Solar Eclipses
demonstrate an understanding that the Moon and Sun appear to be the same size
when viewed from Earth
describe the appearance of partial and total solar eclipses
describe, using diagrams, the mechanisms causing solar eclipses
demonstrate an understanding that the duration of total solar and lunar
eclipses are different and that they do not occur every new and full Moon
Occasionally, on our journey around the Earth, the Moon blocks our view of the Sun. It is a remarkable coincidence that both objects are of such a size and at such distances from us that they appear almost exactly the same size to us in the sky. Because the ratio of the distances to the Earth is the same as the ratio of their diameters they both take up the same angle, about half a degree, in the sky.
Why eclipses are rare
The diagram below (not to scale) shows that the plane of orbit of the Moon is about 5 degrees to the ecliptical plane which is why solar eclipses do not occur once a month but are very rare. Nevertheless the Moon in the sky stays close to the ecliptic and passes through the constellations of the zodiac.
Because the Moon does not rotate around the Earth in a plane exactly parallel to the ecliptical plane this does not occur once every 29 days or so. Solar eclipses are actually quite rare. If you get a chance to see a total solar eclipse do your best to do so. They are spectacular and well worth any effort.
have a look at http://www.eclipse.org.uk/ for information about eclipses past, present and future.
| Here is an animation of what one sees during a total solar eclipse.
REMEMBER - never observe the Sun directly with any kind of optical instrument. Special glasses must be worn when observing an eclipse. 1. The Moon partially covers the face of the Sun (first contact to second contact). One can't actually see this with the naked eye until the very final stages although it does get noticeably darker and colder. 2. The diamond ring effect. 3. Totality. At this point, which lasts about a minute, the corona is visible. One may also see Bailey's beads. (The last bright flashes of sunlight peaking through the rough lunar surface) 4. Another diamond ring. 5. Partial covering of the Sun as in 1(third to fourth contact). |
The Corona
This is a region of very low density gas at a very high temperature which extends millions of kilometres into space. It is normally invisible but can be seen against the dark background of space during a solar eclipse. The streamers we see are due to the effects of the Sun's magnetic field on the charged particles in the corona.
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 pics NASA |
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| The Corona | Bailey's Beads | The Diamond Ring |
Solar eclipses last for just a few minutes as the shadow of the moon on the Earth is small and moves across the face of the Earth quickly.
Lunar eclipses last much longer as the Earth's shadow can cover the whole moon.
Annular Eclipses
Because the distance between the Sun and the Earth varies we may see a total eclipse, where the Sun is completely covered, or an annular eclipse, where it is not completely covered and appears as a disc. This happens because the Moon's orbit is not a perfect circle but an ellipse so its apparent size varies.
