Early this Tuesday morning, on 21 December, the day of the winter solstice, there will be a total eclipse of the Moon. (Image of the 2008 Lunar Eclipse by Alun Halsey)

The Moon first enters the outer, penumbral part of the Earth's shadow at 05:29 UT, and the partial eclipse begins at 06:32 UT.  The eclipse first becomes total at 07:40 UT, reaches maximum at 08:17 UT, and ends at 08:53 UT.

The entire eclipse will be visible after local midnight from Canada, the USA, central America and the north-western tip of South America. The eclipse occurs at the Moon's descending node in eastern Taurus, four days before perigee.  The Moon will be full at 08:13 UT.  Further information on this eclipse may be found at:

http://eclipse.gsfc.nasa.gov/OH/OH2010.html#LE2010Dec21T

From southern parts of the British Isles, the initial umbral phases will be visible, but the Moon will be dropping down into the western sky as dawn approaches.  From such locations, when totality begins at 07:40 UT, the Moon will be very low in the west-north-western sky, close to the horizon and in a rapidly brightening sky.  From locations in Scotland and Northern Ireland, totality will be visible in its entirety, but the Moon will be low down after the time of greatest eclipse (08:17 UT).

From London, sunrise is at 08:04 UT with moonset just seven minutes later.

The table below lists the times of moonset for various locations in the British Isles:

Location                  Moonset (UT)

Brighton                         08:08

London                          08:11

Bristol                            08:21

Birmingham                    08:25

Swansea                       08:28

York                               08:30

Liverpool                       08:35

Newcastle-upon-Tyne   08:39

Dublin                            08:48

Belfast                           08:55

Glasgow                         08:57

Aberdeen                       08:58

The umbral phase lasts from 06:32 UT until 10:01 UT.  During totality, the Moon tracks through the northern part of the Earth's umbral shadow, so for those observers watching it high up in a clear sky, the southern half of the totally eclipsed Moon will most likely appear considerably darker than the northern part.

One never quite knows how dark or how bright a lunar eclipse will be.

Everything depends on the conditions in the Earth's upper atmosphere through which all light falling onto the shadowed Moon has to pass. There have been eclipses when the Moon has been difficult to find even with a telescope, while at other eclipses it has remained bright red or vividly coloured.

For observers in the British Isles, the very low elevation of the Moon during the total phase means that it is not possible to predict the exact brightness distribution in the umbra, so observers are encouraged to estimate the brightness using the Danjon scale at different times during totality. Note that it may also be necessary to assign different Danjon values to different portions of the Moon (i.e., north vs. south).

For an explanation of the Danjon scale of lunar eclipse brightness visit:

http://eclipse.gsfc.nasa.gov/OH/OHres/Danjon.html

The 2010 December 21 total lunar eclipse belongs to Saros 125, a series of 72 eclipses in the following sequence: 17 penumbral, 13 partial, 26 total, 9 partial, and 7 penumbral lunar eclipses.

There will be two total lunar eclipses in 2011, on 15 June and 10 December, but neither will be visible in its entirety from the British Isles.

The BAA Lunar Section will be pleased to receive observations of the lunar eclipse.  See the Section's webpage at http://www.baalunarsection.org.uk/ for more details.

John Mason

BAA Press and Publicity Officer