What is an Eclipse

An eclipse, also known as an occultation, is the temporary blockage of either sun or moon as seen from earth. Although an occultation occurs any time three celestial bodies move into direct alignment, we commonly use the term “eclipse” to refer only to occultations of the sun and moon with respect to the earth.

Thus an eclipse occurs only when the earth, moon, and sun are in direct alignment. However, not every new and full moon becomes an eclipse. The moon’s orbit is at a 5 degree angle relative to that of the earth around the sun; and so the moon usually passes slightly above or below the penumbral cone. Only when the earth is exactly between the sun and the moon does the moon move into the earth’s shadow, causing a lunar eclipse. Similarly, only when the moon is exactly between the sun and the earth does part of the earth move into the moon’s shadow, causing a solar eclipse. Because of this movement into shadow, a solar eclipse can only occur while the moon is new; while a lunar eclipse can only occur while the moon is full.

There are three types of eclipse: total, partial, and annular. The distinction refers to whether the sun or moon is completely or partly obscured, which in turn depends on which part of the shadow the eclipsed object intersects, and also on the relative size of sun and moon at the time of eclipse. Although the moon is much smaller than the sun, it is also proportionately close to the earth: so that its face usually seems to be roughly the same size as the sun. The shadow itself has two parts: the penumbra, which is the wide cone where some of the sun’s rays are blocked; and the umbra, which is the narrow cone where all of the sun’s rays are blocked. Only within the umbra is a total eclipse possible.

Total eclipses refer to complete obscurement of the main body of the sun or moon. Total lunar eclipses occur when the moon moves into the earth’s umbral shadow: but the moon does not go completely dark. Rather, after the sliver of remaining moon becomes thinner and thinner and finally disappears altogether, the full moon suddenly changes colour to a deep, dark red: radiating the reflected light of the earth. Total solar eclipses occur when the moon moves between the sun and the earth and completely blocks off the central solar disc, leaving only the stunning corona visible. At the moment of entering or breaking totality comes the classic diamond ring effect. As well, “Bailey’s beads” may often be seen at the edge of the moon’s disc, where the sun’s rays intersect lunar mountains. The path of totality is the constantly moving point where the umbra intersects the earth. For lunar eclipses much of the night side of the planet may see the total eclipse for hours; while the point of umbral intersection for solar eclipses is much narrower, only about 270 kilometres at its widest, and lasting only seven and a half minutes at its longest. Only two or three solar total eclipses occur in a year, while some years might go by without seeing a single lunar total eclipse.

Partial eclipses refer to partial obscurement of the main body of the sun or moon. The path of the partial eclipse is the constantly moving cone where the penumbral shadow intersects the earth. Because part of the sun’s disc is visible during a partial eclipse, the corona cannot be seen.

A special kind of partial eclipse is the annular solar eclipse: where the moon moves between the sun and the earth at the point in its orbit when it is furthest from the earth. Since it then appears slightly smaller than the sun’s disc, the result is a circle or “ring” of fire around the obscuring body of the moon. Annular eclipses are the rarest kind of eclipse.

In the past century, eclipses have often been helpful in determining the validity of scientific theory. In fact, it was a total solar eclipse which allowed the precise measurements of Mercury required to support Einstein’s theory of general relativity. The theory predicted that the gravitational well of the sun would cause the apparent position of Mercury to shift, but at these points in its orbit Mercury is normally completely lost in the sun’s rays. During the solar eclipse, Mercury could be accurately observed, and the predicted shift was found.

All types of eclipses are very reliably predictable, and tables for eclipses decades in the past and the future can easily be found on the Internet. It continues to be a matter of much archaeological speculation as to which were the most ancient cultures to have shifted eclipses from dread omens and unpredictable harbingers of change to a predictable astronomical event. Today it is very common for tourist treks or cruises to be planned entirely around desirable total solar eclipse locations, as was the case for Aruba back in 1998. Many indeed plan their vacations around future solar eclipse events.

If you are fortunate enough to be able to observe a partial or total solar eclipse, do not look at the sun directly until the moment of totality. Doing so can cause permanent eye damage and even blindness. Several safe viewing options are available, from using heavy welder’s glass filters to projecting the sun’s image onto a piece of paper or cardboard through your binoculars or even through a pinprick hole in a piece of cardboard (which will reverse the image). It is only safe to look directly at the sun during full totality, when the corona is visible.

Enjoy this wonder of nature, but enjoy it safely.