Fifty-one years ago, the world got a Christmas Eve message from on high.

On Dec. 24, 1968, the astronauts of NASA's Apollo 8 mission beamed home gorgeous images of their home planet as seen from lunar orbit, read some verses from the book of Genesis and wished the people of Earth a merry Christmas and a happy new year.

"We were told that on Christmas Eve we would have the largest audience that had ever listened to a human voice," Apollo 8's Frank Borman said during 40th-anniversary celebrations in 2008, according to a NASA feature about the mission. "And the only instructions that we got from NASA was to do something appropriate."

Related: Apollo 8: NASA's First Crewed Trip Around the Moon in Pictures

That audience was indeed huge. About one-quarter of the world's population saw or heard the broadcast, current NASA chief Jim Bridenstine said in a video published last year to commemorate the 50th anniversary of Apollo 8 .

Apollo 8 was a historic mission. Borman and fellow crewmates Jim Lovell and Bill Anders became the first humans ever to orbit a world beyond Earth. And Anders took one of the most famous photos of all time on that Christmas Eve — the iconic "Earthrise" image, which is widely credited with helping to spur the modern environmental movement.

Apollo 8 was also the first crewed flight of the huge Saturn V moon rocket , which launched the Apollo 11 crew on their epic mission to the lunar surface in July 1969. (The first crewed mission of the Apollo program, Apollo 7, launched to Earth orbit atop a Saturn IB in October 1968.)

Apollo 8 launched on Dec. 21, 1968 and splashed down here on Earth six days later. The crucial engine burn that rocketed the mission homeward from lunar orbit occurred a few hours after the famous Christmas Eve broadcast.

When contact with mission control was re-established on Christmas morning, Lovell broke the news of a successful burn by saying , "Please be informed: there is a Santa Claus."

"That's affirmative," Ken Mattingly, the capsule communicator at mission control who was on duty at the time, responded. "You're the best ones to know."

Mike Wall's book about the search for alien life, " Out There " (Grand Central Publishing, 2018; illustrated by Karl Tate ), is out now. Follow him on Twitter @michaeldwall . Follow us on Twitter @Spacedotcom or Facebook .

(Image credit: All About Space)