A solar eclipse is a fascinating phenomenon — our Sun, which is normally so reliable, is suddenly stained black by the Moon. Any solar eclipse is an interesting event, but a total solar eclipse is the most spectacular astronomical phenomenon that you'll ever see.
In the middle of the day, a shadow moves over the Sun — for a brief minute or two, the sky darkens to the same level as on a moonlit night; animals and birds are silent; everything seems to be in suspension. The Sun has vanished, but its outer atmosphere — the corona — is visible as a ghostly halo around the black disc of the Moon, with streamers and ribbons of faint light trailing off for millions of miles in the Sun's magnetic field.
Looking at the Sun, at any time, is dangerous; and that applies during a solar eclipse, just as it does on any normal day. So don't forget to read about eclipse eye safety.
As we've said, a solar eclipse always occurs at the New Moon. Now, a New Moon happens every month, when the Moon is between the Earth and the Sun, so that the near side of the Moon — the side we can see — is in shadow. Usually the Earth and Moon aren't lined up right to cause an eclipse; but once in a while (basically once every 6 months), the Sun, Moon, and Earth are lined up well enough so that the Moon's shadow falls on the Earth. This is what causes a solar eclipse. (You can learn why an eclipse doesn't happen every New Moon by reading about eclipse cycles).
As explained in Mechanics of Solar Eclipses, the Moon's shadow during a solar eclipse has two parts:
This means that a partial eclipse is usually seen over quite a large area of the Earth; but when a total eclipse occurs, it is only visible from a small part of the Earth (whereas its accompanying partial eclipse is seen over a much larger area). However, the shadow isn't simply a "spot" on the Earth; due to the movement of the Earth and Moon, and rotation of the Earth, the shadow actually races across the Earth's surface at around two thousand miles per hour, causing the Moon's shadow to "write" a long track across the Earth.
As an illustration of this, the following map shows the area of the Earth which will be touched by the Moon's shadow during the total solar eclipse on 21 August, 2017. The large green area is the area touched by the penumbra, which first touches the Earth in the west, where the Sun is just rising, moves west-to-east, and leaves the Earth again a few hours later, in the east where the Sun is just setting. In this area, a partial eclipse will be seen.
The narrow red band is the area touched by the umbra; that is, the track of the total eclipse across the Earth, known as the path of totality. As you can see, it starts in the Pacific, races eastward across the US, and departs the Earth in the Atlantic Ocean.
The reason that the partial eclipse area is rounded at the north is that the penumbra almost falls off the "top" of the Earth and into space. The reason that this doesn't happen exactly at the North Pole is that the eclipse occurs in summer, so the Earth is tilted towards the Sun at the north.
What you will see depends, of course, on the type of eclipse (partial or total); but also on where you are relative to the eclipse. The following sections describe what you can see during an eclipse; since a partial eclipse and the partial stages of a total eclipse are essentially the same, both are described here.
If you are standing near or under trees, you may see multiple images of the crescent Sun being projected on the ground by the "pinhole camera" effect of the leaves. This is actually something you can see on a normal sunny day, when circular Sun images can be projected on the ground; but during an eclipse, the projected crescents can be very distinctive. Try holding up a colander during the crescent phases of the eclipse to see this effect!
As the Moon moves to cover the Sun, events proceed very rapidly. The Moon's shadow may be seen rushing in very quickly from the west. The remaining crescent of the Sun gradually shrinks to a sliver, and then breaks up into distinct points of light, known as Baily's Beads; these are caused by the Sun shining through valleys around the visible face of the moon, for a few seconds before totality.
When only one point of light is left, a beautiful diamond ring effect may be observed, with the last brilliant point of light transfixed on the Moon's outline. Then this last glimmer vanishes, as the leading side or limb of the Moon touches the farther limb of the Sun, at a moment known as Second Contact. This is the first instant of the total eclipse.
In the sky above hangs the black disc of the Moon, surrounded by a faint halo, like a negative Sun. The Sun's corona, far too faint to be seen at any time other than a total eclipse, streams out from the Moon in all directions; some streamers reach several times the size of the Sun before fading away.
For a few seconds after the beginning of totality, and again just before the end, the Sun's lower atmosphere, the chromosphere, may be seen, as a reddish glow around the edge of the Moon. Some solar prominences may also be seen, as spectacular arcs of glowing red gas around the Sun.
Apart from the eclipse, the sky at totality is well worth a look. Around the Sun, with the sky nearly dark, some of the brighter stars, and particularly planets, may be seen. This may also be a rare chance to see Mercury, since it is normally too close to the Sun; depending on its position, it may be visible near the Sun.
So what about an annular eclipse? What's that all about? and what about a hybrid eclipse?
This, unfortunately, spoils the show; because the full intensity of the Sun is always visible — even though the area of visible Sun is reduced — it is not safe to look at directly, and the corona, prominences, etc, will not be visible.
Still, an annular eclipse will make an interesting sight. Like a total eclipse, an annular eclipse is accompanied by a partial eclipse covering a much larger area.
A hybrid eclipse is an eclipse that is right on the boundary between total and annular; so much so that the eclipse starts as annular, changes to a total eclipse, then changes back to an annular eclipse again before the end. The path of totality will be very narrow in a hybrid eclipse. People in the path near the middle of the eclipse will see a total eclipse, but normally this will be a very short-lived eclipse; people near the ends of the path of totality will see an annular eclipse.
A partial eclipse can last anything from a few minutes (for a very small eclipse) to a few hours (for a larger eclipse); the duration you will see will depend on how close you are to the centre of the eclipse, with the longest duration near the centre.
The same applies to the partial stages of a total eclipse; however, the real spectacle is the total phase. To see this, you have to be within the path of totality; then you will see a total eclipse which could last from a few seconds to a few minutes, up to (very rarely) 7 minutes or more. (In the 21st century, the longest total solar eclipse will be on the 22nd of July 2009, in China, at 6 minutes 39 seconds; the next is the eclipse of the 2nd of August, 2027, in Africa and the Middle East, at 6 minutes 23 seconds.)
The longest total eclipse will be seen in the centre of the path of totality; the duration falls off away from the centre, and very rapidly near the edges. So, if you're on the edge of the path of totality, you'll see a very short total eclipse; but get reasonably close to the centreline, and you should see close to the maximum duration.
Annular eclipses can last a lot longer — up to 12 minutes; on the 15th January 2010, an 11-minute annular eclipse will be visible in the far East, the longest in the 21st century.