120 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1944 
been observed by spectroscopic methods that the angular rotation of 
the sun’s surface is much faster at the Equator than near the Poles. 
Adams found the following times of rotation as viewed from a fixed 
star: 
Solar latitude £2: 7) Sif 0° 30° 45° 60° 80° 
Rotation) period = 24, 7 26. 7 28. 0 81.2 35. 3 
The earth revolves about the sun in 365% days, and approximately in 
the same direction that the sun rotates on its axis. Consequently the 
solar rotation appears slower as viewed from the earth, which adds over 
7 percent to the sun’s apparent time of rotation. The effective mean 
period of solar rotation viewed from the earth may be taken as 27 
days. 
FACULAE AND SUNSPOTS 
In a telescope, as shown in plate 1, the sun’s surface is seen to be 
mottled, but at some places to show decidedly brighter areas called 
faculae which are most prevalent in the neighborhood of sunspots. 
Sunspots appear as darker dots on the sun’s surface, but they are 
dark only by contrast. Langley compared the faculae to white-hot 
steel in a converter, which made the molten steel look like chocolate. 
Though sunspots appear small on the enormous disk of the sun, actually 
many of them are so large that the earth, 8,000 miles in diameter, 
would only occupy a corner of one. Sunspots are seldom within 10° 
of the sun’s equator or more than 30° away from it. They, of course, 
rotate along with the surface of the sun at such latitudes, and their 
average time of rotation is about 27 days, as viewed from the earth. 
SOLAR VARIATION AND SOLAR ROTATION 
Sunspots are like machine guns shooting electric ions into space. 
These ions plentifully strike and are captured by the earth’s atmos- 
phere. With ions from other sources they make up that high-level 
electrical reflecting surface in our atmosphere which causes radio rays 
to bounce along the surface of the earth for thousands of miles, in- 
stead of losing themselves at once into limitless space. As the sun 
rotates on its axis the conical! columns of flying ions sent out from sun- 
spots sweep through space. The columns from those spots which are 
nearly central on the sun’s apparent disk encounter the earth for the 
short time of 2 or 3 days. From certain observations we made in 
March 1920, it seems that such a column of ions, 93 million miles long 
between the sun and the earth, by scattering the sun’s rays sometimes 
reduces the intensity of the sun beam at the earth by as much as 5 
percent. Ordinarily such effects are much less, seldom exceeding 1 
percent. But it is easy to see that the rotation of a spotted sun, by ionic 
scattering, may produce successions of small variations of the solar 
