Don’t peek at the answer until you’ve given this a good try! Then, scroll down a bit and the truth will be revealed to you.

NASA

What in the world is it?

A Romulan War Bird firing a phasar head-on? A glint of sunlight off a Diamond Ring? Hmmm, well the last guess is somewhat right. It is a phenomenon known as the Diamond Ring. The effect is commonly seen on Earth during a total eclipse of the Sun. This brilliant bead of sunlight appears in the last few seconds before the entire disk of the Moon (seen in silhouette) obscures the sun. The bead is formed by final rays of the Sun passing through a deep valley on the Moon’s limb.

But this image does not show the Diamond Ring effect during a total eclipse of the Sun by the Moon. It shows the Diamond Ring effect during a total eclipse of the Sun by the Earth!

That’s right. The crew of Apollo 12 took this image shortly before reentry on November 24, 1969 — when their command module traveled behind the Earth relative to the Sun.