GRAFFIAS GRAZE OVER NEW YORK ON 2013 MAY 24
-----------------------------------------
John Pazmino
NYSkies Astronomy Inc
www.nyskies.org
nyskies@nyskies.org
2013 May 3
Introduction
----------
The NYSkies territory is treated to a rare double occultation of a
star AND a graze covering of either or both of them! On the evening of
Friday 24 May 2013 beta Scorpii, Graffias, is covered by the nearly
full Moon standing low in southeast. The extreme edge of the Moon's
shadow cast by the star sweeps over New York City.
The star has two members, beta1 the brighter and southern of the
two, and beta2, the dimmer and northern one. Each throws its own
shadow of the Moon onto Earth, which overlap by about 41 kilometers.
New York City sits in the overlap zone!
There are a variety of amazing scenes no matter where you observe
from. From north to south across the NYSkies territory the scenes are
* a very close miss of both stars
* A graze of beta1 and a miss of beta2.
* A total covering of beta1 and a miss of beta2
* A total covering of beta1 and a graze of beta2
* A covering of both stars
The City, except in its south parts sees the total & miss scene.
The south parts in Brooklyn and the Rockaways sees the total ^ graze.
Staten Island get the brief total & total.
Each scene is spectacular, removing the compulsion to travel away
from home. Where ever you are in NYSkies land, you get a grand show.
On the other hand, the dense transit network our our region does allow
convenient easy travel to a more favorable observing site.
This event requires a telescope to bring out Graffias against the
brilliant edge of the full Moon. Bare-eye and binoculars are not up to
this task. The scope may be of modest size, such as a totable model.
The NYSkies Astronomy Seminar of Friday 17 May 2013 will prepare
observer's for this event, with takeaways charts and tables.
Caution!
------
Occultations are routinely calculated and published with Universal
Time, or similar prime time standard in Greenwich, UK. In this time
the occultation occurs on May 25th. Please be wisely about timezone
shift! Universal Time is four hours AHEAD of Eastern Daylight Savings
Time. In New York, and thruout North America, the calendar date is
Friday 24 May 2013.
If you go out on Saturday the 25th, misreading the published
information, you'll be surprised and infuriated to see the Moon some
13 degrees east of beta Scorpii!
Graffias
------
This star has several names. I use Graffias (GRA-fee-yass) here.
An other common name is Elakrab (ell-a-KRABB), with variant spelling.
Star names are not official and you may use which ever of them you
like or make up your own.
It is the northern of the three main stars in the asterism Caput
Scorpii. The other two in this group are delta (Dschubba) and pi. With
several fainter stars in Caput this is a pretty field for binocular
exploration when the Moon is away.
Graffias was discovered as duplex by Castelli in 1627. Castelli is
a little-appreciated astronomer, commonly missed in astronomy history.
He was a colleague of Galileo who called his attention to several new
features in the sky. One was the prospect of using the newly found
double stars, Mizar is particular, to determine stellar parallax.
Such a measurement would prove the motion of the Earth around the Sun,
favoring the Copernicus model of the solar system. Beyond astronomy
Castelli is the founder of modern hydrology, and his work is still
valid today in engineering of water projects.
Graffias has the Bayer name beta Scorpii and Flansteed name 8
Scorpii. After we recognized the twin nature of Graffias we applied
beta1 and beta2 to its components. Flamsteed 8 applies to the stars as
a unit because apparently Flamsteed didn't resolve it with his simple
weak telescope.
Some facts & figures about the stars are listed here
-------------------------------------
property | beta1 | beta2
------------+-----------------+------
Bayer name | beta1 Scorpii | beta2 Scorpii
Flamsteed | 8 Scorpii | same, not resolved
Bright Star | HR 5984 | HR 5985
Smithsonian | SAO 159682 | SAO 159683
Tycho | TYC 6208-1623-1 | TYC 6208-1622-1
Pos Pro Mot | PPM 231174 | PPM 231175
Zodiacal | AC 2302 | none
H Draper | HD 144217 | HD 144218
DD number | BD-19:4307 | BD-19:4308
CPD number | CPD-19:05945 | none
Visual magn | +2.62 | +4.84
Distance | 400 lightyears | same
Radial vel | -1.0 km/s | same
Spectrum | B1 V | B2 V
RA (2000) | 16h 05m 26.235s | 16h 05m 26.564s
Dec (2000) | -19d 48m 19.63s | -19d 48m 06.84s
B-V index | -0.072 | -0.034
--------------------------------------
There are several other companions in the beta Scorpii system.
Probably none are visible for this occultation. Beta2 is in double
star work 'beta C' because beta1 has an other close companion 'B'.
