The "supermoon" set to rise tonight will be the closest full moon since 1948 and until 2034.
Supermoons occur because the Earth is not always perfectly centered within the moon's orbit. The moon orbits the Earth in such a way that sometimes the moon is very close to the Earth, and sometimes it is farther away. The point at which the moon is farthest from the Earth is the apogee, and the moment at which it is closest is the perigee.
The term supermoon actually originated in astrological circles and is not a scientific term.
But it has come to mean a moon that is full when it is closest to Earth — at perigee. For this reason, supermoons are called "perigee full moons" by scientists or "perigee-syzygy." Syzygy is the term given to the point when the Earth is lined up between the moon and the sun — the conditions that produce the full moon.
There are three supermoons in total for 2016. One was on Oct. 24, and another will rise on Dec. 14. But the moon tonight will be exceptionally close — at perigee the moon will be the closest it has been to Earth in 68 years. The moon will reach perigee within two hours of becoming full, according to NASA. It is particularly rare for these two events to occur so closely to each other.
Since it is closer to Earth, the moon will appear larger in the sky and will reflect more light toward the planet. The moon is expected to appear up to 14 percent larger and 30 percent brighter than the moon at apogee, though as some have suggested, it will not likely seem very different to viewers.
So when should people venture outside to see it?
“I’ve been telling people to go out at night on either Sunday or Monday night to see the supermoon,” said Noah Petro, deputy project scientist for NASA’s Lunar Reconnaissance Orbiter mission, in a NASA release. “The difference in distance from one night to the next will be very subtle, so if it’s cloudy on Sunday, go out on Monday. Any time after sunset should be fine. Since the moon is full, it’ll rise at nearly the same time as sunset, so I’d suggest that you head outside after sunset, or once it’s dark and the moon is a bit higher in the sky."