Tonight – September 28, 2015 – if you’re in the Northern Hemisphere, watch for the Harvest Moon to rise shortly after sunset. What you say? Wasn’t the full Harvest Moon last night? And didn’t it undergo an eclipse? Fortunately, the Harvest Moon isn’t just a one-night event. It’s characterized by rising shortly after sunset for several evenings in a row, especially at more northerly latitudes.
Every full moon rises around the time of sunset, and on average each successive moonrise comes about 50 minutes later daily. But, on these early autumn evenings for the Northern Hemisphere – because of the narrow angle of the ecliptic to the horizon – the moon rises sooner than the average. So, instead of rising 50 minutes later in the days after full moon, the waning moon might rise only 35 minutes later or so for a few days in a row (at mid-northern latitudes).
At far northern latitudes – like at Fairbanks, Alaska – the moon rises about 15 to 20 minutes later for several days in a row.
That fact was important to people in earlier times. For farmers bringing in the harvest, before the days of tractor lights, it meant there was no long period of darkness between sunset and moonrise for several days after full moon. And that meant farmers could work on in the fields, bringing in the crops by moonlight. Hence the name Harvest Moon.
Simply stated, the Harvest Moon is the full moon that falls the closest to the Northern Hemisphere’s autumnal equinox. In the Southern Hemisphere, this September full moon counts as the first moon of spring, because the September equinox is their spring equinox.
The Harvest Moon can come anywhere from about two weeks before to two weeks after the autumn equinox. In some years, the Northern Hemisphere Harvest Moon can come as late as early October. The last October Harvest Moon was October 4, 2009, and will next occur on October 5, 2017.
This year’s Harvest Moon qualifies as a supermoon, too, because the moon turned full at nearly the same time that it reached lunar perigee – the moon’s closest point to Earth for the month (and year).
Don’t like the name supermoon? We do. Sure, they don’t look gigantic in our sky. But some observers do say it’s possible to discern the extra large size of a supermoon in the night sky. And a supermoon is certainly brighter than a full moon at an average, or farthest, distance from Earth.
Also, this month’s supermoon will have a stronger-than-usual effect on Earth’s tides. Look for this full supermoon to usher in wide-ranging spring tides along the ocean coastlines for the next several days. That is, the high tides will climb extra high and the low tides will fall exceptionally low. Why are they called spring tides, when the season is autumn for us at northerly latitudes? The name has the same root as the German word Springen meaning “to leap up.”
The moon reached the crest of its full phase on September 28 at 2:51 Universal Time, to undergo a total lunar eclipse for 72 minutes on Sunday night, September 27-28. By the time you read this post, however – assuming you’re reading it on Monday, September 28 – the full moon and total lunar eclipse will already have passed.
One of you asked:
Is the phase of the moon consistent across the United States? Recently, on a trip to the California coast we saw a full moon, but it did not appear to be in the same phase just one day later in the western Pennsylvania sky.
The moon’s phase does appear the same as seen from across the U.S. – even from across the world – more or less. When the moon is full, for example, it’s more or less full for all of us. So looking up at night unites us all, across the planet. We all see the moon as nearly full around now, for example.
The moon’s phase is continuously changing, though, even if that change isn’t perceptible to the eye. From one night to the next, the moon can definitely appear different in phase from the previous night. What’s more, your perception of the moon might be affected by other things – for example, by whether you’re seeing the moon in twilight or late at night, whether it’s peeking from behind trees or shining in solitary splendor, whether it’s a big reddish moon low in the sky or a smaller whiter moon closer to overhead.
There are seasonal variations, too. Around the time of full moon in spring, the moon rises much later one evening than it does the evening before. To me, that gives the impression of a moon that’s waning rapidly. In late summer and fall, the opposite is true. At middle and far northern latitudes in the Northern Hemisphere, full moons in September and October are characterized by a shorter-than-average time between successive moonrises. These moonrises close to the time of sunset – around the time of the full moon in September and October – are the essence of the Harvest Moon phenomenon.
By the way, Lance Bullion captured the image at the top of this post. It’s the beautiful setting Harvest Moon, colored by the extra thickness of Earth’s atmosphere in the direction toward the horizon. You’ll see a similarly colored rising Harvest Moon this evening, if you catch tonight’s moon shortly after moonrise.
Bottom line: Each September and October, around the time of full moon, the moon rises near the time of sunset for several evenings in a row for us in the Northern Hemisphere. It’s almost as if the months of September and October each have several nights of full moon, instead of just one. This is the Harvest Moon phenomenon. In 2015, observe this phenomenon around September 28-30.