View of Earth as photographed by the Apollo 8 astronauts on their return trip from the Moon. Note that the terminator is straighter than on the outbound pictures. The terminator crosses Australia. India is visible. The Sun reflection is within the Indian Ocean.
[NASA caption]

The photograph was taken with the 250mm telephoto lens during the homeward journey from about 133,000 nautical miles [246,000 km] out in space [from Earth].

“The fact that you can put your thumb up to the window of the spacecraft and completely put the Earth behind your thumb is a concept that gives you the insignificance of your own existence with respect to the universe.”

—James Lovell (Chaikin, Voices, pg. 45)

From the mission transcript when the photograph was taken:

116:45:00 Public Affairs Officer (Mission Control):

This is Apollo Control at 116 hours, 45 minutes. At the present time Apollo 8 is traveling at a speed of 4,993 feet per second [1,522 m/s]. And our altitude is 133,267 nautical miles [246,811 km] above Earth. We’ve had no further contact with the spacecraft since our last report. We are anticipating another attempt shortly to play up some music to the crew. And at this time, aboard the spacecraft, Bill Anders is standing watch. Both Frank Borman and Jim Lovell are in sleep periods at the present time. We will continue to stand by for any call to Anders and for that next attempt to play up some music.