The full moon of September 2019 also carries the title of the Harvest Moon for those living in the Northern Hemisphere.
It will also be the smallest full moon of 2019. But depending on what time zone you live in, the moon will officially turn full on either the evening of Friday the 13th or the early morning hours of Saturday the 14th.
Full moon for different places on different nights
The moon officially turns full when it reaches that spot in the sky where the moon is directly opposite (180 degrees away from) the sun in celestial longitude, a nearly perfect alignment that astronomers call "syzygy." This moment will occur on Saturday at 4:33 a.m. GMT. If you live in the Eastern time zone, that corresponds to 12:33 a.m. on Saturday.
For the rest of the U.S., however, the moon will become full prior to midnight. For Chicago, for instance, it happens at 11:33 p.m. Central time on Friday the 13th. For San Francisco, the moon turns full at 9:33 p.m. Pacific time on that same Friday.
Even though the date of the full moon is given as Sept. 14 for Easterners, the moon will appear "fuller" on Friday evening — since the moment of full phase will be only a few hours away — as opposed to Saturday evening, when the moon will be nearly a full day past its peak fullness. In fact, it will not be full at all on Saturday; it will actually be a waning gibbous moon, although as we will explain a bit later, it likely will still look full to most people.
Harvesting by the light of the moon
Because this month's full moon is the one that arrives closest to the September equinox, we brand it the Harvest Moon. Usually, we associate the Harvest Moon with September, although that is not always the case. Sometimes, when the full moon occurs during the first week of September, we must wait until October for the Harvest Moon. Between 1970 and 2050, there are 18 years when the Harvest Moon comes in October. The last time was in 2017 and next time will be in 2020. On average, October Harvest Moons come at three-year intervals, although the time frame can be quite variable, and there can be situations where as much as eight years can elapse (the next such example will come between 2020 and 2028).
Many think that the Harvest Moon remains in the night sky longer than any of the other full moons we see during the year, but that is not so. What sets this month's full moon apart from the others is that farmers at the climax of the current harvest season can work late into the night by the moon's light. So, for several days before and after the full moon, the moon hangs in the sky like a great, glowing lantern and prolongs the light far after sunset. It rises about the time the sun sets, but more importantly, at this time of year, instead of rising at its normal average of 50 minutes later each day, the moon seems to rise at nearly the same time each night.
Below we've provided some examples for 10 North American cities.
Time at moonrise
|Location||September 12||September 13||September 14|
|Albuquerque, NM||6:58 p.m. MDT||7:28 p.m. MDT||7:57 p.m. MDT|
|Chicago, IL||6:51 p.m. CDT||7:18 p.m. CDT||7:43 p.m. CDT|
|Denver, CO||6:58 p.m. MDT||7:26 p.m. MDT||7:52 p.m. MDT|
|Edmonton, AB||8:01 p.m. MDT||8:18 p.m. MDT||8:35 p.m. MDT|
|Houston, TX||7:05 p.m. CDT||7:38 p.m. CDT||8:09 p.m. CDT|
|Los Angeles, CA||6:45 p.m. PDT||7:15 p.m. PDT||7:44 p.m. PDT|
|Miami, FL||6:59 p.m. EDT||7:33 p.m. EDT||8:05 p.m. EDT|
|Montreal, QC||7:01 p.m. EDT||7:25 p.m. EDT||7:48 p.m. EDT|
|New York, NY||6:53 p.m. EDT||7:21 p.m. EDT||7:46 p.m. EDT|
|Seattle, WA||7:23 p.m. PDT||7:46 p.m. PDT||8:07 p.m. PDT|
In actuality, over this three-night interval for our relatively small sampling, the rising of the moon comes, on average, less than 27 minutes later each night. A quick study of the table shows that the night-to-night difference is greatest for the more southerly locations (Miami, located near 26 degrees north latitude, sees moonrise come an average of 33 minutes later). Meanwhile, the difference is less at more northerly locations (at Edmonton, Alberta, located at 53.6 degrees north latitude, the average difference is just 17 minutes).
The reason for this seasonal circumstance is that the moon appears to move along the ecliptic — the apparent path that the sun, moon and planets follow across the sky — and at this time of year when rising, the ecliptic makes its smallest angle with respect to the horizon for those living in the Northern Hemisphere.
In contrast, for those living in the Southern Hemisphere, the ecliptic at this time of year appears to stand almost perpendicular (at nearly a right angle) to the eastern horizon. As such, the difference for the time of moonrise exceeds the average of 50 minutes per night. In Sydney, for instance, the night-to-night difference amounts to 55 minutes.
Small ... and slow
Because the Harvest Moon arrives at its most extreme apogee (its farthest distance from Earth) about 14.5 hours before full phase — when it will be 252,511 miles (406,377 kilometers) from Earth — this also makes it the smallest full moon in apparent size this year: a "micro" moon in popular parlance.
In fact, it is roughly 14 percent smaller compared with the perigee "supermoon" of Feb. 19. If it were somehow possible to place both full moons side by side in the sky, the size difference would be readily apparent. But for most, our September Harvest Moon will not look much different from any of the others that have appeared so far this year.
In fact, because the moon is at its farthest point from Earth in its orbit, it is also moving at its slowest. As a result, take note of how long its disk appears perfectly round, before and after the moment of full moon. This interval is always longest around apogee, where you might perceive a fairly round disk for three consecutive nights, especially when — as will be the case for those in the Eastern U.S. — full moon occurs around midnight. Therefore, to the casual skywatcher, it will seem that the moon will be "full" for three nights in a row! The defect of illumination on the moon's disk — that dark sliver near the edge — will be very slight on the nights of Thursday, Sept. 12 and Saturday, Sept. 14.
And a final thought: By mid-December, the daily lag in moonrise will have increased to an hour or more, but for now it is less than half of that.
Maybe that's what Shakespeare's Juliet meant by "the inconstant moon."
Editor's Note: If you take an amazing photo of the Harvest Moon that you'd like to share for a possible story or image gallery, please contact managing editor Tariq Malik at firstname.lastname@example.org.
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Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications, and he is also an on-camera meteorologist for Verizon FiOS1 News in New York's lower Hudson Valley. Follow us on Twitter @Spacedotcom and on Facebook.