The Solar System

CHAPTER 8 | THE SUN 161

The Solar Constant

Even a small change in the sun’s energy output could produce
dramatic changes in Earth’s climate. Th e continued existence of
the human species depends on the constancy of the sun, but we
humans know very little about the variation of the sun’s energy
output.
Th e energy production of the sun can be monitored by add-
ing up all of the energy falling on 1 square meter of Earth’s sur-
face during 1 second. Of course, some correction for the
absorption of Earth’s atmosphere is necessary, and you must
count all wavelengths from X-rays to radio waves. Th e result,
which is called the solar constant, amounts to about 1360 joules
per square meter per second. Is the sun really constant? A change
in the solar constant of only 1 percent could change Earth’s aver-
age temperature by about 1°C (roughly 2°F). Th is may not seem
like much, but during the last ice age Earth’s average temperature
was only about 5°C cooler than it is now.
Some of the best measurements of the solar constant were
made by instruments aboard the Solar Maximum Mission satel-
lite. Th ese have shown variations in the energy received from the

Sun

Computer model of HD 12545

Bright spot

4800 K

Dark spot

3500 K

Average

temperature

4500 K

(a)

(b)

Emission by ionized calcium is

associated with sunspots and can

be detected in spectra of other stars.

0.1

0.2

0.2

0.1

1970 1975 1980

Year

1985 1990

Calcium flux

Calcium flux

The star 107 Piscum has

spots and varies in a cycle.

The star Tau Ceti appears

to have no spots.

■ Figure 8-16

(a) Although all stars other than the sun are much
too far away to have their surface details imaged,
astronomers have found clear evidence that other
stars have spots. Detailed analysis of absorption
lines in the spectrum of the star HD 12545 allow
astronomers to map the location of large spots.
(Model: K. Strassmeier, Vienna, AURA/NOAO/NSF) (b) In
addition, long-term studies of calcium emission
show that some stars have active regions like those
around sun spot groups on our sun. (Ca II emission
adapted from data by Baliunas and Saar)

sun of about 0.1 percent that lasted for days or weeks.

Superimposed on that random variation is a long-term decrease

of about 0.018 percent per year that has been confi rmed by

observations made by sounding rockets, balloons, and satellites.

Th is long-term decrease may be related to a cycle of activity on

the sun with a period longer than the 22-year magnetic cycle.

Small, random fl uctuations will not aff ect Earth’s climate,

but a long-term decrease over a decade or more could cause

worldwide cooling. History contains some evidence that the

solar constant may have varied in the past. As you saw on

page 157, the “Little Ice Age” was a period of unusually cool

weather in Europe and America that lasted from about 1500 to

1850.* Th e average temperature worldwide was about 1°C cooler

than it is now. Th is period of cool weather corresponded to the

Maunder minimum, a period of reduced solar activity—few

sunspots, no auroral displays, and no solar coronas visible during

solar eclipses.

*Ironically, the Maunder minimum coincides with the reign of Louis XIV of

France, the “Sun King.”