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October has here, and this year, it’s going to look a little different. While in previous years, each year’s October hunter’s moon was a round orb of two or three hours in duration, this year the moon will be slightly larger and a little further away from the Earth than it would normally be in its average orbit.
The name for this eclipse is the hunter’s moon, and it’s been around for decades.
The moon gets its name because in October, the full moon comes closest to Earth in its orbit. Near full moon, there’s a slightly shallow dip in the moon’s line of sight to Earth. It’s that extra dip in the line of sight that produces the relatively narrow, narrow crescent to the moon that appears at full moon.
It’s interesting that the calendar actually refers to the hunter’s moon as the apogee for the full moon, because the moon doesn’t actually make the move that it described. From the relative positions of the moon in its orbit around Earth, it’ll be getting closer and closer until it reaches apogee.
However, because the eclipse has been happening since Aug. 21, this hunter’s moon was actually supposed to be called the apogee moon.
From the angle of the moon’s orbit, when it reaches apogee it gets about four times closer to Earth than it does at full moon. However, the Earth rotates, which means that its side will also get closer to the Earth. In other words, the the prime area to look at the full moon is going to be before the new moon, and the prime area to look at the full moon is after the new moon.
You can’t see this shift in the size of the full moon because the Earth is tilted, but you can see that the full moon is noticeably closer to Earth in an old moon perspective. On Sept. 30, 2019, the full moon will go from being a rather huge size on the right side of the screen, to looking a little bit small on the left. That’s why the hunter’s moon is a “hunting moon,” as opposed to the apogee moon.
There are a couple of important technical points to consider, though. First, the distance between the moon and Earth is not fixed, but moves over time. The discrepancy between the apogee and full moon for a given year will vary somewhat, but even within years this distance varies.
Second, like previous full moons that passed over mid-September, the relative phases of the moon (which measure just how much of the moon is actually illuminated) also occasionally overlap. Sometimes, for reason known only to space weather, this overlap happens between full and new moons, with the full and new moons appearing somewhat different sizes to the human eye.
Sometimes in these events, as in this event in 2018, the surrounding solar eclipse can make these two parts of the moon look a little weird to the human eye. That’s because the two thirds of the moon’s surface which doesn’t get illuminated by the sun, is clouded by the solar eclipse and doesn’t reflect the light with the same intensity as the exposed parts of the full moon.
With only four days to go until it’s here, next Monday night looks like a potential prime viewing opportunity for a fair bit of the moon. The moon arrives at its crescent phase on Monday morning, and the eclipse begins later that evening. According to BestEclipseCharts.com, you should be able to see the entire event in both the night sky and the early morning sky.