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
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
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.
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
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.
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.
me understand how the Sun and Moon can appear to be the same size
Help me understand why eclipses happen
Show me what I'd see in a total solar eclipse
The StarChild site is a service of the
High Energy Astrophysics Science Archive
Research Center (HEASARC),
Dr. Alan Smale (Director), within the
Astrophysics Science Division (ASD) at
StarChild Authors: The StarChild Team
StarChild Graphics & Music: Acknowledgments
StarChild Project Leader: Dr. Laura A.
Curator: J.D. Myers
Responsible NASA Official: Phil Newman