Penumbral lunar eclipse

2016 Sep 16

Dominic Ford – originally published on In-The-Sky.org

There will be a penumbral eclipse of the Moon, visible from London in the eastern sky. The Moon will lie 6° above the horizon at the moment of greatest eclipse.

The eclipse will last from 17:55 until 21:54, and maximum eclipse will occur at 19:55 (all times given in London time).

Like other lunar eclipses, penumbral occur whenever the Earth passes between the Moon and Sun, such that it obscures the Sun’s light and casts a shadow onto the Moon’s surface. But unlike other kinds of eclipses, they are extremely subtle events to observe.

In a penumbral eclipse the Moon passes through an outer region of the Earth’s shadow called the penumbra. In this outer part of the Earth’s shadow, an observer on the Moon would see the Sun partially obscuring the Sun’s disk, but not completely covering it. As a result the Moon’s brightness will begin to dim, as it is less strongly illuminated by the Sun, but it remains illuminated.

The geometry of a lunar eclipse

The geometry of a lunar eclipse. Image courtesy of F. Sogumo.

Although the Moon’s light dims considerably during a penumbral eclipse, this is only perceptable to those with very astute vision, or in carefully controlled photographs.

This is a rare occasion when the whole of the Moon’s face will pass within the Earth’s penumbra, and so the reduction of the Moon’s brightness will be more perceptible than usual. Such events are called total penumbral lunar eclipses, and are rare because the statistical chance that the Moon will enter the Earth’s umbra at some point is very high once it has passed fully within its penumbra, and this makes an eclipse a partial lunar eclipse.

Eclipses of the Moon are visible anywhere where the Moon is above the horizon at the time. Since the geometry of lunar eclipses requires that the Moon is directly opposite the Sun in the sky, the Moon can be seen above the horizon anywhere where the Sun is beneath the horizon. The map below shows where the eclipse of September 16 will be visible.

Map of where the eclipse of September 2016 will be visible.

The chart below shows the Moon’s path across the sky, with the Earth’s umbra (dark gray) and penumbra (light gray) shown. The table below lists the times when each part of the eclipse will begin and end.

The path of the Moon through the Earth's shadow in September 2016.
Local
time
UTC
17:55 16:55 Moon begins to enter the Earth’s penumbra
19:55 18:55 Greatest eclipse
21:54 20:54 Moon leaves the Earth’s penumbra

This eclipse is a member of Saros series 147. The exact position of the Moon at the moment of greatest eclipse is as follows:

Object Right Ascension Declination Constellation Angular Size
The Moon 23h40m -03°18' Aquarius 32'44"

The coordinates above are given in J2000.0.


The details of this observing event were provided courtesy of In-The-Sky.org

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