THE waxing and wane of a lunar eclipse may be an awe-inspiring sight, as the image of what we think of as our massive planet is projected on to a celestial screen that appears, from our vantage point, to be no bigger than a pound coin.
It made Thomas Hardy stop and think when he saw one 90 years ago:
And can immense mortality but throw
So small a shade? And Heaven's high human scheme
Be hemmed within the coasts yon are implies?
Given the necessary clear conditions, you may experience these emotions first hand tonight: the moon will be totally eclipsed for an hour and 10 minutes, beginning at 3.20 am tomorrow.
Eclipses of the moon happen when the sun is, as it were, behind the earth, and the full moon passes for a time through the earth's shadow. The diameter of this shadow is about 5,000 miles, more than double that of the moon itself, so the moon may take up to an hour and three-quarters to pass through it. Lunar eclipses can be seen anywhere in the world that the moon happens to be above the horizon at the appropriate time.
If the earth had no atmosphere, the eclipsed portion of the moon would be totally black and invisible but, as it happens, our atmosphere bends the passing rays of light a little into the shadow, so that the dark portion typically displays a copper-coloured tinge.
A solar eclipse at any particular spot, on the other hand, is relatively rare. It occurs when the moon intervenes between us and the sun, but since the moon is tiny by comparison and casts only a small shadow about 150 miles in diameter, the eclipse is visible only along, the narrow path traced by this shadow as it moves rapidly from west to east across the earth's surface.
There are two periods each year, lasting several weeks, when eclipses are possible, and these "eclipse seasons" as they are called slip backwards by about three weeks per calendar year. They currently lie in April and September-October. In most years there are four eclipses of the sun or moon; the maximum possible number is seven (four solar and three lunar), while in some years there may be as few as two (both solar).
Nineteen ninety-six, as it happens, is an average year in this respect. There was one eclipse of the moon on April 3rd, and the second will occur tonight; the April solar eclipse was confined to the southern hemisphere, but the next is awaited eagerly in Ireland on the afternoon of October 12th.