Astro Bob: Watch the Geminid meteor shower peak tonight Dec. 13-14
The year's richest meteor shower peaks on Monday night, Dec. 13-14.
Every mid-December, Earth and its residents plow headlong through a stream of dust and small pebbles shed by the asteroid 3200 Phaethon (FAY-eh-thon). The material strikes the atmosphere at around 79,000 miles an hour (127,000 kph) at altitudes between 45 and 62 miles (76-100 km). Each speedy encounter creates a streak of glowing air several miles long by some 30-65 feet wide (10-20 meters) called a meteor or shooting star.
When you drive your car through rain or snow at night, the drops and flakes appear to stream from a spot in the distance called the vanishing point. It's a perspective effect. The same happens when the moving Earth (the "car") passes through the asteroid stream — the particles appear to radiate from a spot in the sky called the radiant. Earth encounters Phaethon's debris trail every year in mid-December, when meteors stream from the direction of the constellation Gemini the Twins, not far from the bright star Castor. That's why we call them Geminids.
The Geminid meteor shower is active much of the month because the Earth takes time to cross the stream — think of a boat crossing a wide river. We encounter the densest part of the debris trail on the night of Dec. 13-14 (Monday night-Tuesday morning), when up to 120 meteors per hour might be visible. That's an idealized number based on pristine skies with the radiant directly overhead. In real life, where most of us live with some degree of light pollution, that translates to about 50-60 per hour from rural-suburban skies. Of course, the closer you are to a big city, the fewer meteors are visible. If you can, try to drive to darker skies.
The moon also plays into how many meteors will be visible. This time around, it's in waxing gibbous phase and about three-quarters illuminated. That means lunar glare will handicap the shower during the evening hours up until about 3 a.m. local time. You'll still see Geminids as soon as it get dark but only the brightest ones, with counts reduced to around 20-30 per hour.
Good news. The shower is known for Venus-or-brighter meteors called fireballs, so all is not lost even in moonlight. You'll just have to be more patient.
Since Gemini rises in the northeastern sky, set up a reclining chair and face to the southeast or north for the best views before midnight. That way you'll get a good mix of short-trailed meteors that appear near the radiant and longer-trailed ones that streak past broadside. Remember to face away from the moon to preserve your night vision as best you can. Naturally, you'll need to dress warmly, so you're comfortable enough to relax and enjoy the celestial show. I wear my full winter gear, with chemical handwarmers tucked in my gloves, and covered to the neck with a wool blanket. Just thinking about it makes me feel all toasty.
Best shower views will be between about 3-6 a.m. local time on Tuesday morning the 14th when dark skies return after moonset. Because of Earth's rotation Gemini will have shifted into the southwestern sky at this time, so turn your chair to face south or north to bag a nice meteor mix.
Rates vary throughout the night. Sometimes two or three meteors flash back to back followed by "quiet" spells that can last five minutes or more. Hang in there. Every meteorless minute that passes only increases the anticipation of seeing Phaethon's next fiery javelin. Should poor weather interfere, try the following two mornings. Meteor counts will be lower, but activity will continue until about the 21st.
The weather in my region looks iffy. But I still plan to set the alarm for 4 a.m. and hopefully share two hours with the Geminids. I trust you will, too. In return we'll get to experience the childlike pleasure of watching cosmic sparks race across the sky.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob .