harm a satellite, as they crash into its
fragile fuselage and guts.
When the space program matured with significant, and expensive,
facilities in orbit, the threat of meteor attack was considered. The
major effort to understand and assess this threat was in preparation
for the Leonid storm of the late 1990s and early 2000s.
It actually happened. The Olympus-1 telcomms satellite was hit by
a Perseid in 1993. It damaged the electronics for the gyroscopes.
Efforts to regain stability used up most of the craft's fuel. The
operator got Olympus-1 into a graveyard orbit and abandoned it.
Also in 1993 the launch of shuttle Discovery was held off until
after the Perseids were over. This was a precaution against possible
harm to the craft with its human payload. As a matter of standard
practice, the Shuttle is inspected after each flight and meteor hits
are found in the tiles and windows.
In 2009 Landsat-5 satellite was hit by a Perseid, causing it to
overspeed its gyroscopes. After a spell of tumbling, ground control
stabilized it to resume normal operations. Landsat-5 is still in
service, but can be victim to a future meteor strike.
As seen from an orbiting satellite, the radiant wanders in an
aberration ellipse centered on the geocentric radiant. Each lap of
this ellipse takes one orbital period of the satellite, A satellite in
low Earth orbit has a speed of 7-1/2km/s relative to the Earth center.
One in geostationary orbit moves at 3km/s.
Consider a low-elevation satellite in a orbit facing the meteors,
inclination 90 deg degree between the orbit plane and the direction of
the meteors. The satellite velocity vector is orthogonal to the
meteor's. The aberration is, with the 60km/s speed of a Perseid:
satellite--> O------------------------------> meteor vector
| 60km/s
7-1/2km/s |
\|/
satellite vector
(aberration) = atn((7.5 km/s)/(60 km/s)), low Earth orbit
= atn(0.1250)
= (7.13 degree)
In this extreme case of a faceon orbit, the satellite is exposed
around the edge of Earth to the meteors for its entire orbital cycle.
It has no relief for half a period by hiding behind the Earth. For
higher orbits, with lower speed, the aberration is less. Depending on
the craft's purpose, vulnerable components, manoeuvering, it is a
potential victim from a meteor collision.
The energy of a meteor is awesome, despite the minuscule mass of
the particle. A Perseid of 0.1g mass has a kinetic energy, relative to
Earth center, of 180,000 joule. This is about the energy of a 1-1/2-
ton automobile hitting you at 55km/h. Such a collision is often fatal
for the pedestrian!
On the satellite, this energy is concentrated in a mere point of
contact. The meteor may plow clear thru the craft without stopping.
Enough slowing occurs to transfer substantial energy to damage or
destroy the craft's fragile internals.
Perseus
-----
Perseus is the pont tying the fall and winter constellations. To
the west are Andromeda, Cassiopeia, Pegasus of fall. These are during
your watch in the overhead to south region of the sky. To the east,
below Perseus, are Auriga, Taurus, Gemini (as dawn comes).
You should take advantage of your hours under the night sky, given
clear mild weather, to inspect these constellations and surrounds.
Have to hand observing guides, charts, binoculars, scope to plan
excursions between meteor vigils
The tables below are for ONLY for targets in Perseus. All are
discernible, even if weakly, from New York City. The two open nebula
are a challenge and will almost for sure be veiled by the summer-
soaked sky of the Perseid season. You may have to wait for the darker
drier sky of autumn in a couple months.
--------------------------------------
DOUBLE STARS
Perseus has many attractive double stars. The ones here are the
more showy ones for viewing from the City. For the three whose
separation and position angle vary on timescale of decades, I give
their epoch. The other two seem fixed over a century's span.
The color is the schematic tint for the spectral class of the
primary star.
------------
Designatn RA2000 DC2000 Cns MagA MagB Colors Sep PA Year
--------- ------- ------ --- ---- ---- ------ ----- --- ----
epsilon 03 57.9 +40 01 Per 2.9 8.1 b-w 8.8 10
eta 02 50.7 +55 54 Per 3.3 8.5 ora 29 301 2002
omicron 03 44.3 +32 17 Per 3.8 8.3 b-w 1.0 24 2004
psi 01 05.6 +21 28 Per 5.6 5.8 31 159 1972
56 04 24.6 +33 58 Per 5.9 8.7 4.2 250
---------------------------------------------------------
OPEN CLUSTERS
The Milky Way slices right thru downtown Perseus. Expect to find,
specially with binoculars, heaps and piles of stars here and there,
scattered from Cassiopeia thru Auriga.
Please take in the other clusters besides just the Double Cluster
and M34. It surprises new astronomers how well these off-road clusters
can show up from the City.
Mel 20 is the gaggle of stars accompanying alpha Persei, Mirfak,
as a bound association. Its magnitude +1.2 is the aggregate of the
dozen or so stars in the association.
