Lunar Eclipses  
 

Did you see the lunar eclipse on September 16th? Most of the people I spoke to seem to agree that it was a very impressive sight and an interesting xperience. It is at these events that the non-astronomer gets to think a little about the motions of the bodies in the solar system, something we all seem to ignore. For instance, we talk about the sun and moon rising and setting and somehow imagine them to be in motion when in fact it is the Earth's spin that produces that effect. One has to mentally visualise the line up of Sun, Earth and Moon to understand what is happening at an eclipse.

I had not read any media reports of what to expect so I went out with no pre-conceived ideas. The moon was a bright creamy colour as it started to move into the umbra. It seemed to me that at no time was the moon completely dark - it was always easily visible to the naked eye. When the moon had moved completely into the earth's shadow, it was, for me,  dull brownish-grey with that hint of creamy yellow along the top right hand edge. People who I spoke to the next day all had other ideas of what the moon looked like. They ranged from red to reddish-orange to orangey-brown to brown. Isn't it strange that we all saw the same happening and yet we all saw it differently? I tried to think back on other eclipses that I have seen. They all seem to vaguely merge into a common memory, except perhaps for two very memorable eclipses. Both involved occultations, or rather the one involved a grazing occultation under the most spectacular circumstances one could wish for.

On 4 May 1985 there was an eclipse of the moon. In the middle of the eclipse the edge of the moon just clipped, or occulted, a very bight star, alpha Libra (or Zuben-el-genube). These grazing occultations are only visible if you are at the right place on the earth's surface and the right place for this event was on a line that stretched across the whole of Southern Africa, from the Okavango swamps across the Northern Transvaal (as it was then) right across to St Lucia area. There were many parties strategically placed along this line ; we chose a site on the old main road just past Naboomspruit.

This event occurred in the early evening on a Saturday night. We travelled up at leisure and had plenty of time to position observers at convenient stations. When you are standing around on the side of the road with your telescope set up, a short wave radio tuned as best you can to a time signal or pre-arranged station and all your paraphernalia scattered around, this tends to arouse the suspicions and interest of passers by. One car load of guys who were particularly interested in what we were doing happened to come from the farm next to which we were stationed. It is, of course, always polite and diplomatic to let these visitors have a peep through your telescope at the moon or convenient object ( except when the graze event is imminent) and this we duly did. They then drove off and arrived back a short time later with an arm full of game biltong for the observers!

The graze itself was spectacular. Watching a 1st magnitude star wink out as it moves behind an invisible mountain on the moon's edge is perhaps one of the most spectacular astronomical events one could hope to see. This event had another, more ambitious scientific objective. There is an ongoing discussion as to whether the sun is shrinking or not. As it burns up hydrogen into helium and other elements and in the process loses some 4 million tons of mass every second of every day, logic suggests that it should be getting smaller. Measuring the sun's diameter to an accuracy that could determine this is almost impossible with the instruments that we have on Earth. There is, however, an indirect method which might yield the desired accuracy and that involves using the moon's outline during a total eclipse of the sun. Because the moon's distance from Earth varies between about 357,000km and 407,000km, the apparent size of the moon is sometimes just big enough to cover the sun during an eclipse. One could use this information to measure the sun's diameter accurately. Of course there are other factors which one needs to know, like the distances to the sun and the moon and, most importantly, the moon's diameter. One way to measure the moon's diameter very accurately is to observe occultations and grazing occultations.

With this objective in mind a team of amateur astronomers from America (only 2 actually) travelled to Central Africa to observe the northern limit of the moon's shadow while we observed the southern limit. (Since the light from a distant star is so very nearly parallel, the size of the moon's shadow on the earth is almost exactly the same size as the moon). The story of the difficulties they experienced are enough to fill another issue of Canopus as they had lots of problems, including not being able to identify where they were and then having a sand storm shortly before the graze. In the end they had mixed fortunes and uncertain results. Whether anyone was ever able to combine our observations I do not know.

Locally there were more than 300 timings, which were reduced and written up by Danie Overbeek in an article published in Canopus.

The second eclipse that I will never forget involved an attempt to observe multiple occultations of stars with the 26 ½ inch refractor at the observatory. I had a list of about 30 stars that were going to be occulted during the eclipse and arranged with Jan Wolterbeek to use the 26 ½ inch as some of the stars were quite faint. He was, at that time, using the telescope for double star work and had his special eyepiece in, which gives about 750 times magnification! Because of this we devised a plan where he would position the telescope on the star, I would do the observation and then inform him of the whereabouts of the next star to be occulted and he would juggle the telescope around till he had it centred, and so on. Anybody who has ever used that telescope knows what a cumbersome beast it is and how awkward it is to find an object. With the 750x eyepiece it is doubly difficult. In the event, we had partially cloudy skies which then closed in completely. We managed 3 or 4 of the brighter stars before having to close the shutters as the rain was coming in. You win some and you lose some.

The colour of the moon during an eclipse seems to vary from one event to the other. It has something to do with the varying distance of the moon from the earth as well as the size and state of the earth's atmosphere. Sunspot activity affects the earth's ionosphere and causes it to swell, thus altering the lensing effect it has on passing sunlight during an eclipse. At the moment we have just passed through a very deep sunspot minimum. Also, the moon was at perigee just hours before the eclipse and would have passed deeper into the cone of shadow from the earth. There are probably other factors that effect the colour of the moon during an eclipse. That's what makes it so interesting.

Sometimes lunar eclipses can be used for very productive astronomical work. They present opportunities for amateurs to make observations which would otherwise not be possible.

So next time there is an eclipse why don't you plan to do a little serious observing.

BF