StarChild Question of the Month for February 1999


What is an eclipse?



An eclipse occurs when one object gets in between you and another object and blocks your view. From Earth, we routinely experience two kinds of eclipses: an eclipse of the Moon and an eclipse of the Sun.

Eclipses of the Moon

Lunar Eclipse Diagram
Sometimes, as the Earth orbits the Sun, it comes between the Sun and the Moon. When this happens, the Earth throws a dark shadow across the Moon. This is known as an eclipse of the Moon, or a lunar eclipse.

Eclipses of the Sun

Sometimes, the Moon passes between the Earth and the Sun. The Moon blocks the light of the Sun and a shadow of the Moon is cast on the Earth's surface. This is an eclipse of the Sun, or a solar eclipse.
Solar Eclipse Diagram
There are three types of a solar eclipse: total, partial, and annular. During a total eclipse, the Moon completely covers our view of the Sun. A total eclipse is only visible from a narrow strip (about 150 km wide) of the Earth's surface at any one time. From the areas outside this narrow strip, the Sun appears to be only partially covered and a partial eclipse is seen. A partial eclipse will also occur if the Sun, Moon, and Earth are not precisely lined up. The eclipse cannot be total unless the center of the Moon's shadow is able to strike the Earth. The eclipse type that occurs when the Moon is at its farthest distance from the Earth is an annular eclipse. The Moon then appears too small to completely block out the disk of the Sun.

Cartoon image of the Sun Animation of solar eclipse.
During a solar eclipse, the Moon actually casts two shadows toward Earth. One shadow is called the umbra which becomes smaller as it reaches the Earth. This is the dark center of the Moon's shadow. The second shadow is called the penumbra. This shadow becomes larger as it reaches the Earth. A total solar eclipse, or a complete blocking out of the Sun's light, can only be seen by those who live in the area covered by the umbra. People who live in the area of the Earth covered by the penumbra will see a partial eclipse.

A total solar eclipse can only occur when two events happen at the same time. The first event is a new Moon. This phase of the Moon occurs when the Sun is almost directly behind the Moon, and we see only a sliver of the Sun's light reflected by the Moon. During this time the Moon and the Sun appear close together. The second event that must occur is that the Moon must be in the right position, directly in the line of sight between the Earth and the Sun. These two events occur at the same time about once every year and a half. A total solar eclipse will be visible from Europe and Asia on 11 August 1999.

Star Help me understand how the Sun and Moon can appear to be the same size

Star Help me understand why eclipses happen

Star Show me what I'd see in a total solar eclipse


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