Astronomy - June 2015

WWW.ASTRONOMY.COM 55

reaches our planet’s surface. Anybody in
the lighter outer region of the shadow
(which astronomers call the penumbra)
will see a partial solar eclipse.
The lucky individuals under the dark
inner shadow (the umbra) will experience
— a much better word than “see” — a total
solar eclipse. Sometimes, only the Moon’s
penumbra falls on Earth, and the eclipse is
partial everywhere. Not in August 2017.
A question people often ask is, “Isn’t the
Sun a lot bigger than the Moon, so how
does the Moon cover it so exactly?” Yes, the
Sun’s diameter is approximately 400 times
larger than that of the Moon. What a coin-
cidence that it also lies roughly 400 times
farther away. This means both disks appear
to be the same size.
Regarding timing, all solar eclipses hap-
pen at New Moon. Unless the Moon lies
between the Sun and Earth, it can’t block
any of our star’s light. The only lunar phase
when that happens is New Moon.
But why doesn’t a solar eclipse happen
at every New Moon? The reason is that the
Moon’s orbit tilts 5° to the plane formed by
Earth’s orbit around the Sun, which astron-
omers call the ecliptic (because that’s the
only place eclipses can occur).
Most of the time, our satellite is either
north or south of the ecliptic. But during
each lunar month, the Moon’s orbit crosses
that imaginary plane twice. Astronomers
call these intersections nodes.
Solar eclipses only occur when the Sun
and the Moon lie at the same node. Unfor-
tunately, during most lunar months, the
New Moon lies either above or below one
of the nodes when the Sun is there, and no
eclipse happens. On average, a total solar
eclipse occurs somewhere on Earth about
once every 16 months.
But the average length of time between
two total solar eclipses at a specific location
on Earth is much longer: 330 years in the
Northern Hemisphere and 550 years for
locations south of the equator.
The difference between the hemispheres
is due to two factors: 1) More eclipses occur
during summer months (more hours of
daylight); and 2) the Northern Hemisphere
lies farther from the Sun during its sum-
mer, making our daytime star a smaller
target (hence, easier to cover).
The maximum length of totality also
varies from one eclipse to the next. The

Michael E. Bakich is a senior editor of

Astronomy. He will be conducting a massive

public viewing party for the eclipse in St. Joseph,

Missouri. See http://www.fpsci.com for details.

The 13th NOAA Geostationary Operational Environmental Satellite took this series of images during

the partial solar eclipse October 23, 2014. The shadow moved over Alaska, western Canada, and the

far northwestern United States. NASA/NOAA/SSEC

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