As the moon makes its monthly rounds across the sky, it often passes by the stars, obscuring them for up to an hour.
Such an event is called an occultation (derived from the Latin word occultism, meaning 'to hide'), and it can be a surprising sight, especially if the star is bright. The star appears to creep up to the edge of the moon, lingers on the edge for a minute or two and then, without warning, abruptly blinks out. Later it reappears just as suddenly on the other side of the moon. The suddenness with which occultations occur was one of the first proofs that the moon has no atmosphere. If our natural satellite were covered with an atmosphere, a star approaching its edge would appear to gradually fade away, just as you set behind the western horizon from here on Earth.
If you live in the Pacific or Mountain Time Zone, be sure to set your alarm before sunrise on Monday, January 8, because early in the morning a waning crescent moon, 11% illuminated, will occult the first-order red supergiant . star Antares, one of the brightest stars in the sky. This should be worth waking yourself out of bed in the early morning hours to watch if you live in one of the far western states.
Unfortunately for those living further east, in the central and eastern states, this stellar eclipse occurs after sunrise in clear daytime skies. From these areas, Antares will appear very close to the moon's lower left as they slowly rise across the east-southeast sky before sunrise. But as they gradually get closer, the increasingly bright twilight sky will eventually overwhelm Antares before the moon has a chance to hide it.
Related: Night sky, January 2024: what you can see tonight
Region of visibility
For Westerners, the action begins when Antares disappears behind the bright edge of the moon. For those living in the northwestern states, the moon will barely be above the horizon, so poor visibility will exacerbate the problem. From Seattle, for example, Antares' disappearance comes right after the moonrise.
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Further south, the time of disappearance will appear higher. From San Francisco the pair is 6 degrees high and from Los Angeles 11 degrees above the horizon. (Your clenched fist, held at arm's length, is about 10 degrees wide.) The moon and star will make a striking sight as the latter disappear near the southern tip of the crescent moon.
Because Antares will disappear behind the bright edge of the moon, binoculars or a telescope will likely be needed to observe this event.
However, those same Northwestern states, as well as Northern California, will be in a prime position to see the star's return, which will be spectacular wherever the moon is in the dark sky; an event that is more easily observed, because the moon will be higher above the horizon and the star will not be blurred by the bright glare of the crescent moon. Dawn will have already begun for most of Washington, Oregon, the northwestern corner of Nevada and the northern third of California, but the sky will still be quite dark when Antares comes into view again from behind the dark edge of the moon . If you see well, its sudden onset should be visible to the naked eye. Binoculars will pull in the star well, and a small telescope will provide a beautiful view.
Those who live as far east as a line running from approximately southernmost California to southeastern Montana will see Antares appear during clear dawn; the further east you go, the clearer the sky becomes. For those who live east of a line running from approximately southeastern Arizona to northwestern South Dakota, Antares will unfortunately reappear after sunrise. Nevertheless, the moon should still be visible and provided it is not too blurry, a good telescope should also show Antares as a tiny dot against the blue sky.
A "cunning companion"
What makes this event even more interesting for those who will be watching under dark skies is Antares' magnitude 5 companion star, which was discovered in Vienna, Austria by Professor Johann Tobias Bürg during the Antares occultation of April 13, 1819. The faint secondary star is located 2.7 arcseconds due west of Antares and will always appear five seconds before Antares itself. (To understand how small 2.7 arcseconds is, the apparent width of the moon has an average diameter of 1800 arcseconds. So Antares and its companion are separated by 1/667 of a lunar diameter, or 0.00075 degrees!). The American astronomer Mary Proctor (1862-1957) called it 'the cunning companion of Antares'.
The companion star is described as appearing emerald green or a "green hue" compared to Antares' ruddy glow. But the coming occultation will provide a rare, if brief, opportunity to gauge the true nature of the companion, free from any contrast illusions. In February 1991, I had the opportunity to see the faint companion appear behind the Earth-lit edge of a waning crescent moon and judged it to be pale blue-white in color. What color do you see on January 8?
The companion's disappearance will be invisible, because it takes place between the glare of Antares and that of the bright limb of the moon.
What to expect
In the table below, we provide viewing conditions for seven selected cities, showing times when Antares disappears behind the bright edge of the moon ("in") and when it comes back into view from behind the Earth-lit dark edge of the moon ( "out"). The time of sunrise is also mentioned. Note that from Phoenix, Denver and Winnipeg, Antares will emerge after sunrise.
Also included is a map showing Antares' path as it passes behind the moon. The star will disappear behind the bright crescent, while it will reappear behind the Earth-lit dark edge. Those in the Far West looking for the early rise of the Antares companion star are focusing on the part of the moon's disk where the companion star and, five seconds later, the much brighter Antares will appear.
The next time?
If unstable weather prevents you from seeing this 'Antares Eclipse', there are two more chances to see one more in the coming months, but in either case the viewing area will be limited mainly to the Piedmont and southeast coast regions. , as well as Florida and parts of the Gulf Coast. The first case involves a last quarter ("half") moon on March 3 and, like our upcoming event this week, will take place again in the early hours of the morning. The other event will occur during the more suitable late evening hours of May 23, but the moon will be nearly full.
Antares occultations are taking place in a series, which began on August 23, 2023 in much of North America, and will continue each month in different parts of the world until the current series ends on August 27, 2028 in Central Asia.
If you want to see Antares, moon occultations, or other night sky views, our guides to the best telescopes and best binoculars are a great place to start.
And if you want to take photos of Antares' lunar occultation or the night sky in general, check out our guide to photographing the moon, as well as our best cameras for astrophotography and the best lenses for astrophotography.
Joe Rao is an instructor and guest lecturer in New York Hayden Planetarium. He prescribes on astronomy Natural history magazinethe Farmer's almanac and other publications.
