BAN #227: Gilligan’s epic, A comet’s antitail

15 June 2020   Issue #227

[Spiral Galaxy M81 image credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona]

Subscribers are brave and sure.

Apropos of nothing

Not everything needs to be themed

I have no words, nothing I could possibly add to this.

Pic o’ the Letter

A cool or lovely or mind-bending astronomical image/video with a short description so you can grok it

The comet C/2017 T2 (PANSTARRS) is a moderately bright binoculars object right now, visible in northern skies. You’ll need a dark sky and big binocs or a small telescope to see it, to be clear (the link above has finder charts, as does this one). The fact that it’s currently moving through the bowl of the Big Dipper should make it considerably easier to spot!

But if you have cloudy skies, or no equipment, or like me you’re just lazy, then why not gape in wonder at an image of it taken by Damian Peach?

[The comet C/2017 T2, near the galaxies M81 (right) and M82 (right of center). Credit: Damian Peach]

WOW! The two galaxies to the right of it are M81 and M82 (or just scroll to the top of this issue to see M81 writ large), and you should look them up because they’re gorgeous. You can also see lots of “cirrus”, wispy dust floating around in this part of the sky that I’ve written about before.

The comet is green, likely due to the diatomic molecule C2 (two carbon atoms bound together) which, though not hugely abundant in comets, glows very strongly when excited by sunlight.

You may notice the comet appears to have two tails, pointing in opposite directions! That’s a weird geometric effect, with the one pointing ahead of the comet called the antitail. The comet is shedding material as ice on it turns to gas due to heat from the Sun. This stuff blows more-or-less away from the Sun and back along the comet’s orbit, forming a flat fan. When the comet passes through the orbital plane of the Earth we see the fan edge-on so it looks like a line, and if the geometry is right some of the stuff behind the comet appears to be ahead of it in its orbit:

[Why we see a comet’s antitail; not to scale and really drawn in an exaggerated way to emphasize the geometry. Credit: Phil Plait]

See how this works? The tail isn’t physically ahead of the comet, but for a short time things line up that as it spreads out behind the comet we can see it on both sides of the comet head.

Astronomy is tricky sometimes. Things are far away and we see them projected on the sky, flattening the 3rd dimension. That can make it confusing to tell what’s what, like what side of a galaxy is closer to you, and whether something is in the foreground of a nebula or the background. In this case, it makes comets spiky.

And hey, if you want more gorgeous images of comets, planets, and more, follow Damian on Twitter. His stuff is always amazing.

Blog Jam

What I’ve recently written on the blog, ICYMI

[The crescent Earth, part of an astonishing time-lapse video of the Earth over a single day. From Friday’s article. Credit: JAXA / JMA / Seán Doran]

Monday 8 June, 2020: If this alien planet exists, it might be Earth-like. Or it might not.

Tuesday 9 June, 2020: Titan has had *enough* of Saturn, leaving the planet 100X faster than expected

Wednesday 10 June, 2020: Quarter-million-light-year long threads in an active galaxy have astronomers… engaged

Thursday 11 June, 2020: Less than a million years ago, a supernova exploded just *600* light years away!

Friday 12 June, 2020: The 75 seconds the Earth stood still

Et alia

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