On Saturday evening, March 8, 2008, skywatchers across the southern United States might see a relatively bright star wink out briefly as one of
the two known moons of asteroid 45 Eugenia covers it. By observing this rare event, you could help determine the location of the moon relative to
the main asteroid more accurately than can be done directly even with the largest telescopes on (or near) Earth.
Eugenia itself will briefly hide the same star for skywatchers in a path across Mexico, whereas both moons will do so for parts of the southern
No matter which of these events you are positioned for, the time interval to be watching is 5:42 to 5:45 Universal Time on March 9th, which is
Saturday evening in North America. In local time, this is the three-minute interval that starts at 9:42 pm PST, 10:42 pm MST, and 11:42 pm CST.
WEATHER UPDATE (inserted early March 8th): The
Astro Meteo (30-hour prognosis) cloud-cover forecast maps show more clouds than before, especially in central California (including the central part
of the Central Valley) and over northern New Mexico. Southern
California looks good (including the southern San Joaquin Valley) but
there are clouds in the mountains and along I-10 in the desert. The
clouds in Texas may be thin enough to observe near Lubbock, but thin
clouds may extend all the way to Dallas; that may cause trouble, given
the event's 16° altitude there. I plan to observe in what should
be clear sky southeast of there, along US 259 and US 59. East of
Texas looks completely clear except possibly some thin clouds near
where Tom Campbell plans to observe.
It's worth pointing out that, back in 1977, astronomers laughed when Paul Maley and I suggested that asteroids
might have small moons based on visual observations of their occultations (eclipses) of stars. In 1994 they stopped laughing when images returned
from the Galileo spacecraft showed Dactyl, the small moon of asteroid 243 Ida. Nowadays, of course, many dozens of moons of asteroids are known.
But on only three occasions have confirmed timings been made when these moons covered stars (and those observations were all made in Japan).
A Three-in-One Bonanza
The star to be covered is easy to spot in binoculars. It lies about 5° east of Aldebaran and the Hyades V of stars in Taurus. At
visual magnitude 5.7, it is the brightest in a unique group of five stars. The target star
is ZC 741 (also designated SAO 94227 or HIP 23043) and is located at
right ascension 4h 57.4m,declination +17° 09' (equinox 2000.0).
The occultation by Eugenia itself, about 215 km in diameter, will last up to 12 seconds in its path crossing northern Mexico over Loreto
(in Baja California Sur), Torreon, Saltillo, and Monterrey.
The occultation by Eugenia's 13-km larger moon, Petit-Prince, will last about 0.7 second in a path across southern California (nominally
over King City and Tipton, but observers all the way from San Francisco to Los Angeles have almost an equal chance to see it), southern Nevada,
northern Arizona (Flagstaff area) and New Mexico, Texas (Lubbock and Dallas/Ft. Worth, but Waco to southern Oklahoma have a chance), northern
Louisiana, and southern Mississippi. This event might also be seen, low in the sky, from the Florida panhandle early Sunday morning, March 9th,
where the local time is 12:42 to 12:45 am EST.
The occultation by Eugenia's smaller moon, called Petite-Princesse, will be more difficult to observe. A mere 6 km or so across, it could
cover the star for roughly a third of a second in a narrow path through northern Mexico and southern Texas. The location of this path is
especially uncertain; it could go almost anywhere between San Antonio and Brownsville, Texas. (It might even go farther south, in Mexico; there's
some chance that those in the northern part of the occultation path for Eugenia could have an event by Petite-Princesse.)
Equipment and Techniques
Binoculars are fine for observing these eclipses by either Eugenia or its moons. Any telescope with an aperture of at least 2.4 inches (60 mm)
will also do. Much information about timing occultations of all types is in the International Occultation Timing Association's free
handbook, "Chasing the Shadow: The IOTA Occultation Observer's
Many imaging enthusiasts have CCD cameras on their scopes, and they may not want to remove them for this event. These people can also contribute
usefully. IOTA's David Herald invites them to look at this technique.
"If this event is well observed," Herald notes, "the profiles of the components will be resolved at the 1-km level, relative positions being
determined to within a few hundred microarcseconds. So I encourage everyone near the predicted paths to join in the group activity and
monitor this event! And remember, the uncertainty in the path location could be a good 100 km or more. So even if you are outside the predicted
paths, you should still monitor the event."
For More Information
On the IOTA website,
go to the box at the bottom of the page for finder charts and other useful links concerning this event.
Also, for making observations with whatever you have available, go here.
Simply reporting to us whether or not an eclipse of the star occurred or at your location can be important (especially if it does occur).
The interactive Google maps on Derek Breit's website (click on
"Spectacular Triple Asteroid Occultation — 45 Eugenia" near the top) also include static maps showing the ground paths, with green lines
showing the predicted central line, blue lines showing the northern and southern limits, red lines showing the standard error limits (1 sigma),
and gray lines showing less likely limits (2 sigma). Read the information in the boxes for the interactive maps if you use them; they can be used
to view the paths on detailed maps, satellite images, and in some cases aerial photography or other maps, to almost any desired level of detail.
Derek Breit's site also includes a list of stations and cities in and near the predicted path, ordered by distance in kilometers from the
predicted central line (distances north of the line are considered negative). For each it gives the predicted time of the center of the
occultation, the probability that an occultation will occur, and the local circumstances (mainly, in this case, the altitude above the western
horizon) of the event.
Notice that the event will occur at about 12° altitude above the horizon along I-35 between Dallas and Oklahoma City, so observers in that
area and farther east, where the altitude will be even lower, need to take care to find large open fields, the east sides of lakes, or otherwise
places with an unobstructed view of the western horizon.
Some Added Requests
Those in the possible region of visibility of these events are urged to publicize them as much as possible, via astronomical-society list servers,
science writers for public media, etc. And if you haven't already, please let us know where you plan to observe from. As soon as we have
information about fixed-site observers, we can direct mobile observers where to go so as to fill in any gaps in coverage.
coordinating plans for events involving the Eugenian satellites.
If you are reading an e-mailed copy of this AstroAlert, be sure to look at the online version at SkyandTelescope.com/AstroAlert for possible late updates.
Good luck with your observations!
Sky & Telescope