Posted by: Chris Cole | February 25, 2009

Comet Lulin

For those of a stargazing disposition, tonight was a bit of a minor but still somewhat anticipated event on this year’s astronomical calendar. Comet C/2007 N3, or “Lulin” to its friends, was co-discovered by two teams of astronomers, in China and Taiwan respectively, in 2007. It is a so-called non-periodic comet; one which cruises in on a wayyyyy eccentric orbit (this particular visitor to the inner solar system is actually going backwards with respect to the planets and all the “normal” asteroids and comets we know) and then buggers wayyyyy off again, either never to be seen again in these here parts, or at best to make a repeat appearance in millions, rather than tens, of years. Other periodic comets, such as Halley’s, have less eccentric orbits, go the “right way” around the sun, and come back regularly (some every few years, or like Halley, every 76 years or so). So spotting Lulin this month is pretty much the only chance you’re going to get.

Lulin is currently (Feb 24th) at its closest approach to the Earth. Tonight it’s a mere 62 million kilometres from us (about 41% of the distance from us to the Sun), moving away from the Sun at a hefty rate of knots, and spewing bright green shit everywhere, which is very pretty from a distance but would be wonderfully toxic up close (the groovy green colour is due to emissions from ionised diatomic carbon and cyanogen gas).

So where the hell is it? At the moment, from Australia, it’s visible just above (north of) and to the left west) of Saturn, as seen around midnight. If you can find Regulus (the very bright star at the “top” of the constellation Leo, as seen from the Southern hemisphere), draw an imaginary line between it and Saturn (the very bright not-so-flickering planet… if you have a telescope or even good binoculars, the rings sticking out each side are a bit of a giveaway) and look for a faint but large fuzzy glow somewhere along that line, much closer to Saturn than to Regulus. As February draws to a close, the comet will move closer to Regulus and further from Saturn. The diagram below shows roughly where to find it in the sky for various dates. I’ve rotated it so it’s almost correct for Aussie observers. Ignore the horizon on the diagram and pretend the ground is along the bottom of the frame.

Lulin's position in the sky for various dates

Lulin's position in the sky for various dates

Below is another diagram showing where to look over the next couple of nights. It’s upside down because most people in the northern hemisphere assume no-one bothers to live in the other half of the world, or if we do, we’re undoubtedly all savages and wouldn’t want or be able to read a sky chart accurate for us, anyway. Bastards. Again, pretend the ground is at the bottom. In reality, at the moment Saturn and Regulus are something like 40 degrees up from the horizon, not close to the ground as the diagram depicts.

Another rough map of Lulin's position

Another rough map of Lulin's position

Lulin is currently just scraping in at 6th magnitude. This is about at the limit of what you can hope to see with your unaided but dark-adapted eyes. And by dark-adapted, I mean sitting totally in the dark for at least 15-20 minutes. Yes, that’s how long it takes. Trust me. And yes, even glancing at the illuminated screen of your mobile phone will fuck that up, and you’ll have to start all over again. Sad, but true. You will also find it easier to see the comet (and any other faint object) if you don’t look directly at it. This is an annoying but again, nonetheless true, consequence of your ocular physiology.

Your fovea, which is the bit of your retina you’re reading this with, is packed with photoreceptor cells called cones, which are awesome for seeing colour in bright conditions. Tragically, they need lots and lots of photons to work well, and so they totally suck for looking at faint things. For that, you need different photoreceptor cells called rods. Happily, you have an ample supply of these, too, but they’re located in every other part of your retina except the fovea, which is the bit that catches the light from things you’re looking straight at. Consequently, if you want to see faint things in the sky, you need to look just a tiny bit away from whatever it is, and it will magically become much brighter and easier to see.

If you happen to have a telescope (even a crappy one will do) or a half decent pair of binoculars, you will easily be able to see Lulin. A decent pair of 7 x 50 binoculars will let you see most objects down to about 9th magnitude (by the way, the bigger the number, the fainter it is… astronomers are a bit weird). The 7 refers to the magnification factor, which is totally irrelevant. The 50 bit refers to the size of the aperture (big front lens) that’s capturing the light. The bigger the better, and most good field binoculars will be 50mm, though even a “compact” 35mm set will do fine for looking at this particular comet.

Well, I’m off to do some of that sleeping type stuff. Below are a couple of examples of what the comet looks like. Both were taken by hobbyist astronomers with small off-the-shelf telescopes, from their backyards.

Comet Lulin caught in the act of glowing green

Comet Lulin caught in the act of glowing green

Another backyard snapshot of Lulin

Another backyard snapshot of Lulin

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Responses

  1. thats awesome! Never really been in to the star gazing, but am definitely keen to start, its amazing how much goes on up there that 90% of the world is oblivious to.


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