The Phantom Moonace
Right: On September 18, 1997, many stargazers in the U. S. were able to watch a lovely early morning lunar occultation as a bright Moon passed in front of Saturn. This animated gif image was captured at the Whipple Observatory atop Arizona's Mount Hopkins. Credit and Copyright: K. Stanek (CfA), W. Colley (Princeton)
Astronomers hope that the event, occurring as it does at a convenient time of night over the United States and Canada, will be observed by thousands of sky watchers. If enough people record the occultation on video, then scientists can use the tapes to construct a very precise map of the moon's limb. Accurate profiles of the lunar terrain are important because they help scientists interpret solar eclipses and address issues like the constancy of the sun's diameter and its long-term energy output.
Want to help? You don't have to be an astronomer to participate. All that's required are a good home camcorder (with 12x or higher zoom), a shortwave receiver, and a view of the moon.
The occultation takes place Friday evening, May 21st, for viewers in western North America and on Saturday morning, May 22nd for observers in the east.
The exact time that the Moon begins to pass in front of Regulus depends on where you live. Generally, the farther west you are, the earlier the occultation will take place. Regulus will vanish behind the Moon's limb before sunset in the far Northwest, during twilight for most of the western and the north-central states, and a little after midnight in the East when the Moon is setting over the western horizon. The occultation will be visible only to observers in northern Mexico, and throughout the United States and Canada.
Occultation times for many US and Canadian cities are tabulated below. If your home town is not listed, simply use the times for the nearest city as a guide. A more complete listing is provided at the International Timing Association web site and there are also an excellent set of timing maps available from Sky & Telescope.
May 21 - 22, 1999
Location Disappearance Reappearance Zone Atlanta GA 12:25 am 1:25 am EDT Boston MA 12:15 am 1:10 am EDT Brownsville TX 11:41 pm 12:25 am CDT Chicago IL 11:09 pm 12:12 am CDT Dallas TX 11:23 pm 12:21 am CDT Denver CO 10:01 pm 11:07 pm MDT Detroit MI 12:11 am 1:12 am EDT HamiltonBermuda 1:30 am ------ ADT Houston TX 11:31 pm 12:25 am CDT Kansas City MO 11:10 pm 12:15 am CDT Los Angeles CA 9:11 pm 9:49 pm PDT Memphis TN 11:20 pm 12:22 am CDT Miami FL 12:42 am 1:33 am EDT Minneapolis MN 11:00 pm 12:05 am CDT Montreal PQ 12:10 am 1:05 am EDT New Orleans LA 11:32 pm 12:27 am CDT New York NY 12:17 am 1:14 am EDT Norfolk VA 12:22 am 1:20 am EDT OklahomaCity OK 11:16 pm 12:18 am CDT Philadelphia PA 12:18 am 1:15 am EDT Phoenix AZ 9:14 pm 10:03 pm MST SaltLakeCity UT 9:52 pm 10:58 pm MDT SanFrancisco CA 8:54 pm 9:41 pm PDT St John's NF 1:39 am ------ NDT St Louis MO 11:14 pm 12:17 am CDT Tampa FL 12:37 am 1:31 am EDT Toronto ON 12:10 am 1:09 am EDT Washington DC 12:19 am 1:17 am EDT Winnipeg MB 10:48 pm 11:54 pm CDT
Above: The path that Regulus will take behind the Moon as seen from various cities, adapted from Sky & Telescope.
Experts suggest that you begin your recording at least 15 minutes before the occultation. For a bright star like Regulus a telescope should be unnecessary. Simply zoom in on the Moon's limb and center the field of view at the point where you expect Regulus to vanish (see the diagram, above). The recording should continue for several minutes after the star disappears. A tripod will provide the best image stability, but hand-held recordings can also provide good data.
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When reporting your observations, it is also important to specify your latitude and longitude as accurately as possible. Ideally the coordinates should pin down your location to within 15 ft. The best way to locate your observing site is with the aid of a GPS receiver. Unfortunately, these are somewhat expensive and not widely available. Another approach is to carefully measure topographic survey maps. In the USA, suitable maps can be ordered directly from the US Geological Survey by calling 1-800-USAMAPS. There are also mapping resources online. For instance, if you are observing from an urban area you can search for your address on Mapblast.com. In addition to a map, it will return a decimal latitude and longitude.
Even if you don't have a GPS receiver or can't obtain a map, the International Occultation Timing Association (IOTA) can often figure your location provided that you carefully measure the distance of your observing site from local landmarks such as buildings or street intersections.
Left: On April 23, 1998 the rising crescent Moon passed in front of Venus and Jupiter. The double occultation was a rare event and only visible from certain locations tracing a path across Earth's surface. This dramatic telephoto picture was taken at one such location, Ascension Island in the South Atlantic. The sunlit crescent is over-exposed revealing the rest of the lunar surface illuminated by faint earthshine. Venus is emerging just beyond the crescent's tip and Jupiter is trailing above the dark lunar edge with a spot of light, Jupiter's moon Ganymede, between the lunar limb and the planet's disk. Look closely at Jupiter and you can see yet another Jovian moon, Io, just visible against Jupiter's glare! More information.
International Occultation Timing Association -- learn more about lunar occultations
The Moon occults Regulus -- from Sky & Telescope
Moon Occults Saturn -- Astronomy Picture of the Day, May 8, 1999
Help Map the Moon -- Astronomy Picture of the Day, Sep. 11, 1998
Occultations and rising moons -- Astronomy Picture of the Day, May 13, 1998
X-ray Moon and X-ray Star -- Astronomy Picture of the Day, Feb. 27, 1996
Sunshine, Earthshine at the Lunar Limb -- Astronomy Picture of the Day, May 30, 1996
The sky on Friday May 21, 1999 -- from EarthSky.com
The Nine Planets: the Moon -- from SEDS
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|For more information, please contact:
Dr. John M. Horack , Director of Science Communications
|Author: Dr. Tony Phillips
Curator: Bryan Walls
NASA Official: John M. Horack