How to put day and night in the same photo?

I started shooting on the morning of December 30th, 2010, by setting my camera on a tripod facing east. For the daytime portion, I followed the sun's trajectory across the sky from sunrise to sunset, moving the camera from east to west. Using an intervalometer, I set it to take a picture every 15 minutes to capture the position of the sun, and I adjusted the exposure with a filter attached to the lens. During one of the shots, when the sun reached its highest point in the sky, I removed the filter to capture the solar halo (seen at the bottom of the top image).

After sunset, I turned the camera towards the west and northwest to capture a smooth transition from day to night. The nighttime sequence was also composed of images taken from different locations, but this time moving from west to east. After capturing the transition, I pointed the camera northwest to capture a half-hour star trail. At 7:30 PM, I repositioned the camera facing north to start another star trail photo that lasted for the entire night — a total of 11 hours. Once completed, I moved the camera to the northeast and captured another half-hour star trail. Finally, I directed the camera towards the northeast to capture another series of transition photos from night back to day.

This stunning piece of work was finally processed on the computer for another 12 hours to complete!