Monday, November 8, 2010

Why do the stars curve like that?

So often, I get questions. Questions of how I do this or why does that happen. I love it when people ask questions like that, and I love it even more when I know how to answer! And once in a while, I know the answer, but there are others who can answer it all better then I can.

For this question, about this image, I can answer, my my all knowing about stars sister, Karen, well she can answer so much better! Fact being, she was the one to originally told me the whole left to right, north to south answer to began with. So, here we go. Why do the stars curve to the left AND to the right?

"What are star trails and why do they look like that?

As the Earth spins around on its axis, the background of stars appears to move from East to West. Over the course of one day, the background of stars will have aparently moved 360ยบ, a full circle. Stars closer to the North and South Poles do not move as rapidly as those closer to the Equator.
Timed photographic exposures or stacked exposures can be done in one of two ways: tracked counteracting the movement of the Earth (no star trails) or the camera kept still, moving with the Earth (star trails). In exposures that show star trails, the stars closer to the poles make a tighter circle around each of the poles. These stars are called "circumpolar" and most latitudes have circumpolar stars or constellations. They are visible through the night and the year. A couple of examples of this for the Northern Hemisphere are the Big Dipper and Cassiopeia.
Therefore, stars in the Northern Hemisphere arc toward the North Celestial Pole (Polaris) and stars in the Southern Hemisphere arc toward the South Celestial Pole (which doesn't have a pole star). Stars closer to the Celestial Equator do not arc either way, but appear to move in a straight line.

Karen I. Meddows-Carey
Harford County Astrnomical Society
Harford County College, BelAir, Maryland
One People - One Planet - One Sky"

The basic translation of this is: The stars that curve to the top left of the image, are over the Northern Hemisphere, the stars that curve to the bottom right, are the stars over the Southern Hemisphere and the stars that are straight, they are over the Equator. The whole thing is pretty cool, if you ask me. The way Earth turns and how we all stay on!

1 comment: