The Christmas Morning Occultation of Spica

The path that Spica will appear to take behind (or next to) the Moon depends on where you are. Interpolate between the cities here to estimate where on the Moon's edge the star will vanish and pop back into view as seen from your location.
Sky & Telescope diagram.
We're not sure what an astrologer might make of the symbolism here, but on the morning of Christmas Day 2005 when kids are getting up to thoughts of presents, the waning crescent Moon will blot out Spica, the brightest star of Virgo. For amateur astronomers, however, the meaning is plain: the event offers a special and memorable way to start Christmas Day.

As shown on the maps below, the occultation happens over most of North America except the southwest and west. For most of the continent, both the disappearance and reappearance occur after sunrise in broad daylight; observers in these areas will certainly need a telescope.

In fact, you'll probably need a telescope to see Spica disappear on the Moon's bright limb even if this happens in darkness; the glare of the sunlit lunar surface will overwhelm the star some seconds before disappearance if you look with only binoculars or the unaided eye.

Unfortunately, this means you will need a telescope for every person watching — unless you rig a camcorder to point into the eyepiece so people can watch the screen. (Be sure to get the whole setup built, tested, and working a day or two beforehand; then put it aside ready to go.)

Spica's reappearance from behind the Moon's dark limb should be more dramatic — but by then more observers will be in daylight, and you'll have to be watching at just the right moment.

Times and Places

For your location, interpolate between the yellow curves to find the Universal Times of Spica's disappearance and (using the second map) reappearance. To convert Universal Time to Eastern Standard Time, subtract 5 hours; to get Central Standard Time, subtract 6 hours; to get Mountain Standard Time, subtract 7; to get Pacific Standard Time, subtract 8. Also indicated are whether the event occurs in darkness, morning twilight, or daytime. Click on each image for the full-size version.
Sky & Telescope diagram by Gregg Dinderman.

You can find the times of Spica's disappearance and reappearance at your site using the maps here. (Click on them to get the full-size versions.) At your location, interpolate between the yellow time lines to get the Universal Time of the event. The maps also indicate whether your location will be in darkness, morning twilight, or broad daylight at the time. Even if the event happens after sunup, you should have no trouble finding the Moon in the southern part of your sky with the naked eye (given clear, crisp air) and picking up Spica next to the Moon with a telescope.

We also list time predictions and other details for many cities in a PDF-format table.

For your location, interpolate between the yellow curves to find the Universal Times of Spica's disap-pearance and reappearance. (To convert Universal Time to Eastern Standard Time, subtract 5 hours; to get Central Standard Time, subtract 6 hours; to get Mountain Standard Time, subtract 7; to get Pacific Standard Time, subtract 8.) Also indicated are whether the event occurs in darkness, morning twilight, or daytime. Click on each image for the full-size version.
Sky & Telescope diagram by Gregg Dinderman.

To see where on the Moon's limb Spica will disappear — and more important, where it will reappear — interpolate between city tracks on the Moon diagram at the top of this page.

Whatever else you'll be doing on December 25th, you can make it a special astronomical day to remember.

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