James Price of Rathkeale College asks :-

What is the size of the Moon's umbra, the shadow cast on the Earth during an eclipse of the Sun?

Alan Gilmore, an astronomer at the University of Canterbury's Mt John Observatory, responded.

A solar eclipse occurs when the moon's shadow crosses over the earth. Because the moon's orbit is tilted to the earth's orbit there are only two intervals in a year when this can happen. In 2004 eclipses of the sun and moon happened around March and September but they progress through the calender on an 18-year cycle.

For the sun to be hidden completely behind the moon one must be on the narrow track followed by the umbra, the dark centre part of the moon's shadow. The diameter of the umbra varies from about 200 km to nothing.

When the umbra is large then the eclipse lasts a long time, up to four minutes. Then we can see the corona, the glowing gas around the sun. Astronomers travel to remote places to be on a total eclipse track.

When the umbra shadow ends before reaching the earth then the sun is not completely hidden. At best it surrounds the moon as a brilliant annulus. So such eclipses are called 'annular' eclipses. They are of no interest scientifically, or as a spectacle.

The varying size of the umbra is mostly due to the moon's varying distance from earth. It varies between 360,000 and 400,000 km. If an eclipse happens when the moon is close then the umbra is biggest; we get a total eclipse. If the moon is distant then the umbra ends before reaching us; we get an annular eclipse. Earth's slightly varying distance from the sun also plays a small part in this. An annular eclipse crosses the South Atlantic ocean on September 22 2006.

The outer part of the moon's shadow, the penumbra, is much bigger; thousands of kilometres in diameter. As it crosses we see a bite out of the sun, a partial eclipse. So, when a narrow strip of the the world is enjoying a total eclipse, a large region is seeing a partial eclipse. This happened on March 29 2006.