Research of eclipses
The first albedo maps of Pluto and Charon that illustrated the light and dark areas as well as surface features on the planetary surfaces were produced during the time period from 1985 through 1990. The data necessary to assemble these maps came from the frequent eclipses between Pluto and its satellite, where scientists would measure the curvature brightness between the two bodies as they visually crossed in front of each other. The data collected also led to the first accurate calculations on the sizes of Pluto and Charon, as well as illustrating the surface reflectivity of the planetary bodies.
In determining the albedo maps of Pluto and Charon, scientists recorded the changing brightness with respect to time as Charon slowly blocked regions of Pluto (and vice versa) during its orbit. In the first eclipses observed, the north polar region was blocked. This was followed by the equatorial region and finally the south polar region. From the data collected, Pluto has a highly reflective south polar cap, a relatively dimmer north polar cap and a mixture of bright and dark features on the equator. The geometric albedo of Pluto is 0.49 to 0.66, much brighter than its satellite Charon, having an albedo of 0.36 to 0.39.
The eclipses enabled scientists to take accurate measurements of Pluto and Charon's sizes. By carefully timing the eclipses from beginning to end (which lasted sometimes up to four hours), preliminary measurements for the diameters of Pluto and Charon were taken. Imaging from the Hubble Space Telescope was also used to measure the sizes and come within one percent of the actual value. Pluto's diameter is 2,274 kilometers (1413 miles) while Charon's diameter is 1,172 kilometers (728 miles). They are separated at an average distance of 19.640 kilometers (12,200 miles).