Image: Moon and Venus in sky  /  Starry Night
From Earth's viewpoint, Venus has shifted to the east since its last date with the moon on Christmas. This is the what the view to the southwest in midnorthern latitudes should be like at 5:30 p.m. Saturday.
By Night Sky Columnist
updated 1/24/2004 12:18:43 AM ET 2004-01-24T05:18:43

On Saturday evening as darkness is falling, take a look toward the west-southwest part of the sky for yet another beautiful celestial tableau formed by a lovely crescent moon and the brilliant planet Venus. Venus will appear to hover above and to the right of the moon.

The pairing is virtually a repeat of their Christmas night get-together. Similar setups occur now on roughly a monthly schedule.

If Venus were stationary and did not appear to move against the star background, then a Venus-moon encounter would occur every 27 days, 7 hours and 43 minutes. This is called a "sidereal month" — the time it takes the moon to circle the Earth once, using the background stars as a reference point.

Since Venus and the moon were together on Christmas night, we might have expected a return engagement this past Wednesday (the 21st) if we tried to apply the "sidereal month rule" to this schedule. Of course that rule didn’t work because Venus is not stationary, but moving in its own orbit around the sun.

Moving objects
From our earthly viewpoint, Venus has appeared to shift considerably to the east against the star background. Back on Christmas night, Venus was in the constellation of Capricornus the Sea Goat. On Saturday it will have appeared to shift more than 35 degrees to the east, where it currently resides in the constellation of Aquarius the Water Carrier.

So the moon had to travel that much more across the sky to catch up to Venus.

Since the moon appears to move across the sky at roughly 13 degrees per day, it needs three more days to catch up to Venus. That takes us to Saturday evening, where once again we will be treated to an eye-catching sight in our western twilight sky between the two brightest objects in the night sky.

Another factor that must be considered is our own Earth’s movement around the sun. If you looked for the crescent moon this past Wednesday night you wouldn’t have been able to see it because the satellite was at new phase and hence too near to the sun to be seen. That’s because during the 27 days that had elapsed since Christmas, Earth’s movement around the sun would have caused the sun’s position in the sky to shift to the east as well ... in this case, right into the very same region that Venus and the moon occupied on Christmas.

By Saturday, however, the moon will be well clear of the sun and readily visible in the west-southwest with Venus.

Venus rising
Venus has been ascending dramatically higher during January and now is setting more than three hours after the sun. It now gleams at magnitude -4, on a scale in which smaller numbers represent brighter sky objects. Negative numbers are reserved for the brightest of all. No other star or planet can come close to matching Venus in brightness, not even brilliant Jupiter, which this week comes up over the east-southeast horizon around the same time that Venus is setting.

During World War II, aircraft spotters sometimes mistook Venus for an enemy airplane. There were even cases where Venus drew antiaircraft fire.

If clouds obscure your view of Venus and the moon on Saturday evening, don’t fret. Another opportunity to see them together will come on Monday, Feb. 23.

A note about observing during winter: Stepping outside to enjoy Venus these frosty nights takes only a minute or two, but if you plan to stay outside longer, remember that enjoying the starry winter sky requires protection against prevailing low temperatures. This has been especially true across the eastern half of the country, where unseasonably cold weather has been the rule during recent weeks. Among the best garments are a hooded ski parka, which is lightweight yet has excellent insulation, and ski pants. It is also important to remember your feet. While two pairs of warm socks in loose-fitting shoes are often adequate, for protracted observing on bitter-cold nights, wear insulated boots.

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