Moon will block bright star Regulus

By Joe Rao

updated 1:41 p.m. ET Oct. 26, 2007

If you live in the southern or western parts of the United States, you'll have an opportunity on Nov. 3 to see a fat waning crescent moon gradually drift toward and ultimately hide the 1st-magnitude star, Regulus, the brightest star of the constellation Leo, the Lion.

This event is called an occultation, a word that is derived from the Latin occultare which means literally "to conceal." And if you are fortunate enough to live in the zone of visibility for this event (see below), that's exactly what you will see; the moon, appearing to temporarily conceal Regulus from your view.

Because the moon is waning, its bright crescent faces forward as it advances eastward against the starry background. Regulus will thus disappear on the moon's bright limb and even a bright blue-white diamond might be briefly overwhelmed by the glare of the lunar surface some seconds before it vanishes. How well you see the moment of the star's disappearance will depend a lot on the size and quality of your telescope and the steadiness of the atmosphere.

On the other hand, the reappearance of Regulus will happen on the dimly earthlit dark limb as the moon leaves the star behind. Here the scene will be much more dramatic. Those blessed with a dark sky background will see Regulus suddenly and dramatically "pop" into view, as if a switch were thrown; a stunning demonstration of the moon's orbital motion and the star's tiny angular size. Binoculars or maybe even just your eyes will be all you'll need (though you might need to block the glare of the moon's bright limb behind a nearby wall or tree limb).

Zone of visibility
If you live anywhere to the south of a line that curves roughly from near Gold Beach, Ore., across the U.S. to near Savannah, Ga., you'll see the moon pass in front of Regulus. If you live north of this line, you'll see the moon slowly glide below Regulus, resulting in a tantalizing near miss! And if your neighborhood happens to lie fortuitously within about a mile or two on either side of the line, you might have an unusual opportunity to see the grazing occultation of a 1st-magnitude star. As Regulus appears to move tangent to the moon, it might just disappear and reappear a few times along the rugged terrain of the moon's bright limb, just off to the left of its upper cusp.

Dark or bright sky?
Another important consideration is whether the occultation occurs against a dark sky, a twilight sky or a daytime sky?

Over California and the desert Southwest, the entire event will occur in a dark predawn sky.

Across Texas, Oklahoma and most of Kansas, Regulus will disappear in a dark sky, but it will reappear during early-to-mid morning twilight.

Over the Deep South, Piedmont, Southeast Coast and Florida, morning twilight will be well advanced and the sky quite bright when Regulus disappears; in fact the sun will have just risen along much of Florida's east coast. And the star's reappearance will take place after sunrise for much of this region.

Full schedule
A detailed set of predictions for over 100 selected cities, including in Mexico and the Caribbean are available on the Web site of the International Occultation Timing Association (IOTA).

For more specific information regarding the location of the graze path relative to your location, you can see an interactive map of the path.

An expedition to observe this event from Utah is being led by Patrick Wiggins. He invites those who might be interested to contact him at . Check out his Web site at http://utahastro.info/.

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