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Solar Eclipse

An Overview of a Solar Eclipse


Solar Eclipse of the Sun

A total solar eclipse is seen on March 29, 2006 above Athens, Greece. In an total solar eclipse, the Moon moves between the Sun and Earth and completely blocks the sun. (Photo by Milos Bicanski/Getty Images)

Milos Bicanski/Getty Images

A solar eclipse is a natural event that occurs when the moon passes between the sun and the Earth causing the sun to become fully or partially covered. Eclipses are a regular occurrence made possible by the elliptical (oval-shaped) orbit of the moon around Earth and Earth’s elliptical orbit around the sun. In addition, the sun’s diameter and distance from the Earth in comparison with that of the moon (the moon is 400 times larger in diameter but is also 400 times farther from the Earth than the moon) make eclipses impressive as the two appear to be the same size when viewed from Earth. Not understanding the relationship between the earth, sun, and moon, ancient peoples would sometime connect solar eclipses with the supernatural.


Eclipse Background and Geometry

Before understanding the basics of a solar eclipse it is important to first understand the moon’s rotation around the Earth. The moon itself is a rocky body about 2,160 miles (3,476 km) in diameter that has no light of its own. Instead, it shines and is viewed from Earth via the sun’s light reflecting off of its surface.

It takes about 29.5 days for the moon to rotate around the Earth. During this time, the moon's changing position with respect to the sun causes it to go through a series of phases. The first of these phases is called new moon. New moon is important to solar eclipses because they can only occur during this moon phase as this is when the moon most directly passes between the sun and Earth. During new moon if the moon and sun line up correctly (diagram) and the moon’s shadow hits Earth’s surface at the same time, some portion of the sun will be blocked, causing an eclipse.

Eclipses do not regularly occur with each new moon though. This is because the moon’s orbit around the Earth is tilted five degrees more than the Earth’s orbit around the sun. Therefore, the moon will normally pass above or below the sun and its shadow will miss Earth. A solar eclipse can only take place when the new moon occurs close to where the moon’s orbit crosses the ecliptic (the plane of Earth’s orbit around the sun). This geometry usually lines up at least twice a year, causing some part of the moon’s shadow to reach Earth and make a solar eclipse viewable from that region. In some years, up to five solar eclipses have occurred.


Types of Solar Eclipses

There are four types of solar eclipses, each determined by the amount of sun that is obscured by the moon. The first of these is the total eclipse which occurs when the sun is completely obscured from view. Instead, the sun’s intense light is replaced by a dark silhouette of the moon that is outlined by the sun’s corona (the super heated plasma extending out from the sun). For a total eclipse to occur, the dark inner shadow directly in front of the moon (its umbra) must reach Earth’s surface. Because one must be within the umbral shadow to view a total eclipse, they can only be viewed in narrow areas where the eclipse is occurring directly overhead.

In addition, the elliptical orbit of the moon around the Earth plays a role in whether a solar eclipse will be a total eclipse because the moon can appear larger than the sun and cover it when it is closer to the Earth (near its perigee).

Another type of solar eclipse is an annular eclipse (photo). These occur when the sun and moon are exactly in line but the moon appears smaller than the sun. During an annular eclipse, the sun appears as a bright ring around the moon. Like a total eclipse, the moon’s elliptical orbit plays a role in whether an eclipse will be annular. When the moon is at its farthest point from Earth (its apogee), it appears smaller than the sun, and can therefore cause an annular eclipse.

The third and most common type of solar eclipse is the partial eclipse. These occur when the sun and moon are not completely aligned and the sun is only partially obscured. Unlike a total or an annular eclipse, these are visible over large portions of the Earth because they are caused by the moon’s penumbral shadow (i.e. - the faint outer shadow that extends out from the umbral shadow). These solar eclipses are common not only because they are viewable from numerous places on the globe, but also because they can occur even when the umbral shadow never reaches Earth’s surface.

The final type of solar eclipse is the hybrid eclipse. A hybrid eclipse is a combination total and annular eclipse that takes place when a total eclipse changes to an annular eclipse or vice versa along different sections of the eclipse’s path.


Solar Eclipse Frequency and Predictions

Each year, Earth experiences an average of 2.4 solar eclipses. The actual number can range from two to five, although, it is rare to have five. The last time five solar eclipses occurred was in 1935 and the next will not be until 2206. Total eclipses are the rarest and there is only one every one to two years.

Solar eclipses are easy for scientists to predict because the moon’s phases and its position within its orbit are easily observable and track able for the future. Using modern technology, agencies such as the United States’ NASA have been able to track and predict the type and exact date of solar eclipses for well into the future. NASA for example has been able to track and predict solar eclipses for the five thousand period between 2000 B.C.E and 3000 C.E. During that time, it estimates that Earth will experience 11,898 solar eclipses.


Viewing Solar Eclipses

Because solar eclipses are easily track able and predicted for well into the future, it is also easy for the public to view them. However, like viewing the sun normally, viewing it during solar eclipses can cause severe eye damage or even blindness. Therefore precautions such as wearing special eclipse glasses and other eye protection or items that filter the sun’s intensity should be used.

To learn more about solar eclipses, visit NASA’s Solar Eclipse Page and the Exploratorium’s Total Solar Eclipse website.


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