Astro Bob: Mercury and Mars make merry on New Year's Eve
As 2021 concludes, Mercury pokes its head into the evening sky, while Mars returns at dawn.
Good bye, Venus! One week left before the brightest planet slips into the solar glow and disappears from sight. Venus will be in conjunction with the sun on Jan. 8 and return at dawn around Jan. 20.
Before it dances off the evening stage, the innermost planet Mercury joins Venus in a farewell pairing now through Dec. 31. Wednesday (Dec. 29), Mercury will stand about 4.5° — the width of your index and middle fingers held together at arm's length — to its lower left 30-45 minutes after sunset low in the southwestern sky.
Then on New Year's Eve, the two planets will be nearly level to the horizon. When you grab your party hat, take the binoculars, too. You might need them to pick out dimmer Mercury to the left of Venus.
Venus is a beautiful, little crescent "moon" in binoculars. Since it's nearly in line with the sun, only the planet's edge catches sunlight from our perspective. And because Venus is a sphere like the Earth that edge describes an elegant crescent. We're also cosmic buddies at this moment, with just 26.1 million miles (42 million km) between us. Mars never gets this close. At best it's 33.9 million miles (54.6 million km) away or almost 25% more distant. That's why Venus' reaches its maximum apparent size right now. From tip to tip, the crescent spans 1 arc-minute across, which is equal to 1/30th the apparent diameter of the full moon.
One arc-minute is also about the resolution limit of the human eye. Some people have claimed to see the Venusian crescent without optical aid, but I've never had any luck. That won't stop me from trying again this week as long as the clouds skedaddle out of here.
Mercury, just entering the evening sky as Venus departs, appears more than 10 times smaller and shows a gibbous or three-quarter-moon phase. You'll need a telescope to see its shape. Not only is Mercury considerably farther away than Venus, it's much smaller, with a diameter of 3,032 miles (4,975 km). To put that in perspective, that's just 872 miles (1,403 km) larger than the moon.
Mercury will hang with us for the first two weeks of the new year and then circle back in the sun's direction and transition into the morning sky. That will leave Saturn and Jupiter the only bright planets visible at dusk. Saturn will disappear in the solar glow in late January, with Jupiter following in mid-February. Come March, the only evening planet will be Uranus, which requires binoculars to see.
Meanwhile, the waning lunar crescent stops by Mars on Friday morning, Dec. 31. The Red Planet has been faint and too near the sun in recent months to see. It's only now reappearing at dawn and thanks to the moon will be easy to find. The two will huddle about 3.5° apart an hour or so before sunrise. Since the visibility of our featured planets depends a great deal on where the sun is, click here to find out your local sunrise and sunset times so you can plan accordingly.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob .