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) to Earth roughly every two weeks. It's considered a supermoon if its nearest point — known as perigee — also happens to be a new moon or a full moon.
According to EarthSky, astrologer Richard Nolle coined the term supermoon more than 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.
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. Some supermoons are closer than others, and one that occurred in November 2016 was reportedly the closest since 1948. This month's supermoon won't get quite that close, but it is scheduled to happen again in 2034.
Meanwhile, the moon is actually being "pushed" away from Earth at a rate of 1.6 inches annually. Several billion years from now, astronomers predict the moon's orbit around Earth will take 47 days, rather than the current 27.3.
4. When should I look for this weekend's supermoon?
The moon will turn full on Dec. 3 at 15:47 UTC. (In the U.S., that's 10:47 a.m. ET, 9:47 CT, 8:47 MT and 7:47 PT.) The full moon will rise over New York City on Dec. 3 at 4:59 p.m. local time, but it won't technically be a supermoon until the perigee a few hours later. If you want to see the supermoon at its peak, aim for the perigee on Dec. 4 at 8:45 UTC (3:45 a.m. ET, 2:45 CT, 1:45 MT and 12:45 PT.)
As Space.com points out, the December full moon — also known as the Cold Moon — will pass in front of the bright star Aldebaran. This "occultation" will be visible from northern Canada, Alaska, eastern Russia, Kazakhstan and a swath of East Asia. If you're in Anchorage, Alaska, you can see Aldebaran disappear behind the moon at 4:38 a.m., then reappear at 5:32 a.m. Most of the U.S. will miss it, but it should be visible for some in the Pacific Northwest if skies are clear. Viewers in Seattle can catch the occultation at 6:09 a.m. and the reappearance at 6:46 a.m.
This is the fourth supermoon of 2017, according to National Geographic, but the first one visible to casual observers. If you miss it, you won't have to wait long for another chance. After this week's supermoon, another one will occur on Jan. 1, 2018.
Editor's note: The story was originally published in March 2011 and has been updated.