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
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
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
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 solar eclipse can only occur near New Moon and
- A lunar eclipse can only occur near Full Moon
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
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
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 5owith respect to the
ecliptic plane. This sounds small, but remember that 5o
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.
information on eclipses, see Espenak's
Eclipse Home Page.
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.