New! PDF template for observing lunar eclipses

About lunar eclipses

The plane of the Moon's orbit is inclined only five degrees to the ecliptic, and from time to time the Moon will pass through the Earth's shadow and become eclipsed.

There are three kinds of lunar eclipse: penumbral, partial and total. The total lunar eclipse is one of astronomy's most spectacular sights. At totality the Moon is completely covered by the umbra, but the Moon never totally vanishes because sunlight is refracted onto the lunar surface through Earth's atmosphere.

The maximum possible magnitude of a total lunar eclipse is 1.888, which equates to nearly two hours of totality.

No two total lunar eclipses are the same. The hues, colour distribution and intensity of the umbra always vary. Much depends on cloud and high altitude dust in the atmosphere, the magnitude of the eclipse and possibly the timing of the event in relation to solar activity.

A frame from a live eclipse webcast by Peter Grego

Observing the eclipse

The observer wishing to study the eclipse is advised to prepare well in advance and pay little regard to any unfavourable Met Office predictions!

Naked Eye -- Annotated line drawings on pre-prepared blanks may be attempted at 15 minute intervals. Notes should be made on the definition of the umbra's edge, and the colours and intensities within it. Using nothing more than a shiny silvery Christmas tree ball a good estimate of the brightness of the totally eclipsed Moon may be made by comparing the Moon's small reflected image with a star or planet of known magnitude. The Moon's mean eclipse magnitude was in this way estimated at -2.9 in November 1975 when compared with Jupiter at -2.4. At the preceding eclipse of May 1975 the eclipsed Moon was estimated at only magnitude 0.7.

The Danjon Scale of eclipse intensity

A scale for classifying total lunar eclipses was devised by the French astronomer André Danjon. Danjon's scale takes into account the brightness and colours of the umbra and serves as a rough guide to the observer, though it is not perfect.

Danjon scale of total lunar eclipse intensity (L = luminosity)

L = 0. Very dark eclipse. The Moon is exceedingly dim, especially at mid-totality.

L = 1. Dark eclipse, grey or brown in colour. Lunar details are distinguishable only with difficulty.

L = 2. Deep red or rust-coloured eclipse. Very dark central shadow, while outer edge of umbra is relatively bright.

L = 3. Brick-red eclipse. Umbra has a bright or yellow edge.

L = 4. Very bright copper-red or orange eclipse. Umbra possesses a very bright, bluish edge.

Danjon attempted to find a correlation between solar activity and lunar eclipses, and after examining a long series of accounts he concluded there was a definite link between solar minimum and the intensity and redness of the umbra. However, a more definite and predictable relationship exists between terrestrial vulcanism and eclipse intensity. The huge amount of dust released by the eruption of Krakatoa in August 1883 seems to have been responsible for the dark eclipses of October 1884 and September 1888 - using Danjon's criteria these particular eclipses ought to have been bright. The darkness of December 1992's eclipse is thought to have been produced largely by the dust released by the eruption of the Philippines' Mount Pinatubo in June 1991, and this too should have been a bright eclipse according to Danjon's scheme.

Binocular and Telescopic Work -- Binoculars give the observer the best overall view of a lunar eclipse. The colours can be striking, and the sight of the Moon surrounded by a dark starry field gives a distinct three- dimensional impression.

The passage of the edge of the umbra over certain prominent lunar features may be timed to the nearest minute. The recommended features (most of them bright spots) include the craters Aristarchus, Kepler, Copernicus, Tycho, Plato, Manilius and Proclus. A contact timing should be made on the feature's immersion and emergence. The width and definition of the edge of the umbra should be noted, along with any observed irregularities in its outline.

A template for visual observation, drawings and notes is available (click on link above)

Photography -- Pleasing results with simple photographic equipment can be obtained. An undriven wide field multiple exposure showing the passage of the Moon through the sky and the progress of the event is recommended for those with a simple camera with a time exposure facility. With a driven telephoto lens a triple exposure can be taken showing the Moon at immersion, totality and emergence phases. This type of photograph is stunning and shows the true extent of the umbra.

Because the brightness of eclipses varies so considerably, no hard guides can be given suggesting optimum exposures for totality. A very dark eclipse may require more than a minute's exposure, whereas a bright eclipse may register with just a few seconds.

It is wise to bracket your photographs and experiment with exposures as the event unravels. The partial phases will need a little more exposure than if you were taking photographs of the Moon's regular phases to reveal anything of the tones within the umbra.

Video -- Finally, why not capture the atmosphere of the event with a camcorder? A must if you're organising or participating in a star party!

Summary

a)Naked eye:

  Definition of shadow edge

  Colour and density of shadow

b)Telescopic:

  Contact timings

  Definition of shadow edge

  Irregularities in umbral outline, width of 'edge'

  Visibility of limb and surface features in different regions

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