The supermoon is coming! But what exactly does that mean? Read below as I dissect the semi-rare celestial event known as the supermoon — and why this weekend's nighttime event is one worth catching.
1. What is a supermoon?
Since its 27.3-day orbit is elliptical, the moon alternates between its farthest point (254,000 miles) and closest point (220,000 miles) roughly every two weeks. What makes the moon a supermoon is when at its closest approach, it also happens to be a new moon or full moon.
According to EarthSky, astrologer Richard Nolle coined the term supermoon over 30 years ago. The term only began to be used recently, however. Nolle defined a supermoon as: "a new or full moon which occurs with the moon at or near (within 90 percent of) its closest approach to Earth in a given orbit."
By that generous definition, there are about 4 to 6 supermoons each year.
This weekend is especially interesting because a hunter's moon and a supermoon are set to combine on the night of Oct. 15 to the morning of Oct. 16 in the Northern Hemisphere. What's so special about a hunter's moon? As Science Alert notes, "The only thing you really need to remember when it comes to a hunter’s moon is how awesome the color will be."
It’s the location of the hunter’s moon so close to the horizon that will make it look so big and orange-red.
2. What can a supermoon do to our planet?
According to scientists, not much. Anytime there is a full moon — when the sun, Earth and moon are near a straight line in space — the gravitational effect on ocean tides is greater. When a supermoon is in play, these forces are exaggerated. That being said, the force is considered too weak to be of great consequence.
Said John Vidale, a seismologist at the University of Washington in Seattle, "Practically speaking, you'll never see any effect of lunar perigee," he told Life's Little Mysteries. "It's somewhere between 'It has no effect' and 'It's so small you don't see any effect.' "
The only concern should be for those near the coast looking to take advantage of lower-than-normal seas. Should a storm also roll along a coastline during a supermoon, there's the possibility of increased flooding because of the tides. It's worth taking precautions during an event like this should such a combo of factors hit.
3. Will the moon ever be closer to our planet?
Yes and no. The moon is actually being "pushed" away from the Earth at a rate of 1.6 inches annually. Several billion years from now, astronomers predict that the moon's orbit around the Earth will take 47 days, rather than the current 27.3.
4. When should I look for this weekend's supermoon?
Hopefully you'll have clear skies. EarthSky says for the U.S., the moon turns full on October 16 at 12:23 a.m. EDT – or October 15 at 11:23 p.m. CDT, 10:23 p.m. MDT and 9:23 p.m. PDT.
Because the moon stays more or less opposite the sun throughout the night, watch for a full-looking moon in the east at dusk, highest in the sky around midnight and low in the west at dawn. On the nights immediately before and after full moon, the moon still looks plenty full to the eye.
The next supermoon will occur on Nov. 14, 2016.
Editor's note: The story was originally published in March 2011 and has been updated.