-----------------------------------------------------------
Designatn RA2000 DC2000 Cns Size Magn CR MEL Other
--------- ---------- --------- --- ----- ---- --- --- -----
Mel 20 03 22 00.0 +49 00 00 Per 185' 1.2 39 20 alpPer
NGC 1039 02 42 00.0 +42 47 00 Per 35' 5.2 31 17 M34
Mel 13 02 19 00.0 +57 09 00 Per 29' 5.3 13
NGC 869 02 19 00.0 +57 09 00 Per 29' 5.3 24 DblClus, hPer
Cr 29 02 37 18.0 +55 59 00 Per 20' 5.9 29 Tr2
NGC 884 02 22 24.0 +57 07 00 Per 29' 6.1 25 14 DblClus, chiPer
NGC 1545 04 20 54.0 +50 15 00 Per 18' 6.2 49
NGC 1528 04 15 24.0 +51 14 00 Per 23' 6.4 47 23
NGC 1444 03 49 24.0 +52 40 00 Per 4.0' 6.6 43
NGC 1342 03 31 36.0 +37 20 00 Per 14' 6.7 40 21
-----------------------------------------------------
GALAXIES
Being in the Milky Way you may not think of Perseus as a home for
galaxies, at least not discernible in small scopes. For the most part,
that's right, but there's one galaxy that I glimpsed personally from
New York -- from Manhattan! -- with a 100mm totable scope. Look for
NGC1023 when the sky is dark and dry. In the Perseid season such
nights may be hard to come by, but try for it.
---------------------------------------------
Designatn RA2000 DC2000 Cns Size Magn OtherName
--------- --------- --------- --- -------- ---- ---------
NGC 1023 02 40 24.1 +39 03 46 Per 8.7'X3.3' 9.2
-------------------------------------------------
OPEN NEBULAE
These ARE challenges for the City! I never positively saw them,
tho I believe I got a hint of glow for NGC1499 from Brooklyn on a
really crisp (and frigid) winter night. Your meterage may vary.
--------------------------------------------------------------
Designatn RA2000 DC2000 Cns Size Magn OtherName
--------- ---------- --------- --- --------- ---- ---------
NGC 1499 04 01 16.0 +36 38 24 Per 160'X40' 6.0 xi Per
IC 348 03 44 30.0 +32 08 24 Per 10'X10' 7.3 HD 281159
-----------------------------------------------------------
Algol
---
I generally pass over variable stars in listing targets because
they require repeated inspection to follow their brightness changes.
Weather and outside life will interfere with a regular scheme of
tracking a given star.
One exception is Algol, beta Persei, here front and center for you
on Perseid night. It's an eclipsing binary star where a bright and dim
star alternately eclipse of each other, causing the combined light
output to vary in a cycle of 2.87 days.
The star orbits are stable to predict the light output but the one
feature of Algol is the fast fall and rise of light when the dimmer
star crosses the brighter. From full brilliance, when the both stars
are shining on us, it takes only 2 hours to fall to a minimum luster.
After 2 hours of eclipse the star brightens, taking a final 2 hour
to regain full brilliance. During a long winter night it's possible to
see the complete down and up phase. The shorter night in the Perseid
season prevents this unless the dip starts at nightfall.
As fate falls on us in 2010, Algol does not do its thing during
the couple nights of Perseid season. The minima occur in daylight or
dawn. If you're game to look on other nights in August 2010, here's
the timetable:
----------------------------
date | UT | EDST | sky
-------+-------+-------+-----
Aug 1 | 00:38 | 20:38 | previous dusk
Aug 4 | 21:26 | 17:26 | day
Aug 7 | 18:15 | 14:15 | day
Aug 10 | 15:04 | 11:04 | day
Aug 13 : 11:52 | 05:52 | dawn
Aug 16 | 08:41 | 04:41 | night
Aug 19 | 05:29 | 01:29 | night - good chance
Aug 22 | 02:18 | 22:18 | previous night - good chance
Aug 24 | 23:07 | 19:07 | dusk
Aug 27 | 19:55 | 15:55 | day
Aug 30 | 16:44 | 14:44 | day
----------------------------
Please note that when the hour falls in day or dusk I skipped a
check for rise/set. It doesn't matter.
Al Maaz
-----
This is a very important star whose proper name is almost never
cited! This is epsilon Aurigae, who stirred up such global excitement
in 2009. It, like Algol, is an eclipsing binary, with a humongous
difference. Its period is, uh, 27.1 YEARS and its minimum lasts 16
MONTHS!
It started its decline to minimum in fall of 2009, reaching it by
January 2010. It'll stay dim thru March 2011, then begin its climb
back to full luminance. During all of 2010, like while the Perseids
are playing, epsilon sits at least luster.
The Perseids is probably the first chance to look at epsilon since
you last inspected it in evening twilight last spring. Renewing your
acquaintance with it now helps you recognize the star when Auriga
migrates into evening sky in a couple months.
Pazmino's Cluster
---------------
You're looking almost straight at my cluster when looking at the
Perseid radiant! The radiant passes two degrees south if it, across
the Camelopardalis-Perseus border. It's a lovely sight in binoculars
and small scope but so far I never got any positive reports of a
sighting by bare eye. It should be within sight at magnitude +6.5.
Be as it may, there it is in all its glitter and sparkle for you.
Conclusion
--------
The annual Perseid meteor shower is not just a pleasing celestial
event, reasonably dependible in its show. It can be a jumpoff for many
astronomy concepts and public erudition, like at indoor talks before
the viewing or preparation on prior days. One or two of these can be
a reserve for viewings ruined by adverse weather.
The topics I discuss here apply to other showers, with their own
parameters. Include them in club and public viewing sessions for them.
The prime caution is to not expect the cited zenith hourly rate!
Be comfortable, in a lawn chair with cool soft drinks, munchies, light
music. You get a pleasing display in clear skies of a score or so
brighter shooting stars, plus an additional score or so of dimmer
ones.