Dimitri Messinis / AP
The full moon rises behind the Temple of Poseidon in Cape Sounion, southeast of Athens, Greece, while tourists watch on May 5. Saturday's event is a "supermoon," the closest and therefore the biggest and brightest full moon of the year.
Tonight's "supermoon" is the biggest and brightest full moon of the year, due to the fact that the moon is near the closest point in its orbital path around Earth. But just how much bigger and brighter does it look? That's a tricky question.
Most reports say the moon looks 14 percent bigger than usual, which is close to the truth but isn't quite right. They also say it's 30 percent brighter than usual, which isn't right, either. James Garvin, chief scientist at NASA's Goddard Space Flight Center, ran the numbers to come up with an explanation that seems to make the most sense.
Read more about Garvin's explanation.
-- Reported by msnbc.com's Alan Boyle
Ralph Lauer / Zuma Press
A supermoon rises over neon hotel signs on Highway 80 in Ft. Worth, Texas.
Mark Blinch / Reuters
The moon rises over the skyline in Toronto, Canada, May 5.
Wilfredo Lee / AP
People fish from a jetty as the moon rises over the Atlantic Ocean, May 5, near Bal Harbour, Fla.
Asmaa Waguih / Reuters
A full moon is seen behind the minaret of Mohamed Ali mosque, in Islamic Cairo, Egypt, May 5.