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18 above the clouds on Venus can see occasional transits of Mercury . NASA ’ s rovers on Mars have returned images , converted to video , of transits of Mars ’ satellites Phobos and Deimos . Both are too small to completely cover the Sun ’ s disk . NASA ’ s Mars orbiters have captured the shadow of Phobos crossing Mars ’ surface as weather satellites have captured the Moon ’ s shadow crossing Earth ’ s surface . The Martian moons ’ transits are frequent for a band centered on Mars ’ equator during “ eclipse seasons ” around the times of the vernal and autumnal equinoxes on the planet . These are the times when the Sun ’ s position in the sky intersects with their orbits , which are in the same plane as Mars ’ equator . Their short orbital periods , hours to a couple of days , allow many events to occur during the season . ( These parameters are not true for Earth ’ s Moon , whose orbital plane is tilted off of Earth ’ s equatorial plane and changing orientation all the time . In addition , the Moon ’ s orbital speed is much slower . The tilt leads to there only being two or three eclipse seasons per year , when the Sun and Moon are both close to the intersection of the projection Earth ’ s equator and the Moon ’ s orbit on the sky .) From Jupiter ’ s cloud tops ( there is no hard or liquid surface ), Jupiter ’ s four largest satellites , Io , Europa , Ganymede , and Callisto , all appear much larger than the Sun when they occult ( eclipse ) it . Jupiter ’ s 65 other satellites ( two new discoveries just announced , with , perhaps , more to be discovered ) are all small and fairly distant so an observer there would see them transit the Sun ’ s disk . The sidebar describes other phenomena that can be observed from Earth . They involve Jupiter and the four large Galilean satellites and phenomena involving each other . As seen from Earth , Saturn ’ s largest satellite , Titan casts a black , pinprick shadow on the disk of the planet during Saturn ’ s eclipse seasons . Observing from Saturn ’ s cloud tops ( there is no hard or liquid surface ), Titan appears much larger than the Sun when it occults ( eclipses ) the Sun . Saturn ’ s 61 other satellites ( with , perhaps ,
COURTESY OF NASA / JPL-Caltech / Malin Space Science Systems / Texas A & M Univ .
This set of three images shows views three seconds apart as the larger of Mars ’ two moons , Phobos , passed directly in front of the sun as seen by NASA ’ s Mars rover Curiosity . For a video of the event , click the image .
more to be discovered ) are all small and fairly distant so an observer there would see them transit the Sun ’ s disk . Saturn ’ s rings would blot out the Sun on the cloud tops during Saturn ’ s eclipse seasons but just what that would look like is somewhat uncertain . NASA ’ s Cassini mission has provided many spectacular images of conjunctions of Saturn ’ s satellites . Observing mutual eclipses , transits and occultations is not impossible from Earth , but difficult . The satellites of Uranus would only be seen to transit the Sun from its cloud tops . Mutual events during eclipse season would be very difficult to observe from Earth . Triton is the largest satellite of Neptune but it would still not create a total eclipse of the Sun . Neptune ’ s many other satellites only do transits during Neptune ’ s eclipse seasons . Mutual events would be extremely difficult to observe from Earth . Though it is considered a dwarf planet , Pluto ’ s five satellites would still have all the events mentioned for Jupiter but even its largest moon , Charon , only transits the distant Sun . The last eclipse season was in the 1980s and the mutual events of Pluto and Charon were used to map each other , at very low resolution , from Earth . The next eclipse season is around 2100 CE . We on Earth are fortunate to be able to see the unique beauty and the phenomena revealed by a total solar eclipse . Go out and watch one !
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