The distance to the system is now usually cited as 400 lightyear
from analysis of HIPPARCOS data. Works before HIPPARCOS, thru the late
1990s, give various distances up to 800 lightyears.
Shadow path
---------
The Moon's shadow is a cylinder formed by the parallel rays of
light from the star at indefinite distance away. On the ground the
shadow is a disc, distorted by the Earth's curved surface, of the same
diameter as the very Moon, about 3,500 kilometer. Standing inside the
zone sweeped over by this shadow you see a total occultation of a star
the Moon passes in front of.
By standing at the north or south limit of the swath you see the
star nicked by the lunar limb. The star blinks in and out between
lunar mountains sliding across it. This is a graze occultation.
For the 24 May 2013 occultation, beta Scorpii is grazed by the
northern limb of the Moon. Because the star is a double star there are
TWO lunar shadows, one for each star. They overlap with TWO parallel
north edges 41 kilometers apart. You see a graze of the one or the
other star, according as the edge line you stand on.
The Moon's shadows enters the NYSkies region from the west-
northwest, races east and southeast directly over the City, crosses
Long Island, and leaves NYSkies over Atlantic Ocean in the east-
southeast. The passage takes only a couple minutes, given that the
Moon's shadow speed on the ground is some full kilometer per second.
The limit lines pass over or near numerous major towns. Select
ones are listed here in alphabet order to generally delineate the
limit lines.
--------------------------------------
NORTHERN LIMIT LINE FOR GRAZE OF BETA1
--------------------------------------
Deer Pk LI, Glen Cove LI, Greenwood Lk NJ, Islip LI, Mamaroneck
NY, Mahwah NJ, Melville LI, New Rochelle NY, Pearl Rv NY, Ramapo NJ,
Ramsey NJ, Ringwood NJ, Spring Vy NY, Suffern NY, Syosset LI, West
Babylon LI, West Milford NJ, Yonkers NY
--------------------------------------
SOUTHERN LIMIT LINE FOR GRAZE OF BETA2
--------------------------------------
Andover Twp NJ, Bayonne NJ, Bensonhurst BK, Canarsie BK, Denville
NJ, Dover NJ, East Orange NJ, Elizabeth NJ, Green Twp NJ, Hanover NJ,
Hopatcong NJ, Livingston NJ, Manhattan Bh BK, Mill Basin BK, Morristown
NJ, Newark NJ, Parsippany-Troy NJ, Randolf NJ, Rockaway Pk QN, Roxbury
Twp NJ,
--------------------------------------
In addition to towns the paths cross many parks, reservations,
beaches. These may be open for public use for the occultation.
These lines are for a smooth lunar globe that skirts the TOPS of
mountains expected to stand along the limb for this occultation.
It may be that a site a kilometer or two SOUTH of the limit line
will capture many blinks of the corresponding star between the
mountains and thru the valleys. Farther south of the limit than 3 or 4
kilometers yields a total, very brief, occultation of that star.
Instrument
--------
You really need a telescope, even a tabletop or totable model. By
eye the Moon's brilliance makes you lose Graffias long before it is
hit by the Moon and next catch sight of it well after the Moon left
it. Binoculars are a poor substitute because still the glare of the
Moon may overpower the star.
The telescope should have an aperture of at least 120mm to see
Graffias comfortably against the lunar disc. If your eyesight be extra
acute a smaller aperture, down to 90mm, may be used. There is no need
for a overly large scope being that there is no detail to be seen on
the Moon or the star that calls for huge aperture or magnification.
A clockdrive avoids the distraction of manually nudging the scope
to follow the star's diurnal motion. Either a conventional equatorial
drive or a stepwise altazimuth drive is good. The latter is what a go-
to scope uses when in altazimuth mode with its base sitting on a solid
level table. .
Tracking on the stars lets the Moon float across the field of
view. Tracking on the Moon lets the stars glide behind the Moon. Which
to use is by your preference,
For a go-to scope be SURE to properly home and align it well
before the occultation. Even with the bright Moon there are enough
target stars to choose from. Some go-to scopes offer a planet based on
its onboard ephemeris. Do NOT use Saturn! Stay with fixed stars.
The expected warm weather in late May relieves you of the concern
about cold-failure of batteries and electronics. On the other hand,
May nights can be humid, giving worry about moisture problems.
Directions in the scope
---------------------
The event occurs at the north limb of the Moon. The Moon glides
eastward over the star. The sky's diurnal rotation is toward the west.
Beta2, the dimmer star, is almost straight north of brighter beta1.
If you be new to lunar observing, please practice exploring the
Moon in your scope one days before the occultation. This will firmly
familiarize yourself with the lunar topography and keep your sense of
directions right way round.
