This year’s Harvest Moon happens tonight, Sept. 8 and it will also be the moment of our natural satellite’s perigee, or the point at which the Moon is closest to Earth. This means it will light up the night sky even more than usual, and will appear bigger. According to the National Aeronautics and Space Administration, this super Moon will be the last of three successive super Moons in Earth’s Northern Hemisphere.
The fullest phase of the Moon will occur tonight when it moves into the spot in the sky directly opposite the Sun. That will be at 9:38 p.m. Eastern Daylight Time (0138 GMT). This full Moon will be the one closest to the September equinox (which is Sept. 23 this year) and that is why it is so designated as a Harvest Moon. In 2010, the Harvest Moon actually occurred on the same night as the equinox itself. That was the first time such a coincidence had occurred since 1991. Because autumn does not begin until Sept. 23, tonight’s full Moon will be the last of the summer season.
This year’s Harvest Moon is relatively early but, in some years, it can happen as late as Oct. 7. Although Harvest Moons in October generally happen about once every four years, this average can be misleading. For example, the last time October saw a Harvest Moon was in 2009 yet the next one will not occur until 2017. On the other hand, after 2017 it will be only three years more years, in 2020, when the next October Harvest Moon will be seen. Tonight’s full Moon / super Moon will be the third in a series of summer super Moons. July and August’s full Moons also happened during the Moon’s perigee. The super Moon in August was the closest of all three.
It is often thought that Harvest Moons stay in the night sky the longest of other full Moons seen throughout the year, but that story is legend. What makes the Harvest Moon (and the full Moon which follows it, the Hunter’s Moon) unique is that the difference between the times of each evening’s moonrise is much shorter than otherwise. From one night to the next, the moon rises about 30 minutes later (at least when seen from about 40 degrees latitude, north or south). So, for the days surrounding the actual date of the full Moon, the period of darkness between sunset and moonrise is shorter.
During this time of year, the daily lag in the time of moonrise is smaller than usual. As the Moon travels along the path of the Sun in the sky known as the elliptic, it makes the smallest apparent angle with the eastern horizon, at least when seen from northern latitudes. Usually, as the Moon moves through its monthly cycle, it rises over the eastern horizon an average of 46 minutes later with each successive night. However, during the three nights of Sept. 7 to Sept. 9, the moonrise happens approximately 38 minutes later each evening (in North America). The actual difference in moonrise times increases as one moves south.
Approximately half of the continental United States should have clear skies for viewing tonight’s Harvest Moon / super Moon. Clouds are forecast, however, for much of the central areas, the desert southwest and the eastern seaboard.
By Gregory Baskin