BAN #381: A comet and planets in the night sky

06 December 2021 Issue #381

December 06, 2021

[The planetary nebula M 2-9, winds from a dying star. Credit: NASA / ESA / Hubble Legacy Archive / Judy Schmidt]

Blog Jam

What I’ve recently written on the blog, ICYMI

[Artwork of a gas giant orbiting very close in to its host star. From Tuesday’s article. Credit: NASA, ESA, and G. Bacon]

Monday 29 November, 2021:  How massive is the Milky Way?

What’s Up?

Look up! There’s stuff to see in the sky!

If you’re on social media (especially astronomy Twitter) you may have heard about a new comet, called C/2021 A1 (Leonard). It was discovered in January when it was still quite a ways out, but has been approaching the Sun and Earth over the past few months. It’s been getting brighter, and it might — might — get to be barely naked eye from a dark site over the next week or two.

If you want to look for it, you’ll need good binoculars or a small telescope, a dark site, and some coffee: It’s up before sunrise but very low to the horizon at dawn, so it’s a tough one. Around Dec. 9 it’ll be too low to see and after that it’ll be very close to the Sun in the sky. Irritatingly, for observers around 40° north latitude, it hugs the horizon as it moves away from the Sun so it doesn’t get any higher in the sky. It’s better for southern hemisphereans.

Sky and Telescope has a viewing guide, and if you search online you’ll find plenty more.

On Friday morning, Dec. 3, the fantastic astrophotographer Damian Peach took this wonderful image of the comet as it passed by the globular cluster M3 (though a tad closer; M3 is 34,000 light years away and the comet was 63 million kilometers away):

[Comet C/2021 A1 (Leonard) as it passed by M3 on Dec. 3, 2021. Credit: Damian Peach]

Whoa. Click that to see it bigger. If you’re on Twitter, follow Damian there!

The comet came from the Oort cloud, the vast halo of icy bodies surrounding the Sun out well past Neptune, extending for trillions of km. Interestingly the comet is on a barely hyperbolic trajectory, meaning it may not be bound to the Sun (usually the orbits are extremely long ellipses). I wrote about this in detail when ‘Oumamua was discovered; that’s usually due either to it interacting with a planet on its way in — unlikely, given the orbit is tipped way over with respect to the planets — or measurement uncertainties. Either way, it won’t be back in these parts again for thousands of years, if ever.

[The path of comet Leonard (white line) and its position on Dec. 3, 2021. The solid line is when it’s north (“above”) of the plane of the solar system; the fainter grayish one when it’s south (“below”). As you can see it doesn’t pass close to any giant planets. If you click the following link you can get to an interactive chart where you can tilt and pan on the orbit. Fun! Credit: NASA / JPL-Caltech]

However, first time comets are sometimes very fragile and can break apart when heated by the Sun. That can generate a lot of ice and dust spraying out, which can brighten a comet a lot very suddenly, and fade rapidly as well. So it’s still possible this may put on a show. Or it may not. As comet hunter David Levy has said, “Comets are like cats. They both have tails and do precisely they want.” In other words predicting them is hard.

I personally won’t try too hard to view this one unless it has an outburst — I’ve seen a comet or two do that and it’s pretty amazing — but I do hope others will get some good photos. There have already been lots on Twitter!

And if you don’t go for the comet, you should head outside after sunset anyway. Venus is still shining in the southwest, with Jupiter and Saturn to the east of it, and they make a lovely sight. Funny, too: On December 10 Pluto and Venus will be about ¼° apart, about half the size of the full Moon on the sky! That will be a tough trick to photograph, though, since Venus is nearly 45 million times brighter than the more distant icy world. You’d need a pretty good ‘scope and camera setup to catch Pluto at all, in fact. Still, it’s fun to think about.

Venus orbits the Sun closer in than we do, and is catching up to us right now; it’ll pass us on the inside on January 8. Until then, it gets closer to the Sun and lower every day after sunset, but it also gets closer to us. It’ll appear as a thin crescent through binoculars, and get decently big in late December, an arcminute or so in size (1/30th the apparent size of the Moon). That’s coincidentally the resolution of the human eye, so someone with keen vision might just be able to see Venus as a thin curved line. But if you have binocs, even small ones, go look at Venus! It’s so cool to see it as a crescent.

[The phases of Venus, which change as we both orbit the Sun and the geometry changes. When Venus passes Earth on the inside we see it as a thin crescent, larger than usual because it’s closer to Earth. Credit: Statis Kalyvas and the VT-2004 programme]

You can see Jupiter’s moons, too, and they change every night, so that’s fun too. Saturn is getting pretty far from Earth on the other side of the Sun, but a small ‘scope will easily reveal its rings and a bright moon or two.

As usual, if you want to find all these things, try Stellarium online or download an app (I like Sky Safari, but there are a lot of them). They take a bit of practice but are extremely useful to find out what’s up.

If you can, go outside and look up! There’s a whole Universe out there.

Et alia

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