Be mindful of mirrored imaging from the peculiar optics of your
telescope! It may be helpful to make a mirrored moonmap to be sure you
can identify features along the lunar edge. Many lunar observing
computer programs have this feature, else do the flip in an image
editor.
Moon's glare
----------
To temper the glare of moonlight, use as high a power as practical
to dilute the lunar disc and still have a firm optical image of the
stars. The amount of magnification is largely governed by the quality
of the air and of the instrument. Too high a power may make tracking
the stars too tedious, even with a well-regulated mount.
For catching the blinks of the star at a site on one of the limit
lines, use the highest possible magnification that your scope, eyes,
and sky can accept. Too low a power may miss the lash of the star next
to a bright mountain side.
One worry is a hazy night where the Moon is a diffuse or frosted
disc. The sky around her may be so lighted that Graffias is veiled out
of sight. As long as the lunar disc shows its features distinctly you
should have a good view of the occultation.
Observing sites
-------------
This event does not require travel away from New York City. The
view from any where in the City, even from Manhattan, is spectacular!
You need a clear view of the low southeast and south sky. You must see
the Moon! Scout out a nearby park or waterfront or roof if you can not
observe from your home.
Do not view thru closed windows for the distortion they cause in
the telescope image. Many office towers have tinted windows that may
not be obvious in bright sunlight. They will dim the stars to vanish.
If you have friends or family near either graze line, go and ask
for a visit to show them the occultation.
The NYSkies area has a dense transit network with stations at
towns all along the graze lines. Take a bus or train into the graze
zone. You may view from the transit station or adjacent field. Only
totable equipment is allowed on transit vehicles.
Service on Friday nights on almost all routes is at least hourly
in both directions, allowing for a comfortable trip with moderate
deadtime. You still have to keep in hand a current timetable. Off-peak
fares should be in force after about 20h EDST.
A reverse ride may be the trick for observers in the Catskills,
Hudson Valley, mid to north Westchester, Connecticut, mid to east Long
Island.
To use a park or other public land you do need your own wheels.
Transit gets you to the main gate or a nearby busy street, but not to
any interior section. You either have a long walk to drag your gear or
make do with viewing from the gate or street.
Public viewing
------------
Several astronomy centers have regularly scheduled starviewing
sessions on Friday the 24th and will for sure feature this occultation
for their visitors. Special sessions for the show may be arranged. As
NYSkies learns of them they are posted into the NYSkies yahoogroup.
Because the stars blink in and out of view in an instant, only
fleeting views can be offered per instrument. With many telescopes and
a smaller audience, this can be worked out.
With modern digital imaging equipment is to present live
projection of the occultation for a larger audience to enjoy. Lacking
live imaging, the starwatch may have a computer running a planetarium
program. The simulated scene of the Moon crossing beta Scorpii runs in
real time to match the sky outside. This is handy for intermittent
clouds. If the planetarium shows beta1 and beta2 separately, the
simulation is much more realistic.
There is no special preparation to attend an occultation session.
What's needed for any other mid spring night starviewing is all you
need. You may want to have your camera to hand to take a shot of a a
projection screen or thru a telescope eyepiece.
Scenarios
-------
Depending on your location you get one of several amazing views:
-----------------------------------------
location | beta1 | beta2
--------------------------+-------+------
north of north limit line | miss | miss
on north limit line | graze | miss
between limit lines | total | miss
on south limit line | total | graze
south of south limit line | total | total
-----------------------------------------
The duration of the occultation increase with locations farther
south of the southern limit line.
Because beta Scorpii is a wide double star, some calculations are
issued for each star, with TWO limit paths. The NORTHERN one is the
graze limit for beta1, the SOUTH component of the pair. The SOUTHERN
line is for beta2, the NORTH member. Most computations so far seem to
apply only to beta1, the southern and brighter star.
When calcs are issued for 'beta Sscopii' as a single star, they
are for beta1. The limit line is the one passing north of New York
City, thru Westchester county. There is no southern line foe beta2.
One curious factoid is that with the angular diameter of beta
Scorpii (either star), its disc projected on the ground in the City's
reach of the graze line, is TWO to THREE meters in diameter! A person
three meters away may see one set of blinkings during the graze while
you see a different series.
Manhattan view
------------
You must have a clear view of the low southeast and south sky. You
may have to seek out a site, like a riverfront park, for this event.
Mind well that directions in all occultation, and other astronomy,
litterature are geographic, NOT along Manhattan's street grid.
'Downtown' within the grid is toward azimuth about 210 degrees, more
southwest than straight south.
