CRUX SEEN FROM NEW YORK CITY
--------------------------
John Pazmino
NYSkies Astronomy Inc
www.nyskies.org
nyskies@nyskies.org
2020 October 18 initial
2020 Decembner 26 current
Introduction
----------
The NYSkies Astronomy Seminar on 2019 September 20 explored the
effects of precession, the slow steady gyration of the Earth's axis.
his motion, taking some 25,000 years to complete one cycle, shifts the
equatorial coordinate rid across the stars, causing their right
ascension and dclination values to drift over the centuries. We
studied starcharts, plotted by computer planetaria, showing the
displacement of various constellations over Nw York City at several
epochs past and future. One was for Crux or Southern Cross. We saw
that in past eras Crux did rise (and set) over the City but
precession then pulled it south. It now skims under the south horizon
at culmination by some ten degrees. This is why for us Crux is a
'southern' constellation.
I was playing with precession now and examined the takeaways from
that Seminar. There's more of a story than we had time to discuss at
the Seminar.
Constellation Crux
----------------
I give here a few highlights of the constellation, it, like
others, is amply described thruout the astronomy litterature.
Crux, Latin for 'cross', is a modern constellation from the 17th
century. As such it lacks a classical mythology. Before then its stars
were included with Centaurus. Some observations of the cross figure of
Crux as a new asterism were made by 16th century explorers in the
southern hemisphere. Bayer in 1603 lettered stars now in Crux with
those of Centaurus.
It wasn't generally recognized as a distinct constellation until
the late 1600s. When Crux was split off, the Bayer sequence in
Centaurus was messed up and relettered by later astronomers. Crux got
its own Bayer letters.
Crux is centered at about 12h 30m right ascension and -60 d
declination.It is the smallest in area of the constellations, only 68
square degrees. It sits 1/4 way from Hadar, in Centaurus, to Caanopus
in Cerina. Its downtown, the very diamond or whatever, is marked by
four bright stars. Clockwise from the south end of the diamond they
are Acrux, Mimosa (or Becrux), Gacrux, delta Crucis. Thre east-west
lenth of the figure, between Mimosa and delta, is 4 degrees (rounded).
North-south from Acrux to Gacrux is 6 degrees. The official border of
Crux is a rectangle aligned with the equatorial grid about 9 degrees
north-south and 88 degrees east-west.
The Cemtauri, stars Toliman and Hadar to the west of Crux, aim to
the cross asterism. Occasionally i hear of Crux combined with the
Centauri as an other new asterism.
Crux isn't as handy a starfinding tool for southern stars as the
Big Dipper is in the northern sky. Its asterism is too small for good
lineups toward other stars and is itself embedded in bright stars. The
only major, but important, alignment is along the north-south arm
toward the south celestial pole.
As much as I try from my visits to the southern hemisphere,
typicly fr comet or eclipse chasing, i can not at first sight call the
figure a cross. It's a diamond or rhomboid. it also isn't all that
spectacular, being surrounding by the Milky Way and bright stars of
Centaurus and Carina.. On such trips I at times point out the Coalsack
first and then the rhomboid/cross/diamond figure of the Southern
Cross.
Stars in Crux
-----------
Downtown Crux is the very cross/rhomboid/diamond astrrism with
CCorners at Acrux, Becrux, Gacrux, delta Crucis. These are clockwise
from the southern,brightest, star of the figure.
beta is also known as Mimosa while delta has no wisely used proper
name. I guess 'Delcrux' didn't catch on?
Because the far south sky wasn't explored until the 1500s it was
not incorporated into the culture of mid northern societies. One
result is that few southern stars, even bright ones, have classical
proper names.
When stars in the far south were selected for ocean sailing, to
help navigate at night, finagle names were given to the brighter
stars.Crux now has Acrux, Becrux Gacrux, standing for alpha, beta,
gamma Crucis. A couple other examples are Delvel (delta Velorum) and
Epscen (epsilon Centauri).
The table here of the four cardinal stars of the asterism is
adapted from wikipedia's full list of Crux stars. The widely spaced
comites of alpha Crucis are listed separately.
--------------------------------------------------
Stars in the Crux asterism
----------------------------------
Row 1; star - HD number - HIP number - proper name
Row 2: RA (2000) - DE (2000) - Mapp - Maabs - LYdist - Spectrum
---------------------------------------------------------------
alpha1 108248 60718 Acrux A
12h 26m 35.94s -63ø 05' 56.6" +1.4 -3.6 321 B0.5-IV
alpha2 108249 ---- Acrux B (Maapp of A+B = +0.9)
12h 26m 6.50s -63ø 05' 58.0" +2.09 -2.88 321 B1-V
beta 111123 62434 Becrux, Mimosa
12h 42m 43m32s -59ø 41' 19.4" +1.25 -3.92 352 B0.5-III
gamma 108903 61084 Gacrux
12h 31m 09.93s -57ø 06' 45.2" +1.60 -0.56 88 M4-III
delta 106490 59747 ----
12h 15m 08.76s -58ø 44' 56.0" +2.79 -2.45 364 B2-IV
--------------------------------------------------------
Just these stars shows the difference between a casual lineup of
stars in an asterism and a physical proximity of stars in an open
cluster. The Crux stars range from 88 to 364 lightyears away and would
form a completely other figure when viewed from a direction other than
Earth.
