Lesson 6 Notes

SOLAR AND LUNAR ECLIPSES

A shadow cast by Earth or the Moon is shaped like a cone which points away from the Sun the dark inner portion of the shadow cone is called the umbra. The lighter outer portion of the shadow is called the penumbra. These shadows always exist, but we only notice them during an eclipse.

An eclipse occurs when one object moves into the umbra or penumbra of another objects shadow. The Sun, Moon, and Earth have to be aligned for an eclipse to occur.

TSE = Total Solar Eclipse -- Observers in the narrow path of totality (up to 200 miles wide) get the chance to experience daytime darkness and a magnificent view of solar corona. (Duration -- in minutes and seconds -- is the maximum time a stationary observer can observe totality.)

pse = partial solar eclipse -- Visible over wide areas of the globe; direct observation of the Sun (with safe filters or indirect observation device) shows crescent images of Sun. (Coverage percantage indicates maximum obscuration of solar disk.)

ase = annular solar eclipse -- Actually a special case of a partial solar eclipse; within the narrow path of annularity, observers get the chance to see a thin ring of sunlight surrounding the dark Moon. (Duration -- in minutes and seconds -- is the maximum time a stationary observer can observe annularity.)

atse = annular/total solar eclipse -- A combination eclipse that starts out annular, becomes total in the central section of the path, and returns to annular near the end. This type of eclipse always has a very narrow path. (Duration -- in minutes and seconds -- is the maximum time a stationary observer can observe totality.)

TLE = Total Lunar Eclipse -- The full Moon is engulfed in the Earth's complete shadow (the umbra), during which the Moon appears a dull reddish color. (Duration -- in hours and minutes -- is the maximum time the Moon remains in the complete shadow.)

ptle = partial lunar eclipse -- Only part of the Moon is passes through the umbra. (Coverage percentage indicates the maximum portion of the lunar disk in the umbra.)

pnle = penumbral lunar eclipse -- The Moon passes only through the partial shadow of the Earth (the penumbra). Darkening of the Moon is negligible.

Lining Up the Sun-Earth-Moon System –

For a solar or lunar eclipse to occur, the Sun, Earth, and Moon must be in syzygy ( in a straight line ). There are between two and five solar eclipses each year and up to two or three lunar eclipses each year. Lunar eclipses are more commonly seen because everyone on the nighttime side of Earth can observe them, while only those in the Moon’s shadow can witness a solar eclipse. Because the umbra of a solar eclipse is relatively small, each location on Earth is only likely to see a total solar eclipse approximately every 350 years.

The Structure of a Shadow -

When the Earth blocks the light coming from the Sun, the shadow out behind it has a dark and a light part. If you were standing in the dark area (called the "umbra"), the Earth's disk would completely shut off the light from the Sun and you would be in near total darkness. If you stand in the lighter part of the shadow (called the "penumbra"), the Earth's disk is off to the side or else too small to completely block out the Sun. There is a portion of the Sun, appearing as a crescent or a thin ring, that is still delivering some light to your position, producing only a partial or semi-dark shadow.

The illustration at the left shows how these various parts of the shadow are produced. The umbra (labelled "U") is a dark cone behind the Earth that comes to a point. The penumbra (labelled "P") is the lighter cone that expands out behind the Earth. The middle part of the penumbral area is an extension of the umbra, sometimes called the "antiumbra" and indicated by the letter "A". Notice the green surface with the two dotted circles that I drew in, cutting the shadow in a cross sectional view. If you examine the shadow from the vantage point of this cross section, you'll see the dark circular umbra in the center, surrounded by the lighter penumbra.

If you are in the umbra, looking back at the Sun, all you will see is the night time half of the Earth. The Sun is totally obscured. This is called a "total eclipse".

Standing in the penumbral part of the shadow, the Sun is only partially blocked out. A crescent of sunlight is still visible, creating a "partial eclipse". Per BioPSciences

Eclipse Times –

The Moon’s orbit crosses the plane of Earth’s orbit at only two points, called nodes. Each month, as the Earth-Moon system makes its way around the Sun, these nodes move slightly westward because of the Sun’s gravitational pull. If the Moon crosses ‘s plane at either node at the same time that a new or full moon occurs, the Moon, Earth, and Sun are in near or perfect alignment, and an eclipse results.

Sketch and label illustration in lab text.