n
HANK YOU, THANKSGIVING
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John Pazmino
NYSkies Astronomy Inc
wwww.nyskies.org
nyskies@nyskies.org
2015 November 26
Introduction
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New York in 2015 entered a new season of lunar occultations of
Aldebaran. As the Moon sweeps thru the zodiac, she passes over or miss
stars according as the lunar ecliptic latitude and the observer's
location on Earth. For many years the Moon missed Aldebaran. Now she
is in a run of almost monthly hits, altho some ae not seen from the
City by timezone or geographic latitude.
The occultation on Thanksgiving morning, 26 November 2015,
occurred in clear chilly air. The Moon was almost full and was low in
the west at the start of dawn.
Preparation
---------
For this event I needed a a window facing west with no
obstructions like trees or roofs. Of a couple choices, one window was
perfect with the Moon in open sky over low skyline.
I observed from indoors, to avoid the hassles of setting up
outside so early in the day. My plan was to watch the occultation,
then turn back to bed for a few more hours of sleep.
At the window on the preceding evening of the 25th I moved an end
table under the sill to support a table-top telescope. I set the scope
case next to the table, ready to unpack it quietly and quickly in the
dawn hours.
Because my cats roam around at night I dared not try to view thru
a completely open window. The pets jump to the sill and could easily
slide off with no positive barrier to contain them. In the stead, I
then washed the inner glass pane and then latched open the screen
and outer glass panes. I finally closed the inner pane, protecting the
cars and giving me, hopefully, a clean view thru it of the
occultation..
I placed a small folding chair next to the end table. My observing
station was now ready to rumble. Eons ago I learned that observing
must be done, in so far as practical, under comfort. On the whole,
being seated at the telescope is a huge step toward that comfort.
At my station
-----------
I got up from sleep at 5 AM and dressed in running slacks and
sweater. The air was thoroly chilly but probably not freezing. I threw
over me a heavy wool bathrobe. I went to the window in dark to keep
my night vision.
The sky was clear. I didn't purposely hunt for stars. I did notice
the Pleiades to the right of the Moon. The Moon, being shortly after
full phase, was dazzling. I could not see Aldebaran at all by eye.
I unpacked the scope and set it on its drum base onto the end
table. I never thought of attaching its equatorial stand because I can
follow the Moon by the axis control knobs. I rotated the eyepiece to
face my chair and sat down.
I got the Moon in the eyepiece right away. She was almost
blinding! I scanned the disc for any limb shadow, didn't see any, and
browsed around at the rays and spots here and there.
The occultation
-------------
here was Aldebaran a good way left of the Moon. It was a lucid
twinkling point. No other Hyades stars were noticed.
I was looking thru a single sheet of window glass that made some
distortion in the image. I found that the most pleasing view was with
low power, covering the whole lunar disc. Higher power merely showed
more window distortion.
On & off for the next half hour I checked on Aldebaran. The axis
knobs helped me to chase the Moon as she drifted out of the field.
Aldebaran creeped closer and closer to the Moon's eastern edge. I
mean astronomical east, the leading, edge The sky was starting to
brighten with dawn.
At 5:46 AM EST, by a wall clock, Aldebaran snapped out of sight
behind the Moon. I think it was gone a second earlier as twinkling
sometimes blanked it out momentarily. After a few more seconds of
inspecting the lunar edge, I knew the star was in fact behind the
Moon. I didn't try for a proper timing of the ingress, preferring to
enjoy the view at leisure.
The wait
-----
Aldebaran would return to clear sky in about 45 minutes. As I laid
down to rest in bed, the sky continued to brighten. By a quarter after
six, again by the wall clock, it was obvious that the sky was already
too bright to see any stars. The Moon was still in open sky over
roofs, a lot easier to examine with a bright sky around her.
The surface markings were far easier to study with the lunar glare
abated by dawn.As a matter of technique, many astronomers observe the
Moon in twilight to suppress the dazzling of full night.
I called it quits since thee was for me no hope of spotting
Aldebaran at the egress.
Closing the station
-----------------
I made sure my cats were no where near me and quickly opened the
inner pane, pulled down the screen and outer panes. Then I closed the
inner pane. I packed the scope, whose case was near the radiator. Heat
for the house wa starting to come up.
I left the packed scope by the end table and went back to sleep. A
couple hours later, in bright morning daylight, I woke up for the day.
put away the scope case and straightened the table, removed the chair.
The cats were already scampering around for breakfast.
Selenehelion?
-----------
The almost Full Moon for this occultation set shortly after
sunrise. Was this a selenehelion? Stricta mente it wasn't because a
classical selenehelion requires that the Moon be in eclipse when seen
together with the Sun at sunrise or sunset. We had a true selenehelion
in New York during the 8 October 2014 lunar eclipse at sunrise. The
sky near the Moon was mostly cloudy, spoiling the view for most
observers in the City.
By a relaxed definition the Moon seen with the Sun at sunrise or
sunset may be within a stated tolerance of the exact Full Moon. Under
this definition we can have several selenehelia each year.
I mysyself allow +/-6 hours centered on the Full Moon hour. The
tolerance for relaxed selenehelia is a personal choice but it can not
excede /-12 hours, else the Moon is really too far from Full phase.
For the Aldebaran occultation the Full Moon occurred on 2015
November 25 17:45 EST.. Even adding the maximum reasonable leeway,
+12 hours, this gives the latest hour for a possible selenehelion of
05:45 on the 26th. This happened to be, wholly by coincidence, when
Aldebaran entered the occultation. By sunrise, 06:54, the window for
selenehelion was closed and the Moon herself set at 07:20.
