Eclipses

Why there is not one every month

The Moon’s orbit is tilted about five degrees to the Earth’s. There is a new moon and a full moon every month, but only when one of them falls close to where the two orbits cross do the Sun, the Earth and the Moon line up closely enough for a shadow to land. That happens in two seasons a year, roughly six months apart, which is why eclipses tend to come in pairs a fortnight apart: a solar one at the new moon and a lunar one at the full moon either side of it.

Those crossing points are the Moon’s nodes, and a season is the stretch when the Sun stands near enough to one of them. It lasts about five weeks, and a lunation is about four weeks and a day and a half, so no season can pass without at least two eclipses, and now and then one holds three. What the nodes are, and how astrology reads them, is on the Nodes page.

The pattern repeats every eighteen years, eleven days and eight hours — the saros — which is how eclipses were predicted long before anybody could calculate an orbit. Those eight hours are why each repeat of a solar eclipse lands about a third of the way round the world from the one before.

Total, annular and partial

The Sun is about four hundred times wider than the Moon and about four hundred times further away, so the two discs look almost the same size from here. Almost, because the Moon’s distance changes. When a solar eclipse falls with the Moon in the nearer part of its orbit, the Moon’s disc is the larger and covers the Sun completely: a total eclipse, with the sky dark and the Sun’s pale outer atmosphere showing round the black disc. When the Moon is in the further part, its disc is too small, and a ring of Sun is left burning round it: an annular eclipse, which never gets dark and is never safe to look at. Either way the full effect is seen only along a track at most a few hundred kilometres wide, and totality at any one place never lasts more than about seven and a half minutes; two or three is more usual. On both sides of the track, over a far wider area, the same eclipse is partial.

The Moon in the Earth’s shadow

The Earth’s shadow has two parts. The outer part, the penumbra, is where the Earth hides only some of the Sun, and a Moon that passes through it alone dims so slightly that most people notice nothing. The inner part, the umbra, is where the Sun is hidden entirely. As the Moon moves into it a curved edge of darkness crosses the disc, and that curve is the outline of the Earth. A total lunar eclipse can keep the Moon inside the umbra for more than an hour, far longer than any total eclipse of the Sun, because the Earth’s shadow at the Moon’s distance is well over twice the Moon’s width.

What the drawings are

The discs are placed so that the covered share of the area is the share the calculation gives: for a solar eclipse seen from your place, how much of the Sun is behind the Moon; for a lunar eclipse, how much of the Moon is inside the Earth’s full shadow. The bite is always drawn from the same side, which is not necessarily the side you will see it from — that depends on the hour and the place. The bands underneath are the exact times each stage begins and ends.

The wheel puts each one at the degree the Moon is in at the height of it. They gather at two opposite points, where the Moon’s orbit crosses the Sun’s path, and those points slide backwards through the signs by about nineteen degrees a year. That is why the eclipses of one year fall in a different pair of signs from the eclipses of a few years later.

A partial solar eclipse has no single amount: it covers more of the Sun in some places than others, and none at all in most. Without a place to see it from there is nothing honest to draw, so nothing is.

Looking at one

A lunar eclipse is safe to watch with nothing at all. A solar eclipse is not: looking at the Sun while any part of it is uncovered damages eyes, and sunglasses are no protection. Use certified eclipse glasses, or project the Sun through a pinhole onto card. Only the minutes of totality, where there are any, are safe without them.

Eclipses have been read as omens for as long as there are records of them. Nothing on this page says what one means for anybody. It says when the shadow falls, and where.