Simply stated, the Harvest Moon is the full moon that falls the closest to autumnal equinox. So the full moon that comes tonight – on September 8-9, 2014 – is the Northern Hemisphere’s Harvest Moon. However, in 2014, the September full moon narrowly beats out the October full moon for the honor. Had the September and October full moons occurred 16 hours earlier this year, the October 2014 full moon would have claimed the Harvest Moon title.
Usually, the September full moon is the Northern Hemisphere’s Harvest Moon. But in a year when the full moon falls in early October, the October full Moon can actually be the Harvest Moon. This last happened on October 4, 2009, and will next occur on October 5, 2017.
The moon reaches the crest of its full phase, on September 9, at 1:38 Universal Time. Astronomers define full moon as that instant when the moon lies most directly opposite the sun for the month. Although the full moon happens at the same instant worldwide, it also occurs at all hours around the clock, depending upon one’s time zone. For instance, for the time zones in the continental U.S., the full Harvest Moon will arrive on September 8, at precisely 9:38 p.m. EDT, 8:38 p.m. CDT, 7:38 p.m. MDT or 6:38 p.m. PDT. For much of North America, the moon will turn precisely full on the evening of September 8. (See worldwide map below.) But no matter where you live worldwide, you’ll see a full-looking moon lighting up the night tonight from dusk till dawn.
By the way, this year’s Harvest Moon qualifies as a supermoon because the moon turns full less than one day after reaching lunar perigee – the moon’s closest point to Earth for the month. Look for this full moon to usher in a wide-ranging spring tides along the ocean coastlines for the next several days, whereby the high tides climb extra high and the low tides fall exceptionally low.
So watch for the bright moon tonight, tomorrow night and the night after. The Harvest Moon has been celebrated throughout the centuries and millennia for providing several nights of dusk-till-dawn moonlight. Before the advent of electricity, farmers of old counted on the Harvest Moon to gather crops during the season of waning daylight.
Day and night sides of Earth at instant of full moon
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.
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 2014, observe this phenomenon around September 8-10.