A RING OF LIGHT:
When the Moon passes directly in front of the Sun it doesn't always block out all of the Sun's disc. How can this be? It's because the apparent size of the Moon, and to a lesser extent, the Sun, varies as we see it from Earth, and when the Moon appears smaller than the Sun, it can't cover it completely and so there's always a ring of sunlight around the edge of the Moon, even in mid-eclipse. This is called an annular (Latin: annulus, a ring) eclipse, and that's what we'll have on 1 September 2016. Another way to think of it is that the umbra, or tapering cone of the Moon's shadow, has tapered away to zero diameter before it reaches the Earth.
Actually, annular eclipses are slightly more frequent than total eclipses, because the average apparent size of the Moon is slightly smaller than the average apparent size of the Sun. Of all solar eclipses, annulars make up about 33.2%, and totals make up about 26.7%. We'll consider the rest later.
TOTALS VERSUS ANNULARS:
By pure coincidence, although the Moon is about 400 times smaller than the Sun, it's also about 400 times closer to the Earth, so they appear roughly the same size in the sky, which is why we can have total and annular eclipses.
But the apparent size of the Moon varies because its monthly orbit around the Earth is not circular, but an ellipse, with the Earth at one focus of the ellipse. (See http://www.mathsisfun.com/geometry/ellipse.html). When closest to Earth, the Moon has an apparent diameter of about 33' 31", and when it's furthest from Earth it has an apparent diameter of only 29' 22". (A minute of arc (') is 1/60 of a degree, and a second of arc (") is 1/60 of a minute of arc)
The apparent size of the Sun also varies, because the Earth's annual orbit around it is also an ellipse, with the Sun at one focus of the ellipse. There's nothing odd about this - ALL orbits in astronomy are elliptical, but with some being much closer to circles than others. At the moment the Earth is closest to the Sun in early January, which is when the Sun appears biggest - about 32' 32" in apparent diameter. It's furthest away in early July, when the Sun's apparent diameter is only 31' 28".
The longest annular eclipses obviously occur when the Sun appears at its largest, and the Moon appears smallest. These can reach a maximum duration of 10m 14s, as on 1601 Dec 24. In very long annular eclipses the width of the ring of light is thickest. Conversely, the shorter the duration, the thinner the ring will be. And with a very short duration annular, the ring of light will in fact appear 'broken' due to irregularities in the edge of the Moon (termed the Moon's limb).
The technical way to describe the size and duration of eclipses is 'Eclipse Magnitude'. This is the ratio between the apparent size of the Moon and the Sun. If the magnitude is 1.0000 they are the same size; if bigger than that we will have a total eclipse - thus the one on 2186 July 16 will have a magnitude of 1.0805. If smaller than that, we will have an annular: the annular on 1601 Dec 24 had a magnitude of only 0.9078.
EFFECTS DURING ANNULAR ECLIPSES
Although annulars do not produce the special effects seen during totality, they have an eerie beauty of their own, particularly if you are situated on or near the central line, where the ring appears as a perfect circle, and the symmetry is spectacular. And if the annular is of short duration (i.e. it's close to being a total), the Moon appears only very slightly smaller than the Sun, and we get the beautiful effects of Bailie's Beads almost the whole way round the edge of the Sun.
OUR NEXT ANNULAR ECLIPSE: 2016 Sep 01:
The track begins in the Atlantic ocean, enters Africa in Gabon, crosses the Congos, and then enters Tanzania, where the duration will be almost 3 minutes, and the Sun's altitude in mid eclipse will be over 65 degrees. Mid eclipse will be at 09.08 GMT. The magnitude is 0.9736 - in other words the Moon will appear 97.36% as big as the Sun. So 2.64% of the diameter of the Sun will not be covered, and thus the thickness of the ring, as seen from the central line, will be 1.32% of the total diameter of the Sun.
That may not seem much, but it's very important to remember that the brightness of the ring is essentially the same as that of the rest of the Sun, and looking at it without proper filters will cause damage to your eyes. More on the eye safety aspect closer to the time.As the eclipse begins, it will look just like any other, but it will be fascinating to see as the Moon gets closer and closer to the start of annularity that in fact it won't be big enough to cover the Sun totally. Although the eclipse is not close to being total, we should still see the Bailie's Beads effects at the start and end of annularity. These are caused by sunlight shining through the irregularities in the edge of the Moon's disc - in other words it will be shining down some lunar valleys towards us.
An annular eclipse is a special event in its own right, and definitely one to see if you can!