Solar Eclipses

 

What is a Solar Eclipse?

A solar eclipse occurs when the moon crosses the path between the sun and the earth. If the motions of the celestial bodies were simple, there would be a solar eclipse every month. However, the sun and the moon have complicated motions, which make it difficult for the path of the moon to intersect the path between the sun and the earth. Despite this complication, eclipses are more common than people tend to believe.

The darkest shadow (where the sun is completely covered) is called the umbra. The umbra is narrow at the distance of the Earth, and a total eclipse is observable only within the narrow strip of land or sea over which the umbra passes. The partial shadow is called the penumbra. A partial eclipse may be seen from places within the large area covered by the penumbra. Sometimes the Earth intercepts the penumbra of the Moon but is missed by its umbra in this case a partial eclipse of the Sun is observed. The sun is so bright that even though it is partially covered it can illuminate the Earth.

 

Nature has created a remarkable coincidence, the sizes and distances of the Sun and Moon are such that they appear as very nearly the same angular size from the Earth, but their apparent sizes depend on their distances from the Earth. The Earth revolves around the Sun in an elliptical orbit, so that the distance of the Sun changes slightly during a year, with a correspondingly small change in the apparent size or angular diameter of the solar disc. Also the apparent size of the Moon's disc changes somewhat during the month because the Moon's orbit is also elliptical.

An annular eclipse occurs when the Sun is nearest to the Earth and the Moon is at its greatest distance, the apparent disc of the Moon is smaller than that of the Sun. When this type of eclipse of the Sun occurs, the Moon's disc passing over the Sun's disc cannot cover it completely but will leave the rim of the Sun visible all around it.

The frequency of solar and lunar eclipses

A solar eclipse, especially a total one, can be seen from only a limited part of the Earth, while a lunar eclipse can be seen wherever the Moon is above the horizon.

In most calendar years there are two lunar eclipses (there can be up to a maximum of three) or none may occur.

Solar eclipses occur two to five times a year, five being exceptional. The average number of total solar eclipses in a century is 66 for the Earth as a whole.

From any point on Earth, on the average you will experience no more than one total solar eclipse in three to four centuries.

The situation is quite different for lunar eclipses. An observer at the same location can see an average of one lunar eclipse per year. A total lunar eclipse can last, as long as an hour and three-quarters, but for a solar total eclipse maximum duration of totality is only 7 1/2 minutes. This difference results from the fact that the Moon is much smaller in cross section than the extension of the Earth's shadow but can be only a little greater in apparent size than the Sun.

What will you see?

The first sign of the eclipse on Earth will be when the Moon's disc first touches the Sun, this is known as First Contact. The time of First Contact depends on where you are, relative to a particular eclipse. At First Contact, the Moon begins to take a "bite" out of the Sun, as the day goes on, this "bite" gets larger and larger.

As the moment of totality approaches, and the Sun is nearly covered, shadow bands might be seen; narrow bands of shadow and light racing across the ground. These are multiple images, caused by irregular refraction in the Earth's atmosphere, of the remaining "slice" of the Sun. 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.

As the Moon moves to cover the Sun, events proceed very quickly. 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. Because the Moon's surface isn't equal all the way round, the display of Baily's Beads isn't consistent, but depends on the angle from which the Moon approaches the Sun.

The Moon's irregularity can also have an effect on the duration and width of the total eclipse. 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.

During totality, the sky goes dark; not quite as dark as night, and with strange shadow effects caused by scattered light from the edge of the eclipse; the horizon still appears quite light, and the whole landscape takes on a strange appearance. Birds go home to roost, bees stop flying, and some flowers may begin to close as if for the night; nature seems to hold its breath. 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. Around the Sun, with the sky nearly dark, some of the brighter stars, and particularly planets, may be seen.

After totality the end of the total eclipse is the reverse of the beginning. Totality ends at Third Contact, the moment at which the Moon begins to uncover the Sun once again, a diamond ring, Baily's Beads, and shadow bands may be seen.

After third contact, the Sun is progressively uncovered by the Moon, and normal daylight returns. The shadow on the Sun shrinks, until it vanishes altogether at the moment of Fourth Contact.

Safe viewing of an eclipse

Warning - Observing the Sun is Dangerous. Do not look directly at the Sun, unless you are using either a solar filter or "eclipse sunglasses" specially designed to protect your eyesight.

If you do not have these filters, then project an image of the sun, through a small hole onto a flat surface, and watch the image. If you're not sure what to do, then observe with someone who does know how to safely observe an eclipse.

When the moon partially covers the sun, it is tempting to look directly at the sun. However, the sun is just as dangerous then as it is when the moon is not partially covering it. If you look directly at the sun during the partial eclipse, serious permanent damage can occur to your retina (part of the eye). It is only safe to look at a solar eclipse when the moon completely covers the disk of the sun and the corona is visible. There are safe ways of observing an eclipse, such as using a pinhole camera or projecting a telescope image onto a sheet of paper.

The totally eclipsed sun may be safely viewed without protection; but watch out for the sudden return of sunlight!