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  • Rapt in Awe

    My Journey through the Astronomical Year

    Think of this as a "companion text" to this, the main web site. Not required reading, butI hope you'll find it interesting and helpful.

Look North in October 2010 – Hey, is that really a comet?!

Click image for larger view. (Prepared from Starry Nights Pro screen shot.)

For a printer-friendly version of this chart, click here.

Yep! That’s a comet. And while it’s easy to spot in the lower right corner of our chart, it may just barely be visible to the naked eye this month as it crosses the northern sky. And it certainly will be visible in binoculars – but – and this is important – don’t get too excited. The long tail on the chart is purely a figment of the imagination of the folks making the Starry Nights Pro software. That’s how they represent all comets just to alert you to where they can be found. But in this case I doubt there will be much – if any – tail visible even in binoculars. However, never – never – never try to second guess a comet. They are sly critters. In 2007 a comet that was generally a dud and the public had paid no attention to suddenly brightened in October by a factor of half a million times overnight – and scientists still aren’t really sure why. But oh my, it was quite a show for a couple of months. And it defied all predictions and all comet common sense. So comets are always worth checking on. They change position nightly – our chart shows its location only for the night of October 15, 2010. Generally their behavior is predictable and certainly their orbits are well known, but fairly frequently they do the unexpected in terms of how bright they get.

And it’s because we still have a lot to learn about comets that this one will be visited in early November by a spacecraft, so we’ll get a close-up view of it most likely. That makes it all the more fun to find with naked eye or binoculars. I go into much more detail about it – with more charts – in the “events” post for this month. Meanwhile, back in the rest of the universe, or at least that portion we see when we look north, the great wheel of stars continues to turn, revealing a fresh wonder.

I’m talking about Capella, one of our brightest stars, which you may catch just peeping up over the northeastern horizon about an hour and a half after sunset this month. And it is brilliant! Lot’s of people assume the North Star – Polaris – is the brightest star in the sky. When you see Capella and compare it with the North Star you will get a good idea of what the word magnitude means. There are a solid two magnitudes difference between the two – or if you want to get technical, Capella is 0 and Polaris is 2. Each magnitude difference represents a change of about 2.5 times in brightness. So a zero magnitude star is 2.5 times as bright as a first magnitude star – and 2.5 x 2.5, or 6.25 times as bright as Polaris, a second magnitude star. And while we’re on this subject, the faintest star in the Little Dipper – it’s over in one corner of the cup – is just about magnitude five. That means Capella is 100 times as bright as that star.

You can also figure Capella will probably be about 100 times brighter than the comet, though the comet may appear even dimmer since its light is spread out whereas a star’s light is concentrated in a point. Of course, the comet could fool us and be brighter – or dimmer – than predicted. Capella won’t. It’s pretty reliable.

And while we’re on the subject of magnitude, the Little Dipper does give us a great range. Polaris, as we said, is magnitude 2. The other bright star in the Little Dipper is magnitude 2 also – it’s at the end of the cup away from Polaris. Sharing that end is a magnitude 3 star, and these two are known as the “guardians of the pole” as they make a circle around it every 24 hours. Most of the other stars in the Little Dipper are magnitude 4, with the one star in the corner of the cup being magnitude 5, as noted – well, actually 4.9. There’s a chart with these magnitudes in this post.

How faint a star can you see? Depends upon your eye sight, the light pollution in your area, the transparency of the skies on any given night, how high the star is in the sky, and, of course, your vision and dark adaption. But as a general rule of thumb magnitude 6 has been accepted as the typical naked eye limit. However, if you live in a typically light polluted suburb you may see only to 4, or worse yet, magnitude 3. And observers on mountain tops with really clear skies reliably report seeing stars as faint as magnitude 8. All of this is with the naked eye. Add ordinary binoculars and you will certainly add three to five magnitudes to the faintest star you can see. In fact, you take a huge jump in the number of stars seen just by using binoculars with 50mm objective lenses. That’s why many amateur astronomers with fine telescopes also are likely to have a pair of 10X50 binoculars that they keep handy. I always did, but more recently I stumbled upon some very inexpensive Celestron 15X70 binoculars that I love for quick peaks, though you can’t hold them steady for serious observing.

Believe me, they will give me my first look at the comet – but for a better look I’ll use a small scope, or binoculars that are mounted on a tripod and so steady.

All of this takes away from Capella. It will be a “look east” guide star next month, but you can get to know it this month as you look north. And if you live in mid-northern latitudes, it will become a most familiar site. In fact, where I live – about latitude 42°N – it is in the night sky at some time every night of the year. This is because it is so far north and as it circles Polaris only dips below the horizon for a few hours at a time.

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