Monday, January 21, 2013

Orion and why we have a winter sky

What: Following up from a couple weeks back, I wanted to try and draw out in my head (and then on paper) why we have a winter sky. The sketch above is a view looking down on the northern hemisphere of the earth as it rotates around the sun (counterclockwise). The dark part of the earth represents the "nighttime" side of the earth. It might be hard to tell from the drawing but Orion is supposed to be positioned below the earth's axis of rotation around the sun, such that at night it appears to be in the southern portion of our sky.

From our perspective, the sun is fixed relative to Orion (drawn just below the word winter). Earth's position relative to the two celestial bodies changes slight each night and significantly across seasons. The shaded side of the earth (what we call night) always faces away from the sun. So as the earth rotates around the sun, the direction we face out into the universe changes. As we circle around, Orion falls farther and farther away from being in the section of sky framed by the horizon. And pretty soon by the time summer rolls around, the shaded side of the earth faces away from Orion, and all those other winter stars that would appear on the left hand side of the drawing.

All this is comes from pretty basic observations that I'd never stopped to make. Drawing made all the difference for me in figuring out why we have a winter sky. You can also use the above drawing to infer why we have summer in the northern hemisphere when the north pole points towards the sun and winter when it points away from the sun.

Where: Up up up!

Other notes: The full moon is coming up soon and I plan on heading back out to take a couple more photos of the moon at the ball park to get another set of relative photos to gauge how it's arc in the sky has shifted from last month to this month, if at all.

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