Monday, December 03, 2007

The bright skies of winter

Winter is finally asserting its grip across much of the United States. As I was walking to the bus on this brisk, windy morning, I was thinking about how bright the stars can appear to be in winter. There are bright stars in the winter sky, some of the brightest visible, and this winter the planet Mars will be bright and high in the sky all night long. But there are bright stars in the summer, too, and the planets are often visible in the summer sky. So, why do the stars seem a little brighter in winter?

The answer, for most of the United States, is in the weather. For most of the summer, the skies are quite hazy. I never appreciated how hazy typical summer weather was on the East Coast until I started doing a lot of air travel. In the summer on the East Coast, you often cannot see the ground, despite it being a sunny day! So, all that murk also dims the stars quite a bit -- I am sure this is a major part of the apparently bright stars of winter.

Also in the winter, we are awake for quite some time after darkness falls, so more people see the night sky. When there is a full moon, the moon is highest in the sky during the winter, and so looks brighter, and, if a little snow is around to reflect moonlight, the entire landscape can look very bright, which probably fools our minds into thinking the sky is brighter.

Surprisingly, then, winter is often not the best time for astronomy in the United States. Much of the country is constantly being hit by cold fronts and pressure systems that, even if they don't have clouds associated with them, cause the air to be quite turbulent. This turbulence blurs our images, so the pictures we get aren't quite as sharp. (If you see the stars twinkling a lot, that is the same turbulence!)

And ice and snow storms are common on mountain tops, where our telescopes are located. So, even if the night is clear, we can't work if the dome is covered in snow! Once I had four nights in January on Kitt Peak in Arizona. Two nights we were closed by an ice fog (I put a picture of the Kitt Peak Solar Telescope taken that day above). The third night was crystal clear, and the stars were not twinkling at all, meaning the air was very still and our images would have been sharp. But there was an inch of ice on the telescope dome -- if we had opened the dome, the ice would either have jammed the dome motors or it would have fallen on the telescope, damaging the mirror. So, in spite of some of the best astronomy weather I'd seen, we had to sit around and play cards and watch movies for another night.

So, enjoy the winter skies. This year Comet Holmes is still visible to the naked eye as a fuzzy spot in Perseus, Mars will be bright and high in the sky (being brightest right around Christmas), an, in February, most of the United States will be able to see its third total eclipse of the moon in a year's time!

If you bundle up, you may also see a lot of meteors -- there are several meteor showers this time of year. The Geminids peak during mid-December, and the Quadrantids peak in early January. Both meteor showers produce a lot of meteors, and even some many days before and after the peak. So, chances are good you will see some meteors this time of year.

So, go enjoy the sky this winter! Sure, you have to bundle up to stay warm, but think how much better that hot chocolate will taste after a half hour enjoying the winter sky!

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