An irritating feature of the Southern Cross is a fifth star,
epsilon, which is the main star within the diamond. Ir should be in
the middle, where the two arm intersect. It's off-centered toward
delta. epsilon's excentricity is immediately spotted by even first
time observers.
Crux sits in a dense reach of the Milky Way. Points of interest by
eye or binoculars are Acrux, a wide double star; kappa Crucis, a large
bright open cluster, the Jewel Box; the Coalsack, an opaque nebula
between Acrux and Mimosa. It is larger than the cross figure and
extends into adjacent Centarus and Musca.
None of these deepsky targets, nor any in nearby constellations,
hvve Messier names. All were too far south Messier in Paris to
observe. Many are in the deepsky observations of his contemporary,
laCaille. Southern astronomers sometimes append to the Messsier
catalog.
Northern and southern?
--------------------
The asterism is 'southern' to distinguish it from the Northern
Cross, the downtown of Cygnus. Since this was never an official
/constellation, it being always the constellation of the Swan, there is
no reason to sa "crux Australis'. Yet some astronomers say that
'Crux', the actual constellation name, means 'Southern Cross'.
Contrasted with Crux and Cygnus is the constellation abutting
Crux on the south, Musca. There once were two Musca constellations.
The northern one, Musca Borealis. was next to Aries, as depicted in the
Sky Ceiling of Grand Central Terminal. That one wa absorbed into
Aries, keeping its identity as the Musca Borealis asterism. There then
was no need to flag the southern Musca. It's plain 'Musca'.
Alterae cruces
------------
Near Crux are two asterisms forming cross figures. The Falsa Crux
(false Cross) is about 1/3 from Canopus to Crux. Its corners are iota and
epsilon in Carina and delta and kappa in Vela. It is actually easier to
notice for being much larger than Crux.
About 2/3 from Canopus to Crux is a dimmer, still obvious, Obscura
Crux (Dark Cross). Its corners are beta, theta, upsilon. omega in
Carina. The Obscura Crux is also known as /Diamond Cross from the
figure's near perfect diamond or rhombus shape.
The three cruces are about equally spaced between Hadar and
Canopus: Hadar - Crux - Diamond Cross - Falsa Crux - CAopus.
theta Carinae is the Southern Pleiades, ta large bright open
cluster.
South celestial pole
------------------
Northern latitudes had the stupendous advantage of having its
celestial pole marked by an plainly obvious star, Polaris. Even when
it was several degrees from the pole in early eras, by precession
effects, its position around the pole was easily calculated from the
orientation of nearby Big dipper.
Historians surmise that the mass projects of seafarig by northern
peoples after the Dark Ages was immensely aided by having such a pole
marker.
The south celestial pole does not have a clearly visible 'south
Star' and lacked one for many past millennia. Apart for the sparse
population of the southern latitudes, the absence of a South Star
impeded development of large scale seafaring.
Astronomers, specially stargazers, need to find the south pole, if
only for aligning starfinding charts.Various pointers toward the pole
were worked out, all approximate, but useful.
One is to use Crux as 'pointer stars' corresponding to our Big
Dipper. The north-south arm of the cross, stepping from Gacrux over
Acrux and continuing for 4-1/2 lengths lands you near the pole.
As a second fix, there is a row of stars in Scorpius-Ara, the
Frying Pan asterism, that also points to the pole. Where they
intersect is closely on the money.. Both of these lineups dissolve
over the mllennia due to precession, but for the rest of our lifes
they work.
An other one is to point at Achenar in Eridanuc. Then point with
the other hand midway between Hadar and Crux. Slowly steadily close
the hands together. They meet at the south celestial pole.
Visibility
--------
From New York Crux never rises. It comes to about 7 degrees, for
its northern star Gacrux, below the southern horizon at culmination.
It sits in the reach f Milky Way commonly called the Southern Milky
Way, spanning constellations Ara thru Vela. This is a boreocentric
name and the limits vary with the observer's latitude. South of
latitude +20 deg the entire Milky Way is visible and there is no
occluded 'southern' reach.
At +33 deg Gacrux skims the south horizon. I suppose, bu don't
know for sure, that astronomers along this latitude parallel have a
'Crux Challenge' like the City's Omega Centauri Challenge. Refraction
may lift Gacrux enough to glimpse it at +34 deg.
The entire cross asterism rises south of +27 deg, making it a
favorite attraction for starparties in the southern belt of the US.
They promote a 'latitude adjustment' for their northern visitors.
South of latitude -34 deg Crux is circumpolar
Crux is diametricly opposite Cassiopeia on the celestial sphere.