For Manhattan here is the timetable of activity on the evening of
2013 May 24 Friday:
-----------------------------------------------------
EDST | event | Sun | Moon | remarks
------+-------------------+--------+--------+--------
19:53 | moonrise | 02 296 | 00 116 | 99% 178E L-1.7 B-1.2
20:13 | sunset | 00 298 | 04 119 | Moon not quite full
20:46 | civil twilight | -- --- | 08 124 | end of daylight
21:27 | nautical twilight | -- --- | 13 132 | full night in NYC
22:05 | beta1 ingress | -- --- | 18 139 | pos 22d, ver 54d
22:11 | mid occultation | -- --- | 19 141 | beta2 close miss
22:16 | beta1 egress | -- --- | 19 142 | pos 34d, ver 352d
00:55 | moon on meridian | -- --- | 29 180 | on May 25th
----------------------------------------------------------
Times in this table may differ from other sources due to assumed
geographic location of 'Manhattan' or 'New York'. You would be on duty
many minutes before the ingress to follow the stars into the Moon and
then watch for their emersion some 11 minutes later.
Sun and Moon columns give the altitude and azimuth of these bodies
for the associated event.
For moonrise the percent of lunar disc lighted by the Sun, Moon
elongation from the Sun, and libration are given. These are fixed for
the whole duration of the occultation.
The libration, L for longitude, B for latitude, are the lon-lat on
the lunar surface for the apparent center of the lunar disc. That's a
real minus signum, not a hyphen, in the figures. They, combined with
external maps and tables, approximate the profile of the mountains and
valleys near the ingress and egress points.
For the ingress and egress the pos and ver angles are given. Pos
is the position angle along the lunar limb measured anticlockwise from
celestial north. This is useful for scopes on an equatorial mount.
Ver is the vertex angle along the lunar limb from the vertex or
top of the Moon, also anticlockwise. This is for an altazimuth
telescope.
There is in the usual ephemeris of occultations a cusp angle, the
angle along the lunar limb from one or the other cusp. With the Moon
almost full there are no obvious cusps. This angle is omitted here.
Other occultations
----------------
Graffias is one degree north of the ecliptic. The Moon can occult
it only when her ascending, like for this instance, or descending node
are nearby. Because the nodes migrate westward in the ecliptic about
19 degrees per year, there can be only a few hits on Graffias before
the node carries the Moon too far away to touch the star. There is
then a long span of misses. The occultation season of Graffias is only
1-1/2 years at each node with an 8 years gap in between. The nodes
alternate in generating occultations, completing a full cycle in about
19 years.
Of all the occultations possible, only some are over the City. The
rest are in other parts of the world. I did not recall the last
Graffias event. I checked and found that there is a good reason why I
didn't recall any.
As far back as the mid 1980s there was ONE occurrence for New
York. It was on 9 January 2002 in daylight with no hope of seeing it
with visual means.
For the future into the mid 2030s there is only ONE hit for the
City, 18 July 2013. It takes place in daylight, being unobservable by
ordinary visual methods.
As fate falls to us, this one on May 24th is the LAST CHANCE to
see Graffias covered by the Moon from New York, specially if you're on
Social Security.
Hierarchy of orbits
-----------------
Beta Scorpii is a wonderful example of the hierarchy of orbits in
double star systems. In the solar system or planetary systems at other
stars, the orbits of the planets can be more or less arbitrarily
arrayed around the central star. The orbiting bodies are of minuscule
mass compared to the central star and their orbits are governed almost
entirely by that star's mass.
In a stellar system the orbiting bodies are of similar mass, a few
solar masses each. Their orbits end up nested in a hierarchy where
each level is many times larger than the next inner one. Graffias is
one such system, whose information was sussed out partly be watching
its components during lunar occultations.
The diagram here shows the beta Scorpii orbit levels with the
orbital periods for each component. The letter designations are a bit
inconsistent in double star work but context clarifies the identities.
A (beta1)
|
+-------+------------------------------+
| | |
D B C (beta2) 16,000yr
6.82dy 610yr |
E 39yr
|
F 10.7dy
Beta1, A, has three companions, B and C and D. C has one companion
E, which in turn has its own companion F. The system, so far, has six
components. Of them, A and C are the obvious pair in small scopes. B
is discernible in large scopes by keen observers.
Other nearby targets
------------------
After the Graffias show you may want to explore the region around
this star. Because of the full Moon, probably all clusters and
galaxies are washed out, even in a clear dark sky. To fill your
observing needs, here are several double stars, targets well suited
for a bright sky condition, within about 20 degrees west, north, and
east of beta Scorpii. All of these stars are within range of small
scopes from New York City.
