  Daily records of sunspot group areas compiled by the Royal Observatory,
Greenwich, from May of 1874 through 1976 indicate a curious history for the
penumbral areas of the smaller sunspot groups. On average, the ratio of
penumbral area to umbral area in a sunspot group increases from 5 to 6 as the
total sunspot group area increases from 100 to 2000 microHem (a microHem is a
millionth the area of a solar hemisphere). This relationship does not vary
substantially with sunspot group latitude or with the phase of the sunspot
cycle. However, for the sunspot groups with total areas <100 microHem, this
ratio changes dramatically and systematically through this historical record.
The ratio for these smallest sunspots is near 5.5 from 1874 to 1900. After a
rapid rise to more than 7 in 1905 it drops smoothly to less than 3 by 1930 and
then rises smoothly back to more than 7 in 1961. It then returns to near 5.5
from 1965 to 1976. The smooth variation from 1905 to 1961 shows no indication
of any step-like changes that might be attributed to changes in equipment or
personnel. The overall level of solar activity was increasing monotonically
during this time period when the penumbra-to-umbra area ratio dropped to less
than half its peak value and then returned. If this history can be confirmed by
other observations (e.g., Mt. Wilson or Kodaikanal) it may impact our
understanding of penumbra formation, our dynamo models, and our estimates of
historical changes in the solar irradiance.
