DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

solar eclipse
solar eclipse The blocking of the solar disk
due to the passage of the moon between the sun
and the Earth. The moon casts a shadow on the
Earth — or from the Earth, the sun is screened
off by the moon. Solar eclipses can come in one
of three forms: total, annular, or partial. A total
solar eclipse occurs when the moon and sun are
in perfect alignment and the relative distances
are such that they have the same angular diameter
in the sky. Because of the eccentricity of
the Earth’s orbit around the sun and in the orbit
of the moon around the Earth, the angular
diameter of the sun is sometimes less than, or
sometimes greater than the angular diameter of
the moon. The annular eclipse also occurs when
the sun and moon are in perfect alignment but
when the moon is more distant, so has a slightly
smaller angular diameter than the sun creating a
bright ring or annulus at the time of maximum
eclipse. Partial eclipses are more common and
refer to the occasions when the moon only partly
covers the solar disk as seen from the viewpoint
on Earth. Of these three, the total eclipse is by
far the most spectacular, occurring on average
once per year somewhere on Earth with durations
ranging from 1 to 7 min. During a total
solar eclipse, the corona and chromosphere become
visible.
solar electromagnetic radiation On average,
the sun emits a total of 3.86 × 10 23 kW
integrated over its surface or 6.3 × 10 4 kW/m 2 .
This radiation can be divided into five frequency
bands:
1. X-rays and extreme ultraviolet (EUV) with
λ