  We are reporting on a new accurate photographic light curve of Pluto for
1933-1934 when the heliocentric distance was 40 AU. We used 43 B-band and
V-band images of Pluto on 32 plates taken on 15 nights from 19 March 1933 to 10
March 1934. Most of these plates were taken with the Mount Wilson 60" and 100"
telescopes, but 7 of the plates (now at the Harvard College Observatory) were
taken with the 12" and 16" Metcalf doublets at Oak Ridge. The plates were
measured with an iris diaphragm photometer, which has an average one-sigma
photometric error on these plates of 0.08 mag as measured by the repeatability
of constant comparison stars. The modern B and V magnitudes for the comparison
stars were measured with the Lowell Observatory Hall 1.1-m telescope. The
magnitudes in the plate's photographic system were converted to the Johnson B-
and V-system after correction with color terms, even though they are small in
size. We find that the average B-band mean opposition magnitude of Pluto in
1933-1934 was 15.73 +- 0.01, and we see a roughly sinusoidal modulation on the
rotational period (6.38 days) with a peak-to-peak amplitude of 0.11 +- 0.03
mag. With this, we show that Pluto darkened by 5% from 1933-1934 to 1953-1955.
This darkening from 1933-1934 to 1953-1955 cannot be due to changing viewing
geometry (as both epochs had identical sub-Earth latitudes), so our
observations must record a real albedo change over the southern hemisphere. The
later darkening trend from 1954 to the 1980s has been explained by changing
viewing geometry (as more of the darker northern hemisphere comes into view).
Thus, we now have strong evidence for albedo changes on the surface of Pluto,
and these are most easily explained by the systematic sublimation of frosts
from the sunward pole that led to a drop in the mean surface albedo.
