Solar eclipses on Pluto
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Eclipses of the Sun on Pluto are caused when one of its three natural satellites – Charon, Nix and Hydra – passes in front of the Sun, blocking its light.
An eclipse can occur only when one of the satellites' orbital nodes, the points where their orbits cross Pluto's ecliptic, is lined up with the apparent position of the Sun as seen from Pluto. Since all three of its satellites orbit in the same plane, the times at which this is possible are the same for all three. There are only two points in Pluto's orbit where this can happen.
Charon typically presents an angular diameter of between 3 and 4 degrees of arc as seen from the surface of Pluto. The Sun appears much smaller, only 40 arcseconds to 1 arcminute. This means that during eclipses with Charon, a large proportion of Pluto's surface will experience a total eclipse, while partial eclipses, where only some of the Sun's surface is blocked, are quite rare.
There are large uncertainties in the diameters of Nix and Hydra, Pluto's two smaller moons, and as a result their apparent diameters (as seen from Pluto) are also uncertain. However, it is known that Nix's angular diameter is 3-9 minutes, while Hydra's is 2-7 minutes. These are much larger than the Sun's angular diameter, so total solar eclipses are possible with these moons.
The last period when eclipses could occur on Pluto was between August 1985 and June 1990. As seen from Earth, Charon also transited Pluto every orbit during this period. By measuring the change in brightness during these transit events, astronomers were able to measure the radius of both Pluto and Charon. Nowadays, telescopes, such as the Hubble Space Telescope have high enough resolution that the radius can be measured directly.
The next period of time when solar eclipses can occur on Pluto will begin around mid-July 2103, and end around March 2117. During this period, solar eclipses will occur at some point on Pluto every orbit of Charon. The maximum duration of any solar eclipse during this period is about 4 minutes 21 seconds.
