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Last updateTue, 18 Sep 2018 1pm

At the turn of every year—the night of Dec. 31-Jan. 1—Sirius the Dog Star, the brightest of nighttime stars (but not as bright as Venus or Jupiter), follows the sun across the sky by almost exactly 12 hours. This means that the blue-white twinkling star reaches its high point in the south on Dec. 31 near local midnight, solar time.

Sirius attains its high point in the south about four minutes earlier each night, or two hours earlier with each passing month. By the start of spring (March 20), Sirius will stand high in the south in evening mid-twilight, only 40 minutes after sunset. In the meantime, enjoy Sirius and the attendant stars of Canis Major, the Greater Dog, marching across the southern sky, acting out the lines of Robert Frost’s poem, “Canis Major”:

The great Overdog

That heavenly beast

With a star in one eye

Gives a leap in the east.

He dances upright

All the way to the west

And never once drops

On his forefeet to rest.

I'm a poor underdog,

But to-night I will bark

With the great Overdog

That romps through the dark.

The moon passes all five bright planets this month, including Venus twice. In the evening sky, within 30 minutes after sunset on Jan. 2, look for soon-to-depart Venus to the lower right of a thin crescent moon low in the southwest to west-southwest. (You’ll need a vantage point without high mountains blocking the view!) A nearly full moon will keep company with Jupiter from dusk until first light of dawn on the night of Jan. 14-15. An hour before sun-up on the mornings of Jan. 22 and 23, watch the waning gibbous moon leapfrog past Spica and the brighter reddish planet Mars just above Spica, well up in the south-southwest. The moon is very closely left of Spica on the morning of Jan. 23, and appears at last quarter phase (half full) on the morning of Jan. 24; on the next morning, the fat crescent appears just below Saturn. Venus, by then a prominent morning “star” low in the east-southeast, will appear to lower left of the waning crescent moon an hour before sunrise on Jan. 29. The next morning, look for the last thin old moon rising to lower left of Venus. Back in the evening sky on Jan. 31, the thin young crescent moon will appear low in the west-southwest at dusk, a few degrees to lower right of Mercury, which pays a brief visit to evening twilight skies in late January and early February.

Venus switches from the evening to the morning sky (with an overlap of a few days), and Jupiter is visible all night early in January. Use binoculars in twilight to reveal Jupiter as a disk, and, all this month, an even larger (in apparent diameter) Venus in a crescent phase. On Jan. 1, Venus is 15 degrees to the upper left of the setting sun. As Venus traverses the near side of its orbit, it will pass only 5 degrees north of the sun on Jan. 10-11. By Jan. 31, Venus will be 29 degrees to the top of the rising sun.

This month, Venus and Jupiter will play hide and seek. The two planets appear in nearly opposite directions in the sky, because Earth overtakes Jupiter on Jan. 5 (creating an opposition of Jupiter, when the giant planet rises at sunset and sets at sunrise), and Venus overtakes Earth on Jan. 11 (creating an inferior conjunction of Venus, when Venus rises and sets nearly together with the sun).

After Venus emerges into the morning sky, see four planets, in east to west order: Venus rising in the east-southeast, Saturn in the south-southeast, Mars in the south-southwest, and Jupiter setting in the west-northwest. Because of our surrounding mountains, you probably won’t spot Venus until after Jupiter has set.

Robert C. Victor was a staff astronomer at the Abrams Planetarium at Michigan State University. He is now retired and enjoys providing skywatching opportunities for school children in and around Palm Springs. Robert D. Miller, who provided the twilight charts, did graduate work in Planetarium Science and later astronomy and computer science at Michigan State University and remains active in research and public outreach in astronomy.

Published in Astronomy