Another eclipse of the harvest moon


For the second year in succession, this month’s full moon is eclipsed. A year ago Britain enjoyed the whole of a dramatic total eclipse, but this time we must be content with a less spectacular penumbral one which is already underway at moonrise (and sunset) on 16 September.

As the full moon closest to our autumnal equinox, due on the 22nd, this is also the harvest moon. For several nights in succession, the bright Moon climbs from low in the eastern half of the sky at nightfall and, according to tradition, its light allows harvesting to continue into the night.

Our diagram traces the Moon’s position one hour after sunset for Britain over the coming days. Its path against the stars, close to the Sun’s path at our spring or vernal equinox, means that it is climbing northwards at its maximum rate.

As a result, instead of the average interval of 50 minutes between one moonrise and the next, the harvest moon rises for us only some 28 minutes later each day. Contrast this with the situation in March, close to the spring equinox, when the Moon’s path falls more steeply (as shown) and the daily change is closer to 64 minutes.

Were it not for the 5° tilt between the Earth’s orbit around the Sun and the Moon’s around the Earth, we would enjoy eclipses of the Sun and the Moon every month. Instead, eclipses only occur during two eclipse seasons each year when the Sun appears to pass the points, called lunar nodes, where the orbits intersect. At least two eclipses must occur within each of these seasons which each last for more than 30 days and shift some 20 days earlier each year as the Moon’s orbit wobbles.

Our 2016 eclipse seasons fall largely in March and September and, true to form, we have two eclipses this month. The first was an annular solar eclipse across central southern Africa on the 1 September while Friday’s penumbral eclipse is the second.

Don’t expect a spectacular dimming of the Moon. Our current eclipse season is drawing to its close and the Moon misses the central dark shadow of the Earth, the umbra. It still, though, sweeps through the southern part of the Earth’s penumbra where some direct sunlight is blocked. While the southern 9% of the Moon’s diameter avoids the penumbra altogether, its northern edge just misses the umbra and should dim noticeably.

The Moon transits the penumbra between 17:55 and 21:54 BST with mid-eclipse coming at 19:54, some 30 minutes after moonrise and sunset for Britain. Although the Moon will be low in our eastern twilight, there should be no mistaking the eclipse in the upper quarter of its disc.

News Source TheGuardianNews

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