Astroblog: The Sky is a Sphere

Tuesday, July 03, 2007

The Sky is a Sphere

The sky is a sphere. Which is why when I try and line up multiple shots taken at slightly different times, I can never get them straight. Because the sky has rotated, rather than simply moved below the horizon, just overlaying the images and lining up on one star, say Regulus, means that anything a little bit away from Regulus will not be lined up.

So to make this mosaic of Venus, the Moon and Saturn over 4 days, I had to rotate the images manually in the Gimp and line up two stars to get things more or less in line (obviously I can't use Venus or Saturn, as they are moving every night). This was a right royal pain, but I more or less got there in the end, and I'm rather pleased with the effect.

Click on the image to enlarge it and make Saturn more obvious, its the dot above the Middle Moon, Venus is the brightest object and Regulus is the dot between the Middle and Upper Moon.

Of course, it would have been technically easier to take the image at exactly the same time every night, but rumbustious small children, drifting clouds and dinner preparations makes it hard to get within 10 minutes of the same time, so that option is out. The images were taken on the 18th, 19th and 21st of June (the 20th was clouded out) in the lead up to the Venus Saturn conjunction. I'll post that image later.

Labels: astrophotography, Moon, Saturn, Venus