Astro Bob: Comet ZTF clips Capella at Full Snow Moon

Although a full moon will interfere, Comet ZTF will pass right next to one of the sky's brightest stars Sunday night.

Full Moon Jan 6, 2023
This fisheye-lens view shows the moon near its highest point in the sky during the Jan. 6 full moon.
Contributed / Bob King

Do you look forward to the full moon? If you're an amateur astronomer hoping to see galaxies and other fuzzy things like comets you don't. You might even snarl at it over your shoulder. A big moon drowns out everything but the brightest stars, so if you want to see something in your telescope that's only visible for a few nights you can pretty much forget about it. On the other hand, a full moon is something to be enjoyed for itself especially at rising, when it's nearly as dramatic as a sunrise.

Full Moon and Earth shadow
The Jan. 6 full moon rises along the fuzzy edge of Earth's shadow (dark, purplish band) as it will do again on Sunday night. Both the full moon and our planet's shadow lie directly opposite the setting sun.
Contributed / Bob King

On Sunday, Feb. 5, the Full Snow Moon will rise in Leo the Lion. As with all full moons it will peep over the horizon directly opposite the sun around the time of sunset. To find out your local moonrise time, use this calculator . Since the sun goes down in the southwest, watch for the moon to appear at northeastern horizon. Because the sun is literally at our back when we face the rising moon, we see its near side fully illuminated.

Full Moon orbit diagram v3.jpg
At full moon our back is to the sun with sunlight fully illuminating the near side of the moon.
Contributed / Bob King

At full moon, sunlight shines straight-on at the moon, the same way it does if you rudely point a flashlight in someone's face. Direct lighting produces no shadows, the reason aging celebrities prefer it in their disturbingly wrinkle-free portraits. So too with the full moon — without shadows to help delineate details, the lunar surface has a flat, pasty look.

Apollo backscatter
With the sun directly behind him, Apollo 17 astronaut Eugene Cernan photographs the bright glow of lunar soil around his shadowed head. Face-on lighting hides shadows and backscatters light. The exact same thing happens when we face the full moon, making it 40 percent brighter than the night before or after full.
Contributed / NASA

Not only is the full moon the brightest lunar phase because the whole face is lit up, at least two other factors increase its brightness even more — shadow hiding and backscattering. Lunar soil particles and rocks cast shadows just like objects do on Earth. At full moon, those shadows fall directly behind the bits and pieces, so they're hidden from our view. Without shadows the moon naturally looks brighter!

Crystalline materials on the moon also scatter light back to our eyes when the sun strikes them face-on, adding to the intensity. Hard to believe but true, these factors (and others) combine to make any area on the moon 40% brighter at full moon compared to just one day before or after full moon.


Comet ZTF E3 Feb 4
This is a 3.5-second time exposure of Comet ZTF and Capella with a 150mm lens I took at 7 p.m. Saturday night, Feb. 4. The comet will be much closer to the star on Sunday night.
Contributed / Bob King

Remember I said that looking at dim, smudgy things in bright moonlight is probably not a good idea? We're going to make an exception. There's been so much interest in Comet ZTF (C/2022 E3) we can't pass up the opportunity to see it Sunday night, Feb. 5 right next to the bright, whitish star Capella.

Normally, I'd say just wait until the moon departs the sky, but I easily found the comet on Saturday night (Feb. 4) using Capella and my 10x50 binoculars. And the moon was only a day before full! While little more than a softly-glowing, faint patch of light, I had no problem spotting it.

Full moon chart Feb 5 2023.jpg
Capella stands high in the southeastern sky during the early evening hours. It's the topmost star in the Winter Hexagon.
Contributed / Stellarium

On Feb. 5, Comet ZTF will appear just 1.5° above (southwest) of Capella in the constellation Auriga the Charioteer . During early evening hours, Capella shines way, way up in the southeastern sky not far from the overhead point. It's also the topmost star in the giant asterism, the Winter Hexagon. I suggest you use binoculars with a minimum of 35mm aperture such as 7x35, 8x40, 7x50 and 10x50.

Comet ZTF map
The green circle shows a binocular field of view that includes Capella, the comet and the Kids. Dark-adapt your eyes as best you can and look for the comet just above Capella on Sunday evening, Feb. 5.
Contributed / Stellarium with additions by Bob King

Before making the attempt, pick a spot where a tree or building blocks the moon from view. Also, allow your eyes about 5 minutes to adapt to the darkness. Once you identify Capella, focus it to a tiny point in your binoculars and then look for a small, puffy patch of light just above and slightly to the right of the star. You can also use the skinny triangle, called the Kids and named for goats, as a pointer.

I'll be out watching, too. If you find the comet, great! Please share your observation on my Facebook . If not, the full moon bathed in dusky pastels will make a fine prize, too.

"Astro" Bob King is a freelance writer and retired photographer for the Duluth News Tribune. You can reach him at
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