This coming Monday morning there is a solar eclipse, it isn’t a total eclipse of the sun, the last of those visible from the UK was in 1999, this is a partial eclipse. It starts 08:49, the point of maximum eclipse is a 10:01 and the moon leaves the solar disk at 11:18.

If you are lucky enough to be in the correct parts of Spain or North East Africa, then you’ll see around 95% of the sun vanish, those of us in the UK will have to be content with less. Here in London the best we’ll get is around 57%. Sky and telescope magazine has a useful chart showing just what you can expect to see depending on your location at the time of maximum eclipse.

If you do intend to watch the ellipse, weather permitting, please do not look at the sun either directly or though smoked glass or the like. There is very real risk of eye damage. Good information on suitable and unsuitable filters for viewing an eclipse are given by NASA.

Eclipses occur because of an interesting coincidence in astronomy, the Moon and Sun have very nearly the same apparent diameter as viewed from the Earth. This allows the moon to cover or partially cover the solar disk leaving the earth in shadow.

The orbit of the moon around the earth is elliptical, not exactly circular. Thus sometimes the moon is closer to the earth (perigee) and sometimes further from the earth (apogee) (note this isn’t the reason the moon sometimes looks large in the sky). So an eclipse occurring when the moon is at apogee occurs with the moon smaller than the sun as viewed from earth. This leads to a bright ring (annulus) of uncovered sun at the moment of maximum eclipse visible to viewers in the area of the antumbra shadow.

A total eclipse of the sun occurs when the moon is a perigee and hence appears larger in the sky. The moon’s apparent diameter is then large enough to cover the whole of the solar disk. Any viewers in the umbra part of the moon’s shadow will experience a total eclipse of the sun, while those in the in the penumbra part will see a partial eclipse of the sun.

The next total solar eclipse visible from the UK is in 2061, so book your holiday in Cornwall now….