We did not have a selenehelion for the Aldebaran occultation. We
did enjoy a very large Moon setting some 35 minutes after sunrise.
Aldebaran occultations
---------------------
I noted that we are in a new season for occultations of Aldebaran.
Stars along the zodiac are occulted only if the Moon's orbit swings
her over the star as seen from Earth. The lunar orbit is inclined
about 5-1/2 degree from the ecliptic, with the Moon completing a
circuit of latitude +/-5-1/2 degree per month. The 'month' is NOT the
sidereal or synodic month. It is the draconic month, the period
between successive crossings of zero degree latitude, either on the
northward leg of the orbit or the southward.
These zero points, the nodes of the orbit, migrate westward thru
the zodiac, against the order of the signs. The Moon crosses a node a
little early relative to the sidereal month. A full migration of the
nodes , a sliding of the whole orbit, takes about 18 years.
The effect is that a star in the zodiac within the latitude
corridor of the Moon is occulted in certain years and is missed in
others. The interval between occultation seasons is about 18 years.
The season lasts from only a couple years for stars far from the
ecliptic, near the limit of excursion of the Moon,like for Aldebaran.
The current season of Aldebaran occultations began with the event
of 2015 January 29 and ends with that of 2018 September 3. This span
covers all occultations over the whole Earth, not just for New York.
New York misses most events for latitude and timezone
displacement. Those geometricly visible from the City are listed here:
-----------------------------------------------------
ALDEBARAN OCCULTATIONS FOR NEW YORK IN CURRENT SEASON
-----------------------------------------------------
T date and hour | %SL | EST | Sun | star | Sep'
-----------------+------+------+-----+------+-----
05 Sep 2015 05:22 | 52- | 23:19 | -42 | 7 | 9 low alt
02 Oct 2015 13:03 | 74- | 09:30 | +36 | 14 | 1 daytime
26 Nov 2015 09:42 | 99- | 06:10 | -+8 | 11 | 9 large Moon
20 Jan 2016 02:29 | 83+ | 22:06 | -57 | 59 | 8
10 Apr 2016 22:19 | 17+ | 18:22 | 0 | 43 | 3 twilight
04 Jun 2016 19:05 | 0- | 15:12 | +45 | 38 | 5 daytime
29 Jul 2016 11:09 | 23- | 05:37 | +7 | 50 | 13 daytime
19 Oct 2016 06:33 | 86- | 01:15 | -53 | 60 | 8
13 Dec 2016 04:27 | 99+ | 23:50 | -73 | 64 | 4 large Moon
05 Mar 2017 02:57 | 47+ | 23:22 | -54 | 14 | 15 brief event
28 Apr 2017 17:33 | 7+ | 12:05 | +64 | 55 | 1 daytime
22 Jun 2017 14:36 | 4- | 09:23 | +53 | 62 | 5 daytime
12 Sep 2017 12:25 | 58- | 08:32 |++32 | 40 | 4 daytime
15 Oct 2017 11:24 | 20- | 05:14 | -11 | 39 | 9
06 Nov 2017 02:29 | 95- | 20:27 | -42 | 21 | 4 large Moon
31 Dec 2017 00:28 | 93+ | 18:48 | +24 | 43 | 11 large Moon
23 Feb 2018 17:12 | 54+ | 11:20 | +38 | 0 | 5 low alt
------------------------------------------
The columns are:
UT date and hour - The time when the occultation is greatest any
where on Earth, which for sure is not in New York
%SL - sunligghted fraction of lunar disc. + for waxing; -, waning
EST - EST of mid event in New York City. Ignores EDST. Watch for
an intervening midnight crossing between UT and EST.
Sun - altitude of Sun at mid event. + for Sun above horizon in
daylight; -, under horizon in twilight or nighttime
star - altitude of Aldebaran at mid event, always above horizon
Sep' - separation in arcmin of Moon and Aldebaran at mid event
-----------------------------------------------------------
Altho we are favored with many Aldebaran occultations in this
current series, some are challenged by impediments. These are noted
after the Sep' column. A few occultations have multiple impediments
but only one of them is noted. Which of these events you may observe
depends on your observing skills and tools and the instant sky
conditions.
Rule-of-19
--------
This occultation obeys the Rule-of-19. Given the date of any one
instance, others occur at 19-year intervals on the same date. The
series continues for several rounds before, like just about all other
'cycles' in lunar motions, falls apart. The expired series is replaced
by a new one.
The previous Rule-of-19 Aldebaran occultation took place on 25
November 1996. The one day discrepancy is a calendar jiggle. New York
missed it for happening after moonset. The Moon was full, 100%
lighted.
The next Rule-of-19 event is on 2034 November 26 at 05:03, for mid
event. We see this one almost a duplicate of the 2015 show with the
99% lighted Moon low in west.
Mind well that the rule, like the similar Saros cycle, does not
give the date of the very next or previous event! Between the three
Aldebaran occultations of 1996, 2015, 2034 there are many others. Each
follows its own Rule-of-19 cycle.
Conclusion
--------
I missed the September 2015 event for low altitude. The Moon was
behind skyline and trees.
I lost the daytime occultation in October 2015. I just could not
ever catch sight of Aldebaran in daylight against the Moon. The sky
was too hazy.
Now om Thanksgiving Day of 2015 I saw half of this occultation. I
missed the reappearance of the star due to overly bright sky.
In spite of these duds, I'll be watching for the future
occultations. Aldebaran occultations are of special meaning for me
because the very first occultation I watched was of Aldebaran on July
19th, 1960 in predawn hours. The Moon ws a waning crescent with
Aldebaran ingress at the bright limb.. Yes, thank you, Thanksgiving!