Total lunar eclipse

2015 Sep 28

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

There will be a total eclipse of the Moon, visible from London in the south-western sky. The Moon will lie 28° above the horizon at the midpoint of the eclipse.

The Moon will be totally eclipsed from 03:12 until 04:23, and there will be a partial eclipse visible from 02:08 until 05:27 (all times given in London time).

Eclipses of the Moon are easy to watch with the unaided eye. A modest pair of binoculars will give a superb view of the Moon’s surface, but are not required. Unlike solar eclipses, lunar eclipses are entirely safe to look at without the need to look through any kind of filter.

They 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. The circular shadow cast by the Earth appears as a ‘bite’ taken out of the Moon during its partial phases. When the Moon’s disk lies entirely in shadow, it often takes on a spectacular reddy-brown color, as some of the Sun’s red light is bent around the edge of the Earth’s globe by its atmosphere.

The geometry of a lunar eclipse

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

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 28 will be visible.

Map of where the eclipse of September 2015 will be visible.

The eclipse will begin at 01:12, when the Moon first enters a 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.

At 02:08, the edge of the Moon’s disk will enter the Earth’s umbra. This is the region of space in which an observer on the Moon’s surface would see the Earth completely obscuring the whole of the Sun’s disk, and would find himself suddenly thrust into darkness.

As an increasing fraction of the Moon’s face creeps into the Earth’s umbra, it will appear to have a growing bite taken out of it. We will see our planet’s circular shadow sweep across the face of the Moon.

Eventually the Moon will pass entirely within the Earth’s umbra at 03:12, and the total eclipse will begin.

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 2015.
Local
time
UTC  
01:12 00:12 Moon begins to enter the Earth’s penumbra
02:08 01:08 Moon begins to enters the Earth’s umbra; Partial eclipse begins
03:12 02:12 Moon fully within Earth’s umbra; Total eclipse begins
03:48 02:48 Midpoint of eclipse
04:23 03:23 Moon begins to leave the Earth’s umbra; Total eclipse ends
05:27 04:27 Moon fully outside the Earth’s umbra; Partial eclipse ends
06:23 05:23 Moon leaves the Earth’s penumbra

This eclipse is a member of Saros series 137. The exact position of the Moon at the midpoint of the eclipse is as follows:

Object Right Ascension Declination Constellation Angular Size
The Moon 00h16m +01°27′ Pisces 33’27”

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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