ASTR 130, O'CONNELL. [Fall 2006] Lecture Notes

ASTR 130 (O'Connell) Lecture Notes

4.2 ECLIPSES

Eclipses are shadow effects. There are two types: lunar eclipses and solar eclipses. Both can be dramatic and beautiful events, for properly situated observers on Earth.

Eclipses occur when the shadow of the Earth strikes the Moon (a lunar eclipse) or the shadow of the Moon strikes the Earth (a solar eclipse for observers in the shadow path). A multi-exposure image of a solar eclipse is shown above.

The figure above shows the shadow configuration for a solar eclipse (not to scale). Anyone along the path where the tip of the Moon's shadow strikes the Earth will experience a total solar eclipse. The shadow moves rapidly across the Earth because the Moon moves rapidly in its orbit.

You can also see that if the Moon moves into the Earth's shadow on the other side of its orbit, it will suffer a lunar eclipse. Click here for a diagram. A picture of a lunar eclipse is shown at the right (click for a mosaic of images taken during a total lunar eclipse).

Referring to the drawing above and the one in Lecture 4 that shows the lunar phases, we can see that see that:

A solar eclipse can only occur near New Moon and

A lunar eclipse can only occur near Full Moon

Eclipses can be total or partial, depending on whether the core or periphery of the shadow is involved. Because of the different sizes of the two shadows, a total lunar eclipse can last up to 90 minutes while a total solar eclipse lasts only up to 7 minutes.

Lunar eclipses are easy to observe because they can be seen from any location on Earth where the Moon is above the horizon. Solar eclipses are observable only from within the path of the Moon's (rapidly moving) shadow on the Earth's surface.

A total solar eclipse is interesting mainly because the Moon just barely covers the luminous surface of the Sun and you can see the surrounding solar atmosphere ("chromosphere" and "corona") which is usually hidden in the glare. The rapid fall of "night" coupled with the appearance of the corona are very dramatic for observers in the shadow path.
As viewed from the Earth, the Sun and the Moon have nearly the same size, about 0.5 degree, (even though they are, of course, of vastly different intrinsic sizes). If the Sun appeared much larger than the Moon, there would never be total eclipses, and if the Moon appeared much larger than the Sun, eclipses would be less interesting aesthetically and scientifically---though they would last longer.
At the right is a time lapse video of a total solar eclipse showing pronounced "diamond ring" phases and the sudden appearance of the corona. The apparent brightening of the Sun during these phases is produced by a change in exposure time.

The Moon, Earth, and Sun must be perfectly aligned for an eclipse to occur, meaning that the Moon must be almost exactly in the ecliptic plane during an eclipse (hence the name of the plane).

But the Moon's orbit is actually tilted by 5^owith respect to the ecliptic plane. This sounds small, but remember that 5^o is ten times the apparent angular diameter of the Sun or Moon.
As a result of this orbital tilt, eclipses follow a complicated pattern in time. On average, there are 2 solar and 2 lunar eclipses each year, but the dates of these change in an 18 year-long cycle.

For more information on eclipses, see Espenak's Eclipse Home Page.

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Last modified September 2006 by rwo

Eclipse images copyright © Fred Espenak. Eclipse drawing copyright © 2000 Harcourt, Inc., from the ASTR 121-4 text by Fraknoi et al. Text copyright © 2000-2006 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 130 at the University of Virginia.