----------------------------------------------------------
cns designatn RA2000 DC2000 MagA MagB sep colors year
--- --------- -------- ------ ---- ---- ----- ------- ----
Lib mu 14 49.3 -14 09 5.8 6.7 1.9 whi 2003
Lib alp1-alp2 14 50.9 -16 02 2.8 5.2 231 whi y-w 2002
Lib 33 14 57.5 -21 25 5.9 8.2 25 ora red 2002
----------------------------------------------------------
Sco 2 15 53.6 -25 20 4.7 7.4 2.1 b-w whi 1991
Sco xi 16 04.4 -11 22 4.9 7.3 7.5 yel 2005
Sco bet1-bet2 16 05.4 -19 48 2.6 4.9 13 b-w b-w 2005
Sco nu1-nu2AC 16 12.0 -19 28 4.3 5.3 41 b-w 2005
Sco nu1 AC 16 12.0 -19 28 4.3 6.4 1.3 b-w 2003
Sco 12 16 12.3 -28 25 5.9 7.9 3.8 b-w 1999
Sco sigma 16 21.2 -25 36 2.9 8.5 20 b-w 1999
Sco alpha 16 29.4 -26 26 1.2 5.4 2.7 red b-w 2000
----------------------------------------------------------
Ser 6 15 21.0 +00 43 5.5 8.8 3.0 ora 2000
Ser delta 15 34.8 +10 32 4.2 5.2 4.0 y-w y-w 2009
Ser nu 17 20.8 -12 51 4.3 8.3 46 whi
----------------------------------------------------------
The colors are the schematic ones for the spectra of the stars.
They are NOT the colors you will -- or should! -- discern ub the
doubles. Some observers claim that when seen in a gray sky, in this
case lighted by the Moon, the colors of double stars are easier to
appreciate. Your experience may vary.
The year is the year for the separation, being that double stars
carry out orbital rotation over the decades.
Rho Ophiuchi is a multiple star. You may spot one or two other
comites next to it. Alpha Scorpii (Antares) usually smothers its
companion behind its own glare. Alpha Librae (Zubenelgenubi) could be
seen as double by bare eye, else use binoculars.
Occultation science
-----------------
As many of us elders know, there was until this century a lively
practice of observing occultations. We used radio time signals from
WWV or CHU shortwave stations, a tape recorder, stopwatch to record
the instants of ingress and egress. This helped refine the lunar
motion and position, in part to assist in the 1960s Apollo project.
For a graze we watched for the multiple blinks of the star as the
lunar mountains cut in front of it. Each on/off, in/out, see/saw was
dutifully recorded. The sequence of the blinks mapped out the
topography of the Moon in the north and south polar regions.
Lunar occultation helped to to discover and study binary stars
that were too close to observe be ordinary optical means. The timings
and photometry for each component of the pair, plus the geometry of
the occultation, gave the separation and orientation of the two stars,
even if they were not seen individually for their tight proximity.
In the 21st century these, and other traditional, occultation
works rapidly became obsolete for home astronomers. While occultation
reports are still collected, there is a much lesser need for them. New
methods, including visits by spaceprobes and nonvisual imaging
methods, displaced occultations in many classical applications.
In spite of the diminished science value of home observations, the
sheer beauty of the event never lapsed. It is also one of the handy
ways to appreciate within a few minutes the real orbital movement of
the Moon. Try taking pictures of the scene. Graffias is in the head of
Scorpius, with the body and heart beneath it around Antares. Use the
techniques for skyscape pictures.
Conclusion
--------
Occultations of bright stars are common with several occurring
over New York each year. They are fun to watch and are a vivid
demonstration of the Moon's real orbital motion thru the zodiac.
Occultations of bright double stars are rarer for the small number
along the zodiac. Usually the two stars are so close together they
blink out behind the Moon within a split second apart. You see a step
decrease in brightness, then the full cutoff of light. At egress first
a dimmer star is seen, then it snaps into full brightness.
Occultations of wide doubles, with well-spaced components, are
rarer with only a couple possible targets in the Moon's corridor. You
see a double ingress and double egress.
Occultations over the City near the shadow edge, creating a graze
event, are common. Almost all are of dim stars with little motive to
make the trip to the limit line.
This one shot instance for beta Scorpii, a bright star, a wide
double, and a graze near the City, is an event you better make effort
to watch! If you are confined to your home, you get one of the five
stunning scenarios explained above. By taking a short ride you get any
other one of the scenarios.
Like for all celestial observing, clouds can wipe out your view.
Even a thick haze or thin cloud can kill the show. On the other hand
you don't need a perfect or extra good sky.