CHAPTER 3 | CYCLES OF THE MOON 41
corona is made of hot, low-density gas that is given a wispy appear-
ance by the solar magnetic fi eld, as shown in the last frame of
Figure 3-10. Also visible just above the photosphere is a thin layer
of bright gas called the chromosphere. Th e chromosphere is often
marked by eruptions on the solar surface, called prominences
(■ Figure 3-11a), that glow with a clear, pink color due to the high
temperature of the gases involved. Th e small-angle formula reveals
that a large prominence is about 3.5 times the diameter of Earth.
Totality during a solar eclipse cannot last longer than 7.5 min-
utes under any circumstances, and the average is only 2 to 3 min-
utes. Totality ends when the sun’s bright surface reappears at the
trailing edge of the moon. Daylight returns quickly, and the corona
and chromosphere vanish. Th is corresponds to the moment when
the trailing edge of the moon’s umbra sweeps over the observer.
Just as totality begins or ends, a small part of the photo-
sphere can peek through a valley at the edge of the lunar disk.
Although it is intensely bright, such a small part of the photo-
sphere does not completely drown out the fainter corona, which
forms a silvery ring of light with the brilliant spot of photosphere
gleaming like a diamond (■ Figure 3-11b). Th is diamond ring
eff ect is one of the most spectacular of astronomical sights, but
it is not visible during every solar eclipse. Its occurrence depends
on the exact orientation and motion of the moon.Observing an Eclipse
Not too many years ago, astronomers traveled great distances to
forbidding places to get their instruments into the path of total-
ity so they could study the faint outer corona that is visible only
during the few minutes of a total solar eclipse. Now, many of
those observations can be made every day by solar telescopes inTotal solar eclipses are rare as seen from any one place. If you
stay in one location, you will see a total solar eclipse about once
in 360 years. Some people are eclipse chasers. Th ey plan years in
advance and travel halfway around the world to place themselves
in the path of totality. ■ Table 3-2 shows the date and location of
solar eclipses over the next few years.
Features of Solar Eclipses
A solar eclipse begins when you fi rst see the edge of the moon
encroaching on the sun. Th is is the moment when the edge of
the penumbra sweeps over your location.
During the partial phases of a solar eclipse, the moon gradu-
ally covers the bright disk of the sun (■ Figure 3-10). Totality
begins as the last sliver of the sun’s bright surface disappears
behind the moon. Th is is the moment when the edge of the
umbra sweeps over your location. So long as any of the sun is
visible, the countryside is bright; but, as the last of the sun disap-
pears, dark falls in a few seconds. Automatic streetlights come
on, car drivers switch on their headlights, and birds go to roost.
Th e darkness of totality depends on a number of factors, includ-
ing the weather at the observing site, but it is usually dark
enough to make it diffi cult to read the settings on cameras.
Th e totally eclipsed sun is a spectacular sight. With the moon
covering the bright surface of the sun, called the photosphere,*
you can see the sun’s faint outer atmosphere, the corona, glowing
with a pale, white light so faint you can safely look at it directly. Th e
Total solar eclipses are rare as seen from any one place. If you corona is made of hot, low-density gas that is given a wispy appear-■ Table 3-2 ❙ Total and Annular Eclipses of the Sun, 2010 to 2019*Maximum Length of Total
Date Total/ Annular (T/A) Time of Mid-eclipse** (GMT) or Annular Phase (Min:Sec) Area of Visibility
2010 Jan. 15 A 7 h 11:10 Africa, Indian Ocean
2010 July 11 T 20 h^ 5:20 Pacifi c, South America
2012 May 20 A 23 h^ 5:46 Japan, North Pacifi c, western USA
2012 Nov. 13 T 22 h^ 4:02 Australia, South Pacifi c
2013 May 10 A 0 h^ 6:04 Australia, Pacifi c
2013 Nov. 3 AT 13 h^ 1:40 Atlantic, Africa
2015 March 20 T 10 h^ 2:47 North Atlantic, Arctic
2016 March 9 T 2 h^ 4:10 Borneo, Pacifi c
2016 Sept. 1 A 9 h^ 3:06 Atlantic, Africa, Indian Ocean
2017 Feb. 26 A 15 h^ 1:22 S. Pacifi c, S. America, Africa
2017 Aug. 21 T 18 h 2:40 Pacifi c, USA, Atlantic
2019 July 2 T 19 h^ 4:32 Pacifi c, South America
2019 Dec. 26 A 5 h^ 3:40 Southeast Asia, Pacifi c
The next major total solar eclipse visible from the United States will occur on August 21, 2017, when the path of totality will cross the United States from Oregon to South Carolina.
*There are no total or partial solar eclipses in 2011, 2014, or 2018.
**Times are Greenwich Mean Time. Subtract 5 hours for Eastern Standard time, 6 hours for Central Standard Time, 7 hours for Mountain Standard Time, and 8 hours for Pacifi c
Standard Time.
h hours.*Th e photosphere, corona, chromosphere, and prominences will be discussed
in detail in Chapter 8. Here the terms are used as the names of features you
see during a total solar eclipse.