Solar Eclipses

Total solar eclipses are more common than usually perceived.

On average there is more than one total eclipse per year somewhere in the world. However, because the shadow of the moon is so small an eclipse occurs in one place very rarely. For instance, though there have been many total eclipses during the 1990s, the last seen in the UK was in 1927 and only seen in the north of England.

Lunar eclipses are far less common but are perceived as being far more frequent. The reason is that when a lunar eclipse occurs it is visible from the entire night side of the world and not localised to a small path.

Though eclipses are far more frequent than you would think, this does not mean that there will be an eclipse every year though, the next one is in July 2001 nearly two years after the 1999 event.

Although the moon orbits the earth every 28 days, the reason we do not get eclipses every month is that the moon's orbit is tilted compared to the earth's orbit around the sun. It is only when the moon is in the direction of the sun and the orbits cross that the moon's shadow will fall on the earth and cause an eclipse.

Of course it is one of the most remarkable coincidences that the sun and moon appear the same size in the sky. There is no reason why this should be but by chance the distances and sizes of the sun and and moon are such that they give apparent sizes of roughly half a degree in the sky. If the moon was smaller or further away it would be too small to cover the sun. If it was large or closer in comparison then it would cross the sun more frequently but the prominences and inner corona would not be seen as they would also be covered.

As both the sun and moon move in elliptical orbits the distances and apparent sizes do vary. Sometimes the moon has a smaller apparent diameter than the sun and a total eclipse will not occur but an annular eclipse where a ring of sunlight surrounds the moon happens, though like a partial eclipse this is not as spectacular.

During the 1999 eclipse the sun was 152 million km away and had an apparent diameter of 31'44" or arc. The moon was 373 thousand km away and had an apparent diameter of 32'00" of arc. There is not much difference but enough to cover the sun totally

The eclipse in July 2001 will be longer as the sun will be farther away at 152.0 million km and the moon nearer at 367 thousand km. The sun will be 31'29" arc in diameter and the moon 32'34" of arc. The 2001 eclipse will have a maximum duration of nearly 5 minutes whereas the 1999 eclipse was under 2.5 minutes.
BTW it is correct that the sun is further away in July than December, the seasons are due to the tilt of the earth's axis and the distance to the sun has an insignificant effect. Of course anyone in the southern hemisphere would not think it odd as their winter is in July.

The length of the eclipse is also dependent on location. Farther away from the centre line the size of the shadow is smaller and the sun is covered for less time.

Also the shape of the shadow is important.
When full on to the earth the shadow is circular and this gives the longest time. When the shadow hits the earth at an angle it is stretched out and thinner so the eclipse does not last as long. This is why the eclipse in the UK was 2m 3s long and in Turkey 2m 15s. The maximum was in Rumania and was 2m 23s in duration.
It does take time for the shadow to travel across the earth, which is why when even allowing for local time differences that the eclipse in the UK occurred 1 hour and 20 minutes before it happened in Turkey. In Cornwall the eclipse was at 10:12 GMT and in Hafik at 11:30 GMT.

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