Three-hour show begins at 9.30 a.m.

Weather permitting, the show for Ireland begins at about 9.30 a.m. when the moon's disc first encounters that of the sun as the two bodies pass in the sky above. Last night the weather forecast was not very promising - yet yesterday's glorious sunshine raised hopes of being able to witness the phenomenon, even if, safely, on television.

It takes about 90 minutes for the moon to reach its point of maximum solar blockage before gradually moving away from the solar disc. The whole thing takes about three hours but the wait is nothing compared with the view, if you are watching from a point where the moon's full shadow touches the earth.

This eclipse will first cast its shadow into the Atlantic and begin tracking eastward. Its first landfall is in Devon and Cornwall in Britain, then France, across central Europe and then on to Turkey and later Pakistan and India, before departing while over the Bay of Bengal in the Indian Ocean.

Ireland's south coast lies about 100 miles outside the "path of totality" - the narrow 70-mile-wide band where the moon will totally obscure the sun and cast the deepest possible shadow.

Those lucky enough to have positioned themselves in the path of totality will experience the full and spectacular view of a total eclipse, when day turns to night. The quality of viewing all depends on the weather, however. If heavy clouds are present, they will obscure everything but the sudden loss of light.

Totality only lasts for 70 seconds or so with this eclipse, and for most of the time, viewers will see only what we will see here in Ireland, a partial eclipse. Viewers in Cork, Killarney or Waterford will see about 95 per cent of the solar disc covered, in Dublin, Limerick and Galway about 90 per cent, and in Belfast, Castlebar, Letterkenny and Derry about 85 per cent.

This blockage will not be apparent, however, because the sun will still be far too bright to observe with the unprotected eye. The light will remain far too intense and will cause eye damage if looked at directly, even as the maximum blockage arrives at about 11 a.m.

For this reason a moderately heavy cloud cover might be an advantage for viewers in Ireland. There are times when the clouds are heavy enough to filter out the brightness but allow the solar disc to show through. Great care must be taken, however, because cloud depth varies minute by minute and too much light might penetrate without warning.

If you miss this eclipse or cannot get to the path of totality, then you will have to hope for a long life. The next eclipse to cross Ireland will not arrive until 2090.

Total eclipses are relatively common occurrences, however, if you are affluent enough to travel to them. A total eclipse will cross South Africa next year.