Harvest Moon 2018 gets up tonight!

With unusually warm summer temperatures persisting across most of the world this year, it seems hard to believe that it is the week of the Harvest Moon, the full moon closest to the equinox of 39. ;autumn.

Saturday, September 22, was the first official day of autumn in much of the northern hemisphere, although it remains officially in summer during the day. The moment of the equinox – the official end of the summer and the beginning of autumn – will not arrive until 21:54. EDT. East of the first meridian, where the equinox occurs after midnight, the first day of autumn will be Sunday (015 GMT, 23 September).

The moon will officially fill two nights later, Monday, September 24, when it will reach the opposite sky (180 degrees) in the sun. This time will happen at 22:52 EDT (0252 GMT on 25th September). The moon will slowly decrease later, making this week a moonlit day. [The Moon: 10 Surprising Lunar Facts]

The version of this year's Harvest Moon falls in September, though, on occasion, this branding can be conferred on the full moon of October. From 1970 to 2020, this calendar quirk occurs a dozen times and was actually produced last year. The Harvest Moon can take place as of September 8 (as in 2014) or October 7 (as in 1987).

Moonrises with the least delay

Our solar day lasts 24 hours. In the sky, the moon seems to be moving east against the background stars faster than the sun. So, the "lunar day" – the time interval that is measured for the moon to make a passage through the celestial meridian to the next – is on average 24 hours and 50 minutes. In addition to crossing the meridian, the moonrise is delayed by an average of 50 minutes per day. Variations from this 50-minute average can sometimes be quite remarkable, especially at a time when the moon is approaching its maximum phase.

Many mistakenly assume that the harvest moon stays in the night sky longer than all the other full moons we see during the year, but that is a mistake. the full moon remains in the longest sky around the December solstice. But near the autumnal equinox, in the days surrounding the full moon phase, the natural satellite rises from night to night with the least delay, as observed from our northern latitudes. He gets up as the sun goes down. But at this time of the year, instead of getting closer to its normal average of 50 minutes later, the moon seems to increase at about the same time each night.

Cause and effect

The reason for the recurring seasonal factor of the moonrise at this time of the year is that the moon seems to move along the ecliptic in our sky. And at this precise moment of the year, the ecliptic makes its smallest angle to the eastern horizon, so that the rising moon appears as moving along a ramp for several days, while is centered on the complete phase. [Harvest Moon Photos: Amazing Full Moon Views]

For example, from latitude 40 degrees north, if we look at the three-night period around the complete phase (September 23-25), the moonrise occurs on average 29 minutes later each night. If you head further south, the night-night difference increases. For Brownsville, Texas, located at 25.9 degrees north latitude, the rise of the moon for the same three-day interval arrives 36 minutes later each night. Conversely, as you head north, the difference becomes much smaller. For London (51.5 degrees north latitude), the average night / night difference is only 22 minutes.

On the other hand, for those living in the southern hemisphere, the ecliptic at this time of the year seems to be almost perpendicular to the eastern horizon. Thus, the average difference of 50 minutes per night for the rise of the moon is exceeded. For example, for Auckland, New Zealand (latitude 36.9 degrees south), the night-night difference is 57 minutes.

Interestingly, for those living near the Arctic Circle (latitude 65.82 degrees north), the moon actually seems to rise almost every night at about the same time as the moon of harvest (only 3 minutes later each night). And for those who live even further north, a paradox: the moon seems to rise earlier! At Prudhoe Bay, Alaska (Latitude 70.3 degrees North), the local Moonrise time, September 23, 24, and 25, will be at 8:35 pm, at 8:28 pm. and 20:21, respectively. Thus, for the 2,174 inhabitants of this community located on the northern slope of Alaska, the moon will appear to increase on average 7 minutes earlier each night.

Origin of the moon of the harvest

Thus, by all the natural signs of the sky, the time of the autumn harvest is again with us. The term Harvest Moon dates back to the preindustrial period, when farmers – devoid of the technology available today – were pressed by the season and hosted a moonlit week to reduce the clement hours of the day. Their fields had to be harvested before the farm could be reunited for the upcoming winter season. The crops had to be housed. The firewood had to be cut. The hours of daylight were rapidly decreasing at this time of the year; apparently, there was not enough time for all the tasks to be done in the sun. The Harvest Moon was a welcome lantern in the early evening sky. [Full Moon Names 2018: From Wolf Moons to Cold Moons]

This year, it is only a beautiful moon of late September that will give rise to a series of bright moonlit nights, at the time of the seasons at the turn. The next full moon, October 24, will be the hunter's moon, traditionally affected by frost and framed by the glorious colors of autumn leaves.

The seasons continue. The summer is weakening and is ending with the rising moon of this weekend.

Editor's note: If you are taking an incredible photo of the Harvest Moon full moon or any other target of the night sky and would like to share it for a story or an image gallery, please contact editor Tariq Malik at spacephotos @ space.com.

Joe Rao is an instructor and guest speaker at the Hayden Planetarium in New York. He writes on astronomy for Natural History magazine, the Almanac of Farmers and other publications, and he is a camera meteorologist for Verizon FiOS1 News in the lower part of the Valley of 39; Hudson in New York. follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.