Mechanics of Solar Eclipses

All solar eclipses involve the Moon passing between the Earth and the Sun, and casting a shadow on the Earth. However, the type of eclipse which can be seen from a given location depends on whether the Moon passes directly, or only partly, between the Earth and Sun; but also on where on Earth you stand to observe it, and on a number of other factors.

This page attempts to explain how solar eclipses work, and the different types of solar eclipse.

The diagrams here, by the way, are drawn to a wildly exaggerated scale; they can not be drawn to a real scale, because the Solar System is just too big! For example, the Sun is about 92 million miles away; but the width of the shadow of a total eclipse on the Earth might be just a few miles.

A Total Eclipse

A total solar eclipse is when the Sun is completely covered by the Moon. This diagram illustrates in more detail what happens during a total eclipse:

Here, the moon passes between the Earth and Sun at a point in its elliptical orbit when it is relatively close to the Earth. As it does so, it casts a shadow. The umbral part of the shadow (the umbra) is the area where the Sun is totally obscured by the Moon. During a total eclipse, the Moon is close enough to the Earth that part of the umbra falls upon the Earth; people standing on those parts of the Earth, within the Umbra, see the Sun's face completely hidden by the Moon -- a total eclipse of the Sun.

The beautiful part of a total eclipse, though, it what is not hidden: the Sun's faint corona, and solar prominences.

Partial Eclipses

A partial solar eclipse is when the Moon covers only part of the Sun, taking a "bite" out of it.

Looking again at the diagram above, you will see that there is an area, outside the umbra, where the Sun is only partly covered by the Moon; this is known as the penumbra, and it covers a much larger area of the Earth than the umbra. Looking at the tip of the right-hand arrow, for example, you will notice that it can "see" the bottom part of the Sun, but not the top part; the Moon is in the way. The area of the Earth which falls within the penumbra sees a partial eclipse of the Sun.

Within the penumbra, the Sun is partly covered, and its light dimmed, to varying degrees; people within the penumbra and close to the umbra would see the Sun almost (but not quite) covered, whereas people on the edge of the penumbra would see the Moon take just a little "bite" out of the Sun, at most. During even a large partial eclipse, the remaining crescent of the Sun is so intense (actually, just as intense as on a normal, clear day) that it is not safe to look at the Sun directly, and in any case it would be impossible to see anything other than the crescent Sun.

As you can see, every total eclipse is accompanied by a partial eclipse falling on a larger area of the Earth, since the umbra is always surrounded by the penumbra. However, it is quite possible for the Moon's penumbral shadow to fall upon the Earth when the umbra misses the Earth completely, and falls away into space. When that happens, parts of the Earth see a partial eclipse, but there is no total eclipse. This happens quite often.

It is important to bear in mind that when you are within a partial eclipse, the photosphere -- the bright part of the Sun -- is still visible. You should never look at this directly, as it is always capable of causing permanent eye damage, even when almost completely covered.

The Annular Eclipse

An annular eclipse occurs when the Moon covers the centre of the Sun, but not its edges, leaving a ring (or annulus) of the Sun visible around its edges. This image illustrates how an annular eclipse can occur:

Here, the Moon in its elliptical orbit is farther from the Earth, and the umbra is too short to reach the Earth. However, the Moon is directly in front of the Sun, so the parts of the Earth underneath it see a partial eclipse where the centre of the Sun, rather than a "bite" at the side, is covered. This leaves a ring -- an annulus -- of the Sun visible round the edges of the Moon. (An annulus is the shape of a circle with its centre cut out.) People off to one side of the eclipse track fall under the penumbra (not shown in this diagram, for simplicity), and see a normal partial eclipse.

As with a partial eclipse, an annular eclipse leaves a section of the Sun's photosphere visible at all times, so again it cannot safely be viewed with the naked eye.

The Hybrid Eclipse

A hybrid, or annular/total, eclipse is an eclipse which is seen as annular by some parts of the Earth, and total by others (and also as a partial eclipse over a much larger area). This image illustrates how a hybrid eclipse can occur:

Here, the Moon is just far enough from the Earth that the umbra can't reach the "sides" of the Earth, so as the eclipse begins, the western portions of the Earth see an annular eclipse as the day begins. In the diagram, observers in the "outer" parts of the eclipse track, coloured green, see an annular eclipse.

As the eclipse path moves on, the umbra has less far to travel to reach the Earth, and is just long enough to reach the "centre" (ie. the part most directly facing the Moon); so observers in the area coloured blue above see a total eclipse.

People standing near, but not in, the annular/total eclipse track, would see a normal partial eclipse.

With the Moon that far from the Earth, the visible total eclipse will be a pretty small eclipse -- i.e. with a narrow track, and short duration. The next hybrid eclipse occurs on April 8 2005, and goes across the south Pacific from near New Zealand to Central America. The total part of the eclipse is visible for 42 seconds at its maximum point, and its track is no more than 27km wide.

It Goes The Wrong Way!

So much for how eclipses happen -- but one question that often comes up is, why does the eclipse go from West to East, when the Sun and Moon go the other way?

Well, the movement of the Moon -- from East to West -- is, in fact, an illusion caused by the Earth's rotation. As a matter of fact, the Moon orbits in the same direction that the Earth rotates; anticlockwise, as seen from above the North pole. But whereas the Earth takes just 24 hours to do one rotation, the Moon takes a month to go round the Earth (actually, the Moon takes 27.32 days to orbit the Earth).

This diagram illustrates the situation -- but remember that it's not even remotely to scale!

In other words, if the Earth was sitting still, the Moon would cross the sky from West to East. It would take 13 days to cross from horizon to horizon, and another 13 days to come around into view again. But the Earth doesn't sit still -- it rotates, every 24 hours, which is significantly faster than this. It's like if you're driving a car and overtake a jogger, they seem to be going backwards relative to you; the Earth rotates faster than the Moon's orbit, so the Moon seems to be going backwards, when it's actually going the same way.

So what happens to "fix" things during an eclipse? Well, the Moon orbits the Earth once a month; but the distance that it travels in that month is a whopping 2,415,256km! This means that it's moving really fast. By contrast, the Earth is a tiny 12,000km across; so for the Moon to cross in front of the Earth -- for its shadow to cross the Earth -- doesn't take long at all; the Moon moves 12,000km in just 3 hours. (The exact time for the eclipse to cross the Earth depends on whether the Moon is crossing over the centre of the Earth or off-centre, and on what part of its elliptical orbit the Moon is in.) So the shadow zips across much faster than the Earth's rotation, which makes its real direction apparent.

To put it another way, the Moon only has to cross a tiny part of the sky -- a small fraction of its total orbit -- for its shadow to cross the Earth completely. This means that for an eclipse, the Moon's own "real" movement is the main cause of its movement; so the shadow goes West-to-East.

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