The Apollo 8 astronauts had a list of 37 stars they could use, but the two they used most were the bright stars Sirius and Rigel. They also measured the angle between stars such as Procyon and the Earth’s horizon using the Command Module’s sextant telescope.

The astronauts were also told by Mission Control which stars to look for in their sextant and at what angles to check that the spacecraft was in the correct attitude with respect to the stars for an engine burn.

To adjust their trajectory, a short 2.4 second burn of the large Service Module engine was made 11 hours into the mission. This gave the crew and Mission Control their first look at the performance of this critical engine, the one that had to work perfectly to get the crew into and out of lunar orbit.