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G. W. Lockwood (Lowell Observatory)
Using a 0.5-m telescope dedicated to precision photometry, we have observed Uranus, Neptune, and Saturn's moon Titan at every apparition since 1972 using the same photomultiplier and interference filters throughout. We usually obtain at least 10 and sometimes many more Stromgren b (472 nm) and y (551 nm) differential observations per season. The overall accuracy of each object's opposition magnitude, normalized to zero degrees solar phase angle and fixed values of heliocentric distance, is 0.005 mag or better, allowing us to map out seasonal and intrinsic variability on decadal time scales.
Titan has completed a full 29.5 year orbit around the Sun since we began observing it, and reveals a closed sinusoidal light curve that constrains models of its unusual flip flopping hemispheric albedo asymmetry. Uranus, now observed over nearly half its 86-year orbital period, shows a basically symmetrical light curve centered on the 1985 southern hemisphere summer solstice, but varies intrinsically as well and has a large north-south albedo asymmetry. Neptune, whose variations appeared to mirror the solar activity cycle during the early years of the program (and which may still do so), has brightened steadily over the past 20 years and is currently at its brightest level in decades as it approaches 2007 southern hemisphere summer solstice.
The long-term light curves of these objects, when combined with imaging by the Voyager spacecraft and Hubble Space Telescope and ground-based data, provide insights into planetary atmospheric activity on time scales from days to decades.
This work is supported by NASA Planetary Astronomy grant NAG5-7853.