IEEE Power & Energy Magazine - November/December 2016 - 73

It's Never Too
Early to Start
Planning for One

Earth (Figure 2). By a rather amazing astronomical coincidence, the sun, which is 400 times larger than the moon,
is also 400 times further away. So both subtend almost
exactly the same angle in the sky when viewed from
Earth, about one-half of 1°. Now the Earth's orbit around
the sun isn't a perfect circle; like all orbital paths, it's an
ellipse, and it varies by about 3%. The moon's orbit around
the Earth varies by a bit more, about 12%. The result is that
not all solar eclipses cause a complete blocking out of the sun.
Sometimes the moon might be at or near its apogee, the far
end of its orbit. Eclipses that occur under those circumstances
are called annular eclipses; the moon's disk is smaller than
the sun's, so the sun appears as a ring of fire, often likened
to a wedding ring, at totality [Figure 3(a)]. There are also
some eclipses where no area on Earth experiences totality; those are partial eclipses. The term "partial eclipse"
is also used to describe an eclipse seen from other than
the path of totality [Figure 3(b)]. Just under half of solar
eclipses are total.
Totality can last as long as about 7 min, but it is usu©istockphoto.com/creativaimages
ally shorter and differs with each eclipse; 2-3 min is typical. That's not very long. But the partial stages along the
path of totality, and the length of time some obscuration occurs in adjacent areas, can be as long as several
hours. The amount of reduction in sunlight depends, of course, on how much sunlight is reduced by the moon
at any given time. In effect, a solar eclipse is caused by the moon's shadow moving across the face of the Earth.
It really moves, too, at over 1,000 mi/h.
The shadow of totality is about 70 mi (112 km) in width, while the diameter of the area experiencing at least
some darkening, the partial eclipse area, is about 4,000-mi (6,400-km) wide. For solar and lunar eclipses, the
shadow of totality is called the umbra, and the shadow of partial darkening is called the penumbra.
If the orbital planes of the moon around the Earth and the Earth around the sun were perfectly aligned,
solar eclipses would occur at every new moon. However, there's about a 5° difference, which makes them
much less common. A solar eclipse occurs somewhere on Earth about once every 18 months. But the path of
totality is so narrow that few areas get to experience one very often. If you stood in the same spot on the face
of the Earth and if you lived long enough, you'd see a total eclipse about once every 350-400 years.
The moon is becoming more distant from the Earth by about 2-4 cm/year, largely due to tidal forces. Wait
around another billion years or so and it will be too far away, and its effective diameter compared to that of the
sun too small, to cause a total eclipse.
november/december 2016

ieee power & energy magazine

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Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - November/December 2016