The behavior of Crux at a given southern latitude is mirrored by that
of Cassiopeia at the same degree of north latitude. As example, at
Wellington, New Zealand, Crux circulates around the south celestial
pole about as Cassipeia does for us in the City.
Precession effect
--------------
New York did see Crux in years past due to the drift of celestial
coordinates form precession. The questions are: When did Crux start
leaving our skies? When did Crux fully leave the skies of new York?
Lo here the two scenarios:
----------------------------------------------
*
g
* *
b d DEPARTURE COMPLETED
------*----- ------*-----
a g
dEPARTURE STARTS
* *
b d
*
a
'-----' is the southern horizon of new York when Crux is on the
meridian. a, b, g, d are short for alpha, beta, gamma, delta Crrucis,
the stars forming the cross asterism.
-----------------------------------
A computer planetarium with accurate algorithms for precession can
find the answer in a few minutes. As fate falls, precession does not
precisely repeat in each cycle. The Earth's axis tilt varies a bit
broaden the precession arc tracked by the celestial poles.
Precession is slow, amounting to only 1.2 degree increase f
ecliptic longitude per century. This is why your grandparent's
starfinder books and charts are pretty much valid today.
With the program set for tour location, get Crux on the screen.
Center and track it to hold it still. Click n wither star and open
its infobox. Read out the declination.
Play with the 'year' in the date/time settings for the threshold
position of the star. When the declination equals and check the
declination. When the declination equals your colatitude, (90deg) -
(latitude), the corresponding year is the threshold year.
Some planetaria do not precess he star coordinates in its infobox
for the star. It's a fixed parameter as at the base epoch. In such
programs a brute force method is needed for the threshold years.
Play with the year but you now must manually slew the sky to put
Acrux or Gacrux at culmination. The true position of the star is the
center of its plotted. When the star is on the horizon at culmination,
its dot is bisected. When this aspect is achieved, the corresponding
year is the threshold year.
All of the above lead to acceptance of +/- 25 years in the year
for departure of Crux from your sky.
For New York's sky I let the latitude be 40.75 degree. Colatitude
is 49.25 degree. I ignored atmospheric refraction. My program factors
in proper motion of stars but this is negligible for a couple
millennia.
For me the Southern Cross starts leaving our sky in year 430 BC.
It completely left our sky in year 720 AD. It took some 1100 years for
Crux to completely leave New York. We have no record of any humans in
the future New York region who specificly recognized this asterism.
Temple alignments
---------------
Many early cultures, mostly in the mid north latitudes of the Far
East, Mid East, Mediterranean regions, built celestial sightlines
into their temples. 'Temple' here means any structure for inspecting
the stars by means of alignment. Usually, due to obstructed sky
exposure by the temple roofs, the sightlines aim toward star risings
and settings.
In the early AD centuries and back into the BC era there were lots
of candidate stars around Crux to build alignments. The choice derived
from the cultural importance of the stars.
The temples were enduring facilities for ceremonies and festivals
for future centuries. They were not 'world's fair' pavilions to be
torn done after several years. Precession dragged the lined up stars
away from the temple sightline.
Later crews of the temple found the star no longer sat on the
sightline. The 'error' in the lineup was laid on mistakes of the
original builder. When the alignment advanced to tolerate, the temple
was rebuilt or abandoned. We know several examples of such temples
with lost lineups. A collateral benefit of these structures is that.
knowing te star they aligned with, we can precess the sky backward and
determine when the temple was built. Inscriptions in it than fill out
the culture's chronology.
As much as early societies were affected by precession, none, as
far as we know, understanded what was going on. One and all they noted
in their chronicles that 'something funny happened to the stars'. did
in their histories that 'something strange happened to the stars'.the
Did the person honored by the temple fall from grace? Did the deity
lose its favor for the society?
Hipparchus was the first to describe precession and Ptolemaeus was
the first to apply precession check his work against earlier star
catalogs.
New stars for temples?
--------------------
After you played with Crux on your planetarium, set the date to
six months away. You now see that stars in Cetus, Eridanus, Sculptor
are pushed higher in altitude vy precession. These stars are more
easily visible as the centuries roll on.
Why not substitute these newly revealed stars for those, in Crux
&c, shoved out of sight by precession?
Look again at the replacement stars. Almost all are dim sparse
stars, And they make no conspicuous asterisms. These were hardly worthy
of including a sightline in a temple. The early cultures lost their
selection of bright stars around Crux and got in the stead a bunch of
weakly shining stars. The stars embodying their deities and masters
are gone for good. Are the deities and masters also gone for good?
Anthropologists and archaeologists debate today if this
permutation in the sky of these people induced their decline and
extinction.
Conclusion
--------
It happens that an exercise for our NYSkies Seminar comes back in
later months or years to spark a new inquiry. In this instance it
leaded me to look into temple alignments and ancient awareness of
precession.
This instance also shows the power of ordinary astronomy software
can be a laboratory to carry out such an inquiry. They don't just
make starcharts.