As this post goes live, it’s only a few days until an annular solar eclipse, like the one pictured above, will sweep across the Americas on 14 October 2023.

Annular eclipses get their name from Latin anulus, “small ring”, which refers to the ring of sunlight that’s visible around the lunar disc, as shown in the image at the head of this post. They’re the poor relations of total eclipses, in which the lunar disc entirely obscures the Sun.

October’s annular eclipse will follow the track shown in red in the map below.

The Moon’s shadow crosses the Earth from west to east, moving at the Moon’s orbital velocity, about a kilometre per second—equivalent to about three and a half hours to cross the full width of the Earth. The noticeable tilt in the eclipse track above, from northwest to southeast, is because of the inclination of the Earth’s equator to the plane of the Moon’s orbit. You can see the same effect in the track followed by the total eclipse of 9 March 2016, here seen in time-lapse from a Japanese weather satellite stationed above Indonesia:

Notice that the tilt in the March eclipse track is the reverse of what we see in the October eclipse map, above. This is because the Earth is more or less on opposite sides of the Sun in March and October.

The large shadow you can see in the video above is the region in which the Moon’s disc partially obscures the solar disc (a partial eclipse). Only right in the middle of that large patch of shadow does the Moon traverse the exact centre of the Sun’s disc. For a short distance either side of that central eclipse (anything from a few kilometres to something more than a hundred) we can still see either a total eclipse, or the ring of light that characterizes an annular eclipse, but the shift in position will make the eclipse lopsided and of shorter duration.

So we’ve got a narrow region in which we can observe either of the two kinds of central eclipse, annular or total, and a much broader region in which we can see a partial eclipse. These regions correspond to separate areas of the Moon’s shadow cone, which I’ve labelled below:

The blue lines mark off the Moon’s entire shadow—if we stand anywhere within that spreading cone, we’ll see the Moon overlapping the solar disc, to a greater or lesser extent. Within the red lines, the entire lunar disc is superimposed on the Sun. These lines mark off two shadow regions—the umbra, within which the Moon completely obscures the Sun; and the antumbra, from which we can see the Moon outlined against the larger solar disc. The umbra is where we need to be to see a total eclipse; the antumbra is where annular eclipses happen. And the whole surrounding shadow area, called the penumbra, is the region from which partial eclipses can be observed.

So why do we see a mix of the two types of central eclipse?

It’s because of a remarkable coincidence—the Moon is about 400 times smaller than the Sun, but is also 400 times closer, making the two bodies appear to have almost the same size in the sky. So small variations in the relative distance of Sun and Moon can make the Moon look either bigger than the Sun, and able to block out all its light in a total eclipse, or smaller than the Sun, and able only to produce an annular eclipse.

Our distance from the Sun varies a little during the course of a year—we come closest (and the Sun appears largest) in early January; and we’re farthest away six months later, in early July. But the distance to the Moon varies more widely during the course of a month, and also over a longer period of a little over 200 days, in a way a described in detail in my recent post about “supermoons”.

So here’s a comparison of the apparent diameters of the Sun and Moon at maximum, mean and minimum:

In the bottom row, I’ve superimposed the lunar disc on the solar. In the middle, the mean lunar and solar discs match to within a hundredth of a degree. At left, the maximum moon serves to completely block the maximum sun (represented by the yellow circle around the edge of the moon); at right, the minimum moon is unable to block even the minimum sun. So when the Moon is at its closest to Earth (called perigee), it will always cause a total eclipse if it correctly aligns with the Sun; but when it’s at its farthest (apogee), it can never cause a total eclipse. (And, obviously, between those extremes we can get a mixture of annular and total eclipses.) Looking back at my shadow diagram, above, the Moon sometimes comes close enough to Earth for the tip of its umbra to reach the Earth’s surface, causing a total eclipse; and sometimes it’s so far away that the umbra doesn’t reach the Earth at all, and we experience an annular eclipse within the antumbra.

You’d perhaps think that, given how closely the mean sizes of Sun and Moon match, we’d have equal numbers of annular and total eclipses. But that turns out not to be the case, because the Moon moves more quickly near perigee, and more slowly when it’s near apogee. So it actually spends more time at distances greater than its mean distance. Which means we end up with more annular than total eclipses.

Looking at NASA’s Five Millennium Catalog of Solar Eclipses, we can see that there are a whole lot of eclipses in which the Moon’s shadow either just misses or barely grazes the Earth’s surface, but if we concentrate on “pure” annular and total eclipses, during which it’s possible to stand exactly in the centre of the Moon’s shadow with a full eclipse track on either side, there are 3827 annular solar eclipses in the 5000-year catalogue, and only 3121 totals.

And, given that the apparent diameter of the Sun is at its greatest in January and least in July, we might expect to see a corresponding periodicity in the ratio of annular to total eclipses—total eclipses should be “easier” to achieve in July, when the solar disc is smaller. And so it turns out to be:

Finally, one other kind of eclipse is worth mentioning—the hybrid eclipse. These occur when the Earth is positioned almost exactly at the point at which the umbra becomes antumbra. In these circumstances, the curvature of the Earth becomes significant, so that the rim of the Earth can lie in the antumbra while the central bulge extends into the umbra. Like this:

So these eclipses start as annular eclipses when they first make contact with the Earth’s surface, evolve into total eclipses as they cross the Earth, and then turn back into annular eclipses just before they lose contact. They’re fairly rare, but not vanishingly so—there are 569 in the Five-Millennium Catalog. The most recent crossed Indonesia in April 2023, but we’ll need to wait until November 2031 for the next one, which will cross mainly open ocean.

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* The steady speed at which the Moon’s shadow crosses the Earth is not reflected in the speed at which the shadow moves across the ground, however. The Earth rotates in the same direction as the shadow moves, and that reduces the ground velocity a bit, depending on latitude. And there’s a geometric effect just after the shadow contacts the Earth’s surface, and just before it leaves, when a small sideways movement of the shadow results in a large movement across the ground, because the Earth’s surface is at an acute angle to the axis of the shadow.