Tonight marks a special occasion for many with their eyes to the sky: the rising of this year’s harvest moon. It's the last full moon before 2016’s autumnal equinox on September 22. But there's also a bit of controversy with some arguing whether this year's harvest moon should also be considered a supermoon.
While the term “supermoon” might sound dramatic, it’s a relatively common phenomenon. Sometimes it can seem as though there’s a supermoon announced every few months, but it isn’t actually an astronomical or scientific term. It was coined in 1979 by an astrologer named Richard Nolle and it really just means that the moon is within 90 percent of its closest orbit to the Earth, Blaine Friedlander and Angela Fritz report for the Washington Post. Both full moons and new moons can be considered “super,” though full ones tend to appear slightly larger than usual.
The controversy over tonight’s harvest moon stems from former NASA astrophysicist Fred Espenak’s interpretation of the supermoon definition. Both Nolle and Espenak have compiled lists of every supermoon in the 21st century, but the dates don’t quite match up.
According to EarthSky’s Bruce McClure, that’s because Nolle’s original definition is somewhat ambiguous. Nolle’s list is based on averages drawn from yearly projections of the moon’s orbit, while Espenak’s is based on monthly means. In this case, Espenak’s list includes tonight’s full moon among the ranks of the supermoons, while Nolle’s leaves it off.
The term “harvest moon” is somewhat similar to supermoon in that it isn’t a scientific term. For some cultures in the northern hemisphere, the harvest moon was seen as a prelude to fall and marked the end of the growing season. Similarly, the first full moon after the autumn equinox is sometimes called a “hunter’s moon,” Deborah Byrd writes for EarthSky.
While some people may report the harvest moon appearing larger or more colorful than usual, it’s different every year. It’s possible reports of these differences could be because people are primed to expect something special from the event. The moon could also appear more orange because it rises slightly closer to sunset than usual, but that's more a matter of timing than seasonality.
While people in the Americas will unfortunately miss out, there is a genuine astronomical event this weekend, too. People in the rest of the world will get a glimpse of a penumbral eclipse tonight, meaning the moon will appear darker as it slips into the edge of Earth’s shadow, Byrd reports. It won’t go completely dark, but the light of the full moon will be somewhat dimmed.
The moon will hit full swing tonight at 3:05 P.M. EDT. And for anyone in the western hemisphere who might want a look at the eclipse, slooh.com has a live broadcast of the event streamed from telescopes around the world, complete with commentary from astronomical experts. It'll be worth a glance as it's the last harvest moon eclipse that will occur until 2024.