May 20, 1999

The Phantom Moonace



The Moon Eclipses Regulus


Astronomers hope that stargazers across North America will video tape this rare naked-eye event on May 21 to gain a unique perspective on lunar terrain.
Credit and Copyright: K. Stanek (CfA), W. Colley (Princeton)
May 20, 1999: Stargazers in North America are in for a treat this Friday, May 21, when the quarter moon passes in front of the bright star Regulus. It's a rare naked eye occultation, the likes of which won't be seen again in most parts of North America until 2005.

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.
Finding Regulus is easy. Go outside after sunset and look toward the southwest. The Moon and Regulus will be shining brightly about 50 degrees above the horizon. Regulus is a 1st magnitude blue-white star located about 85 light-years away. It's thought to be about three times as massive as our sun. Regulus is a popular setting for science fiction stories. For example, the fictional planet Regulus III appeared in the Deep Space Nine episode "Fascination", and events on Regulus V were an important part of the Star Trek episode "Amok Time."

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.
Regulus Occultation Times
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
Times listed as "pm" refer to Friday, May 21, and "am" times occur on Saturday, May 22.
No matter where you live, Regulus will vanish behind the Moon's dark edge and reappear from behind the sunlit side. To see the reappearance you may need a telescope or a good set of binoculars since the star will be buried in the glare of the Moon's sunlit limb. You can use the diagram below to estimate where Regulus will emerge.
occultation paths for different cities

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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It is important To make your observations useful to scientists, you'll need to record an accurate time signal on the audio track of the video recorder. The easiest way to do this is to tune a shortwave receiver to WWV, which transmits time signals at 2.5, 5, 10, 15, and 20 MHz, and position the receiver within listening distance of your camcorder's microphone. To hear what a WWV signal sounds like, you can phone (303) 499-7111. Another good source of time information is the Canadian radio station CHU which broadcasts at 3.330 MHz and 7.335 MHz.

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 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.
Credit and Copyright: Olivier Staiger
When the occultation is over please mail a copy of your tape to Dr. Tony Phillips, 162 Alpine Drive, Bishop CA, 93514. Don't forget to include a brief letter describing your observing site, its location, and the equipment you used to film the occultation. Portions of your tape will be digitized for possible use in future NASA NASA Science News headlines (with full credit to the photographers). Then the tape will be forwarded to IOTA for further scientific analysis.

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.


Web Links

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

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