If you’re out at dawn — and it’s not too hard to be with December’s late sunrises — face east to watch the celestial traffic this week. Tomorrow morning (Dec. 3), the crescent moon will stand just 4.5° above the planet Venus, which is currently at “greatest brilliancy” and shining at magnitude –4.9. These two easy-to-see objects will form a compact triangle with Spica, Virgo’s brightest star, ~5° to the west.
Counterintuitively, Venus is brightest when it’s a thick crescent, not when it’s lit up like a full moon. At crescent phase, the planet is much closer to the Earth, so it’s large and bright. At full phase, the planet lies on the opposite side of the sun from Earth. Even though it’s fully illuminated from our perspective, Venus is so far away that its brightness dims by more than a full magnitude.
Two days later on Dec. 5, you can use the moon to navigate to Mercury because the two shiny objects will be in conjunction low in the southeastern sky about 4° apart. Mercury is just coming into good view at dawn, so be on the lookout for this elusive planet now through the end of the month. In the diagram above, set for mid-northern latitudes about 45 minutes before sunrise local time, Venus stands 25° (two-and-a-half fists) and Mercury about “two fingers” or 3° high. To ensure you find Mercury, bring along a pair of binoculars especially early in the month.
On Dec. 21, the winter solstice, Jupiter will return and join Mercury in a very close conjunction. More on that one in a couple weeks!