Simply stated, the Harvest Moon is the full moon that falls the closest to autumnal equinox, which in 2013 comes on September 22. So the full moon that comes tonight and early tomorrow – September 18-19, 2013 – is the Harvest Moon. The moon reaches the crest of its full phase at 11:13 Universal Time on September 19 (6:13 a.m. CDT, in other words before sunrise tomorrow for most of us in the U.S.).
In eastern Asia, on the other hand, the moon will turn precisely full after sunset on September 19. In that part of the world, the full Harvest Moon will be said to take place on the night of September 19-20, coinciding with the yearly Mid-Autumn Festival, sometimes called the Moon Festival.
So watch for the bright full moon both tonight and tomorrow night. Even on September 20, the moon will still be displaying some Harvest Moon characteristics, such as rising close to the time of sunset.
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 19, at precisely 7:13 a.m. EDT, 6:13 a.m. CDT, 5:13 a.m. MDT or 4:13 a.m. PDT. For all but the eastern part of North America, the moon will turn precisely full before sunrise on September 19, 2013. Although the moon will appear full all night long tonight – from dusk till dawn – astronomers define full moon as that instant when the moon lies most directly opposite the sun for the month. From our vantage point in North America, we can say the Harvest Moon shines on the night of September 18-19.
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 around 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 2013, observe this phenomenon on the night around September 18-20. The September 18 moon is still a waxing gibbous moon when ascends in the east tonight just before sunset. But it will reach the crest of its full phase before dawn tomorrow (September 19) for most of us in the U.S.