The Solar Shrink 
Whether the sun is now — or ever has 
been — shrinking is a matter of dispute 
by Stephen P. Maran 
“Our wishes lengthen as our sun 
declines,” wrote the English poet Ed- 
ward Young (1683-1765). A few as- 
tronomers now believe that the sun 
is indeed declining or, more specifi- 
cally, that it is shrinking, and they 
have revived an almost-forgotten con- 
troversy on the subject. 
Obviously, no dramatic change is 
under way on the sun or we would 
all know about it, since the amount 
of light and heat reaching the earth 
depends on the size and temperature 
of the sun. If the sun pulsated no- 
ticeably, alternately shrinking and 
swelling like the variable star Mira, 
for example, there would be corre- 
spondingly radical trends in the earth’s 
weather. The possibility remains that 
the sun may be shrinking slowly, with 
imperceptible immediate effects. 
However, even a slight but continuing 
change would eventually be reflected 
in a detectable climate change on the 
earth. 
The current controversy was ignited 
in 1979 by two Boston-area research- 
ers, John A. Eddy, then with the Cen- 
ter for Astrophysics in Cambridge, 
Massachusetts, and Aram Boornazian, 
with the private research firm of S. 
Ross and Company. But the debate 
goes back at least to 1809, when the 
great German mathematician and as- 
tronomer Karl Friedrich Gauss dis- 
puted a report that the sun’s size had 
decreased during the latter half of the 
eighteenth century. 
The evidence Gauss took issue with, 
like much of the evidence in the cur- 
rent controversy, consisted of direct 
measurements of the diameters of tele- 
scopic images of the sun. The alleged 
shrinkage under consideration today 
is so small — amounting at most to a 
decrease of a few feet per hour in 
the 865,000-mile solar diameter — that 
finding it depends on very small 
changes in the measurements of the 
images from year to year and decade 
to decade. Nevertheless, the solar im- 
age is blurred by our dusty and tur- 
bulent atmosphere, and critics claim 
that the reported trend may be due 
to changes in the telescopic equipment 
or in the atmosphere or to switches 
in the observing astronomers. In fact, 
analyses of solar-diameter measure- 
ments made at different observatories 
yield different results. 
Some statisticians have found a 
shrinkage in the measurement data, 
some have found none, and some have 
claimed to detect gradual up-and- 
down trends in the solar diameter, as 
though the sun were pulsating like 
Mira, albeit by a vastly smaller 
amount and over a much longer pe- 
riod. The claimed periods, depending 
on which report you read, include ones 
of eleven years (the length, inciden- 
tally, of the sunspot cycle) and of 
twenty-two years (the length of a re- 
lated phenomenon called the solar 
magnetic cycle). In view of the con- 
flicting reports, about the only thing 
we can conclude at the present time 
is that the direct diameter measure- 
ments do not provide unequivocal evi- 
dence for any change in the size of 
the sun. There are, nevertheless, in- 
direct ways of gauging the solar di- 
ameter that may be less affected by 
changes in the observing equipment, 
the observing personnel, and the 
earth’s atmosphere. 
One way to check on any change 
in the size of the sun is to investigate 
the accumulated observations of the 
transits of Mercury. (The transits are 
the passages of that tiny planet in 
front of the sun, as seen from the 
earth. They occur about thirteen times 
per century.) During transits, Mercury 
looks like a black dot against the bril- 
liant solar surface; by timing the du- 
ration of a transit, one can deduce 
the size of the sun. In April 1980, 
a physicist at the Massachusetts In- 
stitute of Technology reported that 
analysis of transit timings made from 
1736 to 1973 showed no solar shrink- 
age, although the data were capable 
of revealing a shrink rate six times 
smaller than had been claimed by 
Eddy and Boornazian. 
Observations of eclipses of the sun 
represent another source of indirect 
information on the solar diameter. 
Surprisingly sensitive determinations 
can be made from very simple ob- 
servations of eclipses. The size and 
orbit of the moon are known with high 
precision, as are the orbit of the earth 
and the earth’s distance from the sun. 
Thus, it should be possible to calculate 
the circumstances of a solar eclipse 
with considerable accuracy. The cal- 
culations can then be compared with 
actual observations. For example, if 
the sun were larger than assumed, then 
the extent to which it is covered by 
the moon would be diminished, and 
the umbra, or shadow of the moon 
cast on the earth during a solar eclipse, 
would be narrower than expected. As 
the umbra moves across the earth, 
it traces out the “path of totality.” 
If the sun were smaller than assumed 
in the calculations, the path would 
be wider than was computed and the 
duration of totality witnessed by a 
fixed observer on, say, the center line 
of the path would be longer than pre- 
dicted. 
In fact, an account of a sixteenth- 
century eclipse was used as an ar- 
gument for the validity of solar shrink- 
age by Eddy and Boornazian, who also 
found an apparent decrease in the di- 
rect measurements made during the 
years 1 836 to 1 953 at the Royal Green- 
wich Observatory in England and 
from 1846 to 1950 at the U.S. Naval 
Observatory in Washington, D.C. 
They cited a description of the eclipse 
of April 9, 1567, written by the Ger- 
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