Two Moments

Remember to turn out and see the Moon totally eclipsed, in the night between Sunday and Monday, September 27/28.

The next two total lunar eclipses don’t come till 2018, and the next that are favorable for Europe or North America come in 2019.

So what time should you be out watching? Bear with the explanation.

The eclipse begins, theoretically, at 0:10 by Universal Time, which is 1:10 AM on Monday by British clocks, 8:10 PM back in Sunday by Eastern North American and 5:10 PM by Pacific North American ones. But what happens then is imperceptible: it is when the Moon touches the extremely faint outer edge of the Earth’s outer shadow or penumbra. The Full Moon seems still its fully brilliant self.

The first exciting moment comes about an hour later, when you suddenly notice a small dark bite out of the Moon’s advancing left edge. That is the moment when you know the Moon is beginning its march through the inner core of total shadow, the umbra.

A second exciting moment comes an hour later again. By then the Moon has slid almost completely into the umbra. A thin rind of bright Full Moon remains. It dwindles to a point and vanishes. The Moon is totally eclipsed. Stars crackle into sight all over the moonglare-freed night.

So it is best to be out and ready at least some minutes before each of these moments. Let’s allow at least five minutes before each of them. No, seven, because that gives some round numbers.

Moment One: Pacific time 6 PM, Eastern 9 PM, British 2 AM.     Moment Two: Pacific 7 PM, Eastern 10 PM, British 3 AM.

This is by clocks still on “Daylight-Shifting” summer time.

There is a trade-off: farther east, you have to be out later into the night; but farther west, the Moon is lower. In the Mountain time zone, the Moon hasn’t quite risen at Moment One, and on the west coast it hasn’t quite risen even at Moment Two; it rises already fully eclipsed.

The picture shows Moment Two. I’ve painted Earth’s shadow – its total shadow, or umbra – onto the picture, as a large dark circle.   This shadow is nothing but a cone of space deprived of sunlight, so really it can’t be seen except where the Moon intercepts it; what is shown is a cross-section of it at the Moon’s distance. (It’s smaller than the Earth itself would be at that distance, because the umbra tapers outward.) Some of the brilliant Full Moon is still out in sunlight, but you can see how it is sliding into the shadow.

The umbra may appear almost totally black, or it may be interestingly varied toward brownish or reddish, especially toward its outer edge, because of sunlight refracted into the cone of shadow by the variably cloudy or smoggy atmosphere around the Earth’s horizon.

The Moon appears surprisingly smaller than we tend to think; if drawn at the size it would have in a photograph, it would strike you as ridiculously small. So it is shown at twice its size. It is also shown as shifted by parallax, that is, where it is as seen from an American location (latitude 40 north, longitude 75 west), instead of from Earth’s center. For the Moon, this makes quite a difference.

Here is a close-up, with the Moon and the umbra at true scale, and not shifted for parallax. The shadow is therefore centered on the ecliptic. This makes clear that the Moon has only just “descended” through the ecliptic plane and that is why the eclipse happens.

It has descended near to the point where the ecliptic and the celestial equator intersect, but that is a coincidence, caused by the time of year when it happens, near to the September equinox.

Another matter of timing is that the Moon at its Full position and its perigee almost simultaneously. (Perigee is at 1:55 UT – between our Moments One and Two, and only 55 minutes before the exact Full instant.) These things are not unconnected. A perigee coming close to either a New or a Full Moon moment squeezes the Moon’s orbit so that the perigee becomes nearer in. This is the closest Moon of this year. It also squeezes the Earth tidally, so that extreme tides are expected soon after.

Detail from the graph of the Moon’s distance in Astronomical Calendar 2015.

This is what people are meaning when they chatter about the “Supermoon,” though it happens every year. The Moon will be at a real extreme of nearness next year, on 2016 November 14.

When the Moon arrives in total shadow and is suddenly no longer a dazzling Full Moon, you may see more stars than are shown in the first picture. Shown, though, are two things fainter than those stars and at the borderline of visibility: Uranus, which the Moon will pass a day later so closely as to occult (hide it), as seen from the southern tip of Africa; and Vesta, the only asteroid that does reach naked-eye findability. Its magnitude at this time, as marked in the picture, is 6.2 – naked-eye in very good conditions.  Vesta is at opposition on Monday – not surprisingly, since it is near to the Moon, and the Moon is Full.

As the Moon sails on through the umbra, the middle of the eclipse comes at 2:47 Universal Time – 3:47 by British clocks, 10:47 by Eastern American ones, 9:47 by Central time, and so on – when the Moon is nearest to the center of the shadow.

As shown best by the illustrations in Astronomical Calendar 2015, page 67.