What Causes an Eclipse?

An eclipse can be defined as "The cutting off of the light of one celestial body by another." There are two basic types of eclipses: lunar and solar. Solar eclipses on Earth happen when the moon moves into a position between the Sun and the Earth. A lunar eclipse happens when the moon passes through the Earth's shadow. When this happens the Sun and the moon are on directly opposite sides of the Earth. Lunar eclipses are visible only at night during certain full moons. What you will see during a lunar eclipse is the Sun turn darker, or sometimes a coppery red color for as long as an hour, but sometimes more.

A solar eclipse is much more spectacular. Solar eclipses occur when the moon passes directly in between the Sun and the Earth. During these types of eclipses a narrow path of darkness appears on the Earth due to the fact that the moon is casting it's shadow on the Earth. During this time daytime will turn to complete darkness for a period of several minutes. The part of the Earth that is completely dark is called the path of totality, or the umbra, while the part of the Earth that is only partially dark is called the penumbra.

Not all solar eclipses are total, there is a second type of solar eclipse; a partial solar eclipse, where only the penumbra touches Earth, the umbra passes just above the North Pole or just below the South Pole, completely missing the Earth.

There is also a third type of eclipse. During this type of eclipse the umbra passes across the Earth, but doesn't last long enough to touch the surface; the shadow dissipates before it hits Earth. This happens when the moon is further out in its orbit. The moon appears smaller and is not large enough to completely cover the Sun, a ring of sunlight still remains visible around the Sun's edge. This type of eclipse is called an annular eclipse.

Why are eclipses rare?

The reason eclipses are rare is because they don't happen every time we have a new moon or full moon. This is because the moon's orbit is somewhat tilted, about 5 degrees. Because of this the moon orbits slightly above or below the line between the Sun and the Earth, only occasionally being lined up perfectly. Only about every six months do either lunar or solar eclipses happen. Out of the two types of eclipses lunar eclipses happen more frequently, and total solar eclipses are the rarest of all types. To see a total solar eclipse, you have to be in the path of totality. This path is at the most 200 miles wide and it never covers more than one-half of one percent of the Earth's total surface area. Out of this area it often covers only the sea and the more remote regions of our planet. There are fewer than 70 total eclipses per century, and a chance to see one is a once-in-a-lifetime event for most of us.

Effects During a Total Solar Eclipse

A total solar eclipse is almost unnoticeable when it begins. As the moon starts to cover more and more of the Sun it looks like someone is taking a "bite" out of the western edge of the Sun. Gradually, more and more of the Sun disappears and several interesting effects can be seen on Earth.

During the first hour of a total solar eclipse it doesn't really get very dark here on Earth. It is not until the last several minutes before totality that daylight disappears quickly.

During a total solar eclipse a small crescent of the Sun remains in the sky for a while, and a curious eclipse phenomenon can sometimes be observed. Thin, wavy lines of alternating light and dark can be seen moving parallel to each other on plain light-colored surfaces. These are called shadow bands, and are the result of sunlight being distorted by the irregularities of the Earth's atmosphere. They can best be observed on a floor or wall.

As that last narrow crescent of the Sun begins to disappear, tiny specks of light still remain visible for a few more seconds. These bits of light surround the edge of the moon and form what looks like a string of beads. These lights are called Baily's beads, and are named after Francis Baily, who was the first astronomer to draw particular attention to them. The beads are actually a few last rays of sunlight that are shining through valleys on the edge of the moon. Baily's beads appear for up to 15 seconds before totality. When one tiny bit of sunlight remains, a beautiful "diamond ring" is created against the outline of the moon, this lasts only an instant and is the signal of the arrival of the moon's shadow. The last ray of sunlight vanishes entirely and totality begins.

Suddenly the sky turns dark and the moon's shadow speeds along the surface of the Earth at speeds up to several thousands of miles per hour. The sky near the horizon will still appear bright but this light is very far away. Most people don't realize it but the darkness produced by an eclipse is not as black as the darkness produced during the night.

In the middle of the incredibly dark sky you will still be able to see the corona of the Sun. This white crown of light shines out in all directions around the dark solar disk. The corona of the Sun is visible only during a total solar eclipse, it is a million times less bright than the Sun itself so during normal times of the day it is not visible at all and is drowned out by the light from the Sun.

Another unusual effect that happens during a total solar eclipse is this. When a total solar eclipse occurs you can see light from the Sun's lower atmosphere for a few seconds before and after totality, this appears as a somewhat purplish glow right along the edge of the moon.

The effects of total solar eclipses effect nature as well:

Find out exactly how long until the next eclipse

©Copyright 1998 Elizabeth Beckett, Holly Bernitt, and Vishwa Chandra.