Know Your Lunar Eclipses

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Know Your Lunar Eclipses

Submitted by An Outdoor Idiots Team Member.

Page 3 of 3 Previous [Index]

Lies, Damn Lies and Geometry

The first thing we did to understand the problem was to get an idea of how the Earth, Sun and Moon all fit together. The principles are clear enough: the Earth orbits around the Sun from a very great distance, and, while the Earth is busy doing that, the Moon is orbiting around the Earth, from an altogether less great distance. But we wanted to get a good feel for the scale of things. So we tried to make a scale drawing, based on the numbers from an astronomy book. When we tried this, we were soon horrified to discover that all the pictures we had been shown in school were in fact malicious lies. Accurate scale drawings of the Earth, Sun and Moon, and how far apart they are, do not seem to exist. Using a combination of astronomical data, mathematics and a graphics program, we made this scale drawing of the three objects, though not in their correct positions in relation to each other, as you can probably see:

Sun, Earth and Moon to scale

The relative sizes of the Sun, Earth and Moon
This picture is a good representation of the relative sizes of the Earth, Sun and Moon. The problem comes when we try to show how far the Earth and Moon are from the Sun. Sticking to the scale in the above picture, the Earth would be about 16 metres away. We don't propose drawing that to scale, so you'll have to picture it in your head. (By the way, computer screen sizes and resolutions vary, so it's worth mentioning that the Sun in the above picture is supposed to be about 15cm across). Sticking to the same scale, the distance from the Moon to the Earth is altogether more manageable - it's about 4 centimetres.

So there you have it as far as geometry goes. Now it's all a question of closing your eyes, and imagining the little blue sphere orbiting around the big yellow one from 16 metres away, while all the time imagining the little dot orbiting the little blue sphere from 4 centimetres away. To make things nice and easy, both the orbits are on roughly the same plane. And to get a sense of timing, the orbit of the blue sphere around the yellow one takes a year, and the orbit of the little dot around the blue sphere takes roughly 27 days. If you haven't got a full year to spend thinking about this, you could speed the image up in your head. Just remember that the little dot rotates around the blue sphere roughly 13 times in the time it takes the blue sphere to rotate around the big yellow one. Hey-presto! A moving, scale-replica of the Sun, Earth and Moon in your head. It's a far cry from the images you get in books, and you can take it with you, wherever you go.

If the model was accurate, we would expect a total lunar eclipse roughly every month, i.e. every time the Moon is at its furthest point from the Sun, with the Earth in between the Moon and Sun. But the model isn't quite accurate. The Moon does not orbit the Earth on exactly the same plane as the Earth orbits the Sun. In fact, the Moon's orbit is tilted by about 5 degrees. Here's a diagram to illustrate this, sticking to the same scale as the previous diagram, and this time also including the correct scale for the distances between the objects:

The Moon's orbit around the Earth

The Moon's orbit around Earth
So it's easy to see why the Earth very rarely completely, or even partially, blocks the sunlight from the Moon. The Moon spends most of its time "above" or "below" the Earth, relative to the Earth's orbit around the Sun. And even when it is at the same "height", i.e. at one of the intersection points in the diagram, there's still a slim chance that it will also be directly behind the Earth in a straight line with the Sun. Having said that, total lunar eclipses do often come in groups, and so you can be waiting for years for one total lunar eclipse to come along, and then suddenly they all come at once. The next one, for example, is due at the end of August of this year.

So that gives us a few months to work towards making the next one better than this recent one, i.e. to make the Moon disappear properly...

Doing it Better Next Time

The Earth does successfully block all direct sunlight to the Moon during a total eclipse. The problem is, there's still plenty of indirect sunlight that manages to reach the Moon. Tiny amounts can reach the Moon after bouncing off other planets and debris in space. But the biggest culprit is the atmosphere of the Earth. When light passes the boundary from the vacuum of space to the Earth's atmosphere (or vice versa), refraction takes place, and it changes direction. It's exactly the same principle that makes a straight stick seem to be bent when half of it is placed under water - whenever light moves from one medium to another, e.g. water to air, or a vacuum to air, or air to a vacuum, it can change direction.

With the scale model in mind, it's easy to see that the light from the Sun doesn't need to be bent very much in order to get around the Earth and reach the Moon.

The fact that the Moon often appears a reddish colour during a total eclipse is also due to the fact that the light it receives has passed through the Earth's atmosphere. A lot of the wavelengths of light that enter the Earth's atmosphere are scattered all over the place by particles in the atmosphere, and never make it to the Moon. The wavelengths of light which are least susceptible to being scattered by particles in the Earth's atmosphere tend to be near the red wavelengths, and so it is largely these wavelengths which finally make it to the Moon.

So anyway, it becomes clear that one option for making the next total lunar eclipse truly a total one is to get rid of the Earth's atmosphere. We've been in touch with various industrial pump manufacturers, and most told us that they aren't up to the job. One thought it might be able to do it, but was unsure about the ethics of it all. We have therefore discounted that option. Another option is to somehow cause a lot of clouds to appear on the side of the Earth that faces the Sun. We put this idea to retired weatherman Michael Fish, and he told us the idea was rubbish, because there's still a lot of atmosphere above the clouds. Mind you, we still haven't forgiven him for making that hurricane happen in 1987, so we are not sure we believe him. Anyway, that leaves us with one tried-and-tested option. We simply need one of these:

A Volcano

Moonstopper Mk. III
Volcanoes can be a good solution, and have worked wonders at ensuring proper total lunar eclipses in the past. While the ash from a volcanic eruption falls back to the Earth within days following the eruption, a lot of gas can remain in the stratosphere. When Mount Pinatubo blew its top in 1991, it is thought the gas it released led to a five-mile-thick layer of tiny sulphuric acid droplets floating around in the stratosphere for over a year. It seemed fairly transparent when looking straight upwards, i.e. stars could be seen at night and all that. However, the light that passes through the Earth's atmosphere en route to an eclipsed Moon does so practically horizontally, and has to pass through hundreds of miles of stratosphere. So during the lunar eclipses of 1992, there was enough of a blockage of sunlight to make the lunar eclipses very good indeed, with the Moon practically disappearing. Now that's what we are talking about.

We're off to give a volcano a helping hand with some dynamite.

Well okay, maybe we'll just go off into the mountains and set up a tent for the next lunar eclipse. That's surely the best way to appreciate them.

If you wish to discuss this nonsense, or if you have a non-soppy Moon-related poem, then you can try the forums here.

If you can't be bothered to enter the forums, then shame on you! But you can still leave a comment below. Please try to avoid the profane ramblings of a madman. That's our job.

Your Comments

On 16 March 2007, Mikey P said:
But what about 'Space 1999'? The moon did disappear in that, didn't it? Or was that 'Starsky and Hutch'...

Page 3 of 3 Previous [Top of Page]

Page 1: Introduction; Stupidity Avoidance

Page 2: Random Eclipse-Related Facts

Page 3: Eclipse Geometry; How to Make a Proper Eclipse

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