The 1998 Penumbral Lunar Eclipses

STOP PRESS see the images and animation here

Introduction

Until this year all lunar eclipses observed and studied by the Calwell Lunar Observatory have been either total or partial, that is where the moon passes through the most dense central part of the shadow cast by the earth, known as the umbra. It was initially thought that it would be impossible to observe or capture images of the less dense outer penumbral shadow during a lunar eclipse where the moon enters only the feint penumbra, as illustrated in the diagrams below.

As there are three penumbral lunar eclipses in 1998, an experiment has commenced to explore the recording of images of the low density penumbra, with the eventual aim of analysing the density and geometry of the elusive penumbral shadow. Method Data extracted from the 1998 Astronomical Almanac for the lunar eclipses of March 13, August 8 and September 6 was used with a computer program, (modified to produce predictions of primary contacts of the moon with the penumbra), to produce contact times, associated data and to illustrate the circumstances of each eclipse.

The third eclipse - 1998 September 6

This eclipses will be visible at the beginning in North America except the extreme east, Central America, South America except the extreme east, Australia except the extreme west, New Zealand, most of Antarctica, extreme eastern Asia, most of the Pacific Ocean and the extreme western Atlantic Ocean; the end will be visible in western North America, Australia, New Zealand, the eastern half of Asia, most of Antarctica, the Pacific Ocean except the extreme eastern South Pacific Ocean and the eastern half of the Indian Ocean. A world map produced by Fred Espenak, shows the area where this eclipse will be visible (in the unshaded parts).

Eclipse contact times

Times have been computed for moon contacts or departure from the penumbra with an estimate of the variation in each of these timings computed assuming a penumbral oblateness equal to that observed for the umbra derived from the analysis of over 30 recent lunar eclipses where a value of 1/102 has been found for the out of roundness due to the earth's upper atmosphere. These variations are shown in Table 1, as a correction to the standard contact times where allowance is made for the oblateness of the earth's geoid (using the classical value of 1/298.257) which ignores the flattening of the atmosphere.

The first eclipse - 1998 March 13

Images captured

Several images have been received from the first of the 1998 penumbral lunar eclipse and have been analysed using GIF Converter, to reveal the penumbral shadow.

X-Sender: obsfunes@relay.openware.com.ar Mime-Version: 1.0
Date: Mon, 16 Mar 1998 13:25:40 +0300
To: minnah@netspeed.com.au
From: OBSERVATORIO ASTRONOMICO FUNES  Subject: Fotografia ECLIPSE PENUMBRAL
X-Attachments: C:\100.TIF;
Attachment converted: 6100:100.TIF (BINA/mdos) (000035F6)
OBSERVATORIO ASTRONOMICO MUNICIPAL DE FUNES 
obsfunes@openware.com.ar
www.openware.com.ar/ObservatorioFunes
 

In the second image the enhanced penumbral shadow (using show edges) is also clearly visible in the top right hand corner, from which the time of capture is estimated to be 02h 43m (Universal Time).

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*See this link for a group of interesting images of this first penumbral lunar eclipse captured by Bengt Ask.

The beginning of this eclipse was visible in North America except Alaska and north west Canada, Central America, South America, Europe, Africa, western Asia and Greenland, the North polar region, parts of Antarctica, the eastern South Pacific Ocean , the south eastern North Pacific Ocean, the Atlantic Ocean and the western Indian Ocean; the end of this eclipse will be visible in extreme eastern Asia, North America, Central America, South America, Greenland, the North polar region, parts of Antarctica, extreme western Africa, the Iberian peninsular, western France, the British isles, the eastern half of the Pacific Ocean, the North American Ocean and the western South Atlantic Ocean, but NOT in Australia. A world map produced by Fred Espenak, shows the area where this eclipse was visible (in the unshaded parts).

The second eclipse

The beginning of the penumbral phase will be visible in eastern North America, Central America, South America, southern Greenland, Africa, Europe, extreme western Asia, most of Antarctica, the eastern South Pacific Ocean, the south eastern North Pacific Ocean, the Atlantic Ocean and the western Indian Ocean; the end of the eclipse will be visible in North America except northern and western Canada and Alaska, Central America, South America, southern Greenland, Africa except the extreme east, most of Europe, most of Antarctica, the eastern South Pacific Ocean, the south eastern North Pacific Ocean and the Atlantic Ocean, but NOT in Australia. A world map produced by Fred Espenak, shows the area where this eclipse will be visible (in the unshaded parts).

Observations

The computed variations for the contact times when the moon first contacts and leaves the penumbra are all quite small, but these estimates need to be confirmed by direct observation, if the penumbra can be discerned when crossing the moon. Timing of penumbral contacts would appear to be achievable, as many such observations were made in the 19 th century by Julius F. Schmidt, the then Director of the Athens Observatory during his epic telescopic observations of 28 lunar eclipses from 1842 January 26 to 1879 December 28 - see Sky & Telescope, March 1977. These historical timings are currently under investigation and detailed analysis of the penumbral primary contact times given for 8 of Schmidt's lunar eclipses have been completed by this Observatory.

Albedo

It may be possible that detection of the change in the albedo of the moon when eclipsed by the penumbra will be a good alternative method for timing of these predicted events. However, a technique for low light level change in the eclipsed moon has been developed by the Argentina images presented above, but such a technique can not be evaluated by this Observatory until the third penumbral eclipse as only the last event will be visible in Australia.

An Invitation

It is hoped that others may like to join us in this experiment - particularly for the next two eclipses if they are visible from your location - by conducting contact timings and/or light level measurement by photographic or video imaging of the eclipsed moon as carried out successfully for the March eclipse by Argentina.