Alert Notice 499: Predicted occultation of Regulus

March 14, 2014: On 2014 March 20 at approximately 02:06 a.m. EDT (06:06 UT), an occultation of Regulus (alf Leo, magnitude 1.4 V) by the magnitude-12.4 V asteroid (163) Erigone will take place. The occultation track includes Bermuda and northwest along a corridor stretching from the mid-Atlantic USA through Ontario, Canada. Unusual things about this occultation include the facts that the track includes major metropolitan areas such as New York City, and the star being occulted is one of the brightest in the Northern sky and is in a constellation pattern that makes it easy to find.

The International Occultation Timing Association (IOTA) website ( has comprehensive information about this predicted occultation and observing instructions, as well as links to other resources. Depending on your equipment, location, and interest, how you observe this event will differ, so please visit IOTA's site to see how to make your observations and where and how to report them. Last-minute weather updates will be posted on the Regulus Facebook page (

Many other sites also have information on the predicted occultation, including the Planetary Society's ( and Sky & Telescope's (

In addition to witnessing a very rare event (Regulus is the brightest star ever to be predicted to be occulted from the USA), the possibilities for science include these three: determining the shape of Erigone, detecting a possible companion of Erigone, and detecting the suspected white dwarf companion of Regulus.

When the precise times that the occultation begins and ends are combined from many locations, a silhouette of the occulting object - Erigone - may be determined.

If Erigone has a companion, it may be possible to detect it from viewpoints outside of the occultation track. Observers from as far south as the Carolinas to as far north as Nova Scotia and Winnipeg are encouraged to watch Regulus from 02:00 to 02:12 a.m. EDT on March 20 and to report whether or not they see it disappear. It is recommended that regardless of how close you are to the occultation track, observe Regulus constantly from at least 2:00 to 2:12 a.m. EDT (06:00 to 06:12 UT) to see if it is eclipsed by a second, unknown object. Report whether you see a disappearance or not.

If you are on the center of the occultation track, it may be possible to observe the suspected white dwarf companion of Regulus. In ATel 5917 (, D. R. Gies (Georgia State Univ.) et al. write: "...This may be the first opportunity to detect the flux of the suspected white dwarf companion of Regulus...The projected separation at the time of the occultation is estimated to be 13.0 +/- 1.7 mas at a position angle above the stellar equator at 168 or 348 deg east from north (Che et al. 2011)...An observer at the center of the occultation track may observe the occultation of the white dwarf 1.7 seconds after that of Regulus or the reappearance of the white dwarf 1.7 seconds before that of Regulus (depending on the actual position angle of the white dwarf), while an observer near one side of the track may see the flux of the white dwarf for the full duration of the Regulus occultation. Although the companion is probably faint, it should be significantly brighter than...Erigone (V = 12.4), so we encourage high speed photometric observations of this occultation to detect the flux of the white dwarf companion."

However you oberve this rare event, please report your observations at and follow the instructions there.

Good observing!

This AAVSO Alert Notice was compiled by Elizabeth O. Waagen.


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