ASTR 1230 (Whittle) Lecture Notes


  • 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.

    Solar Eclipse

  • 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:

  • 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.

  • 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).

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

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    Last modified October 2004 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 © 2000-2004 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.