January 19, 2012

The End of a Handle

Polaris, the north star. Although the exposure was not long enough to see any long streaks, you can see that all of the stars seem to be going around in a bit of a circle. The star in the very center of that circle is Polaris. Currently, it is located about one degree from the celestial north pole, near enough that we don't notice it is not perfect. It is also the end of the handle of the little dipper. You can find Polaris by drawing a line straight up from the two stars at the end of the Big Dippers dipper. It is the next bright star.

I am going to have a go at explaining how the celestial sphere works, and how it lets us pinpoint objects in the sky. Basically it works just like a globe, except that instead of it being something you look at from the outside, we look at it from the middle. To find the celestial poles we simply take the north pole of the earth, and drag a line straight up from it. We do the same with the south pole. The equator we simply keep making a larger and larger circle. Picture a globe with a rod through its poles so it rotates. If you were to take a much larger sphere and put the globe into it, then attach the rod to the larger sphere, that would be the celestial sphere. We are on the inside looking out.

The most common way of describing an objects location on earth is using latitude and longitude. The two are somewhat different in what they are based upon. Latitude is based upon the equator. 0 degrees latitude is exactly half way between the poles of the earth, and 90 degrees is the poles themselves. Longitude is largely an arbitrary measurement. It is now standardized that the prime meridian runs through Greenwich in London, but in the past when navigation was being developed, each country had its own 0 point.

The equatorial coordinate system of the celestial sphere works the same way, although they use different names. Declination is like latitude. Exactly half way between the celestial poles, directly above earths equator lies the celestial equator. From the equator you can go + or - 90 degrees declination. Like on earth, the poles are at + and -90 degrees declination.

Right Ascension is the astronomical name for Longitude. Like longitude on earth, it is measured from a somewhat arbitrary point. Unlike on earth however, the units are not degrees but hours, minutes and seconds. Also unlike longitude, it is measured in only one direction, starting at 0 and going east all the way around to 24. The use of hours is not all that different from the use of degrees, each hour the earth rotates 15 degrees, so that over the course of a full day 360 degrees of rotation and 24 hours have passed. The 0 point for right ascension is the position of the sun in the sky at the exact point of the vernal equinox. This is the point at which it crosses the celestial equator.

This system lets astronomers pinpoint any object in the night sky at any given time, and lets them find it again with no effort at all.

Back to the photo. I took this a few nights ago from the top of Mt Douglas. It is a very nice walk up there. The city is just stunning from up there at night. The exposure is a bit of a strange one. It is actually six individual 30 second exposures all on top of each other. My camera lets me take them and it smushes them all together. If you look really closely at the stars, particularly the ones near the top and bottom, you can actually see the individual exposures, and little blank spots between them where the shutter was not open. I kind of like the effect. The pinky color is from the city lights of Victoria.

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