Harvest Moon 2019: When and how to see September's "micro" moon

(Image credit: Debbie McCallum / NASA)

the full moon of September 2019 Also bears the title of the harvest moon for those living in the northern hemisphere.

It will also be the smallest moon of 2019. But according to the time zone in which you live, the moon will become officially full either the evening of Friday the 13th or the early morning of Saturday the 14th.

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Full moon for different places and different nights

The moon officially becomes full when it reaches that spot of the sky where it is directly opposite (180 degrees) from the sun in celestial longitude, an almost perfect alignment that astronomers call "syzygy". This moment will take place Saturday at 04:33 GMT. If you live in the Eastern Time Zone, this corresponds to 12:33 on Saturdays.

For the rest of the United States, however, the moon will become full before midnight. For Chicago, for example, it happens at 23:33. Central time on Friday 13. For San Francisco, the moon is full at 21:33. Pacific time that same Friday.

Even if the date of the full moon is announced on September 14 for the Orientals, the moon will appear "fuller" on Friday night – because the moment of the complete phase will be only in a few hours – compared to Saturday night, when the the moon will be almost a full day after its maximum fullness. In fact, it will not be complete at all on Saturday; it will actually be a gibbous moon decreasingAlthough, as we will explain a little later, most people will still look full.

Harvest by the moonlight

(Image credit: Stuart McNair)

Because the full moon of this month is the one that comes closest to the September Equinox, we call it the harvest moon. Usually, we associate the Harvest Moon with September, although this is not always the case. Sometimes, when the full moon occurs during the first week of September, you have to wait until October for the harvest moon. Between 1970 and 2050, 18 years ago, when Harvest Moon arrives in October. the the last time was in 2017 and next time it will be in 2020. On average, the October harvest moons arrive every three years, although the timing can be very variable, and it can sometimes happen that eight years pass (the next example between 2020 and 2028).

Many think that the Harvest Moon stays in the night sky longer than the others full moons we see during the year, but this is not the case. What sets this month's full moon apart from others is that farmers at the peak of the harvest season can work late at night in the light of the moon. Thus, for several days before and after the full moon, the moon is suspended in the sky like a big glowing lantern and prolongs the light well after sunset. It rises about as the sun sets, but more importantly, at this time of year, instead of rising to its normal average of 50 minutes later each day, the moon seems to rise almost at the same time each night.

Below are some examples for 10 cities in North America.

Moonrise time

In fact, during this three-night interval for our relatively small sample, the moonrise occurs on average less than 27 minutes later each night. A quick study of the table shows that the night-night difference is greater for the more southerly localities (Miami, located at about 26 degrees north latitude, sees the rise of the moon arriving on average 33 minutes later). Meanwhile, the difference is smaller in the more northerly regions (in Edmonton, Alberta at 53.6 degrees north latitude, the mean difference is only 17 minutes).

The reason for this seasonal situation is that the moon seems to move along the ecliptic – the apparent path that the sun, the moon and the planets follow through the sky – and at this time of year when rising, the ecliptic makes its smallest angle compared 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 be almost perpendicular (almost at right angles) to the east horizon. As such, the difference for the time of the rising of the moon exceeds the average of 50 minutes per night. In Sydney, for example, the night-night difference is 55 minutes.

Small … and slow

As the Harvest Moon reaches its most extreme climax (its furthest distance from the Earth) about 14.5 hours before the complete phase – it will be 406 377 km (402 km away) from the Earth – which in turn also makes the smallest moon in apparent size this year. : a "micro" moon in popular language.

In fact, it is about 14% smaller than the perigee "supermoon" of February 19th. If it was possible to place the two full moons side by side in the sky, the difference in size would be obvious. But for the most part, our September Harvest Moon will not be very different from all the others that appeared this year.

In fact, as the Moon is at its farthest point on Earth's orbit, it also moves to its slower. As a result, note how long his disc appears perfectly round, before and after the moment of the full moon. This interval is always the longest around the climax, where you can perceive a fairly round disc for three consecutive nights, especially when, as will be the case for those in the eastern United States, the full moon occurs around midnight. Therefore, for the occasional skywatcher, it will appear that the moon will be "full" for three nights in a row! The lack of illumination of the lunar disk – this dark ribbon near the edge – will be very light on the nights of Thursday 12 and Saturday 14 September.

And a last thought: in mid-December, the daily shift at the rising of the moon will be spent an hour or more, but for the moment, it is less than half.

Maybe that's what Shakespeare's Juliet meant by "the fickle moon".

Editor's note: If you are taking an amazing photo of Harvest Moon that you would like to share for a story or an image gallery, contact the editorial director, Tariq Malik, at the address [email protected].

Joe Rao is an instructor and guest speaker in New York Hayden Planetarium. He writes on astronomy for Natural History Magazine, the Farmer's Almanac and other publications, and he's also a meteorologist at the camera for Verizon FiOS1 News in the Lower Hudson Valley in New York. Follow us on twitter @Spacedotcom and on Facebook.