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Frequent events with Jupiter and infrequent events among its satellites
It is worth noting that with steady skies and a medium-size telescope , observers from Earth can see a variety of phenomena involving Jupiter and / or its four , large Galilean satellites . Three of the four satellites regularly participate in all three phenomena with Jupiter . Callisto has event seasons , like eclipse seasons on Earth , and is sometimes out of place for these . During Callisto ’ s seasons , all four of the Galilean satellites mutually affect the view from Earth of each other . The common phenomena are transits of two types , eclipses and occultations . A search of the internet or in monthly astronomy magazines will provide times for all of these events when Jupiter is in the night sky . Just be sure to correct the given time to your time zone . Transits are viewed on the illuminated , day side of Jupiter . The shadows of the satellites crossing Jupiter ’ s illuminated hemisphere are more easily seen . An intense black dot appears surrounded by the bright clouds . Over time , the dot moves as the shadow-casting satellite continues around its orbit . A careful observer using a good telescope under steady conditions will be able to see the satellite , itself , in transit , leading or trailing the shadow spot . ( For a few days over the months Jupiter is in the night sky , the satellite may actually overlap its shadow or be found directly above or below it .) Eclipse disappearances and reappearances around Jupiter can be observed with binoculars . They are just like lunar eclipses here on Earth , but Jupiter ’ s shadow is much darker . The satellite is visible near the planet , then disappears . Alternatively , a satellite may appear rather suddenly after it emerges from Jupiter ’ s shadow . It ’ s very difficult to see a satellites disappearance and later reappearance because Jupiter itself hides one event or the other depending on the relative positions of Jupiter , Earth and Sun . During the few days mentioned in the paragraph above , a satellite would be seen to merge with Jupiter and disappear . It would reappear as a growing on the other side of the planet and then separate from Jupiter ’ s cloud tops . During periods when only an eclipse disappearance
COURTESY OF NASA / JPL / University of Arizona This true-color composite frame , made from narrow angle images taken on Dec . 12 , 2000 , by the Cassini spacecraft , captures Io and its shadow in transit against the disk of Jupiter . The distance of the spacecraft from Jupiter was 19.5 million kilometers ( 12.1 million miles ). The image scale is 117 kilometers ( 73 miles ) per pixel .
is visible , the satellite will emerge from occultation behind Jupiter . During periods when only an eclipse reappearance is visible , the satellite will disappear into occultation behind Jupiter . During Callisto ’ s eclipse seasons ( approximately every six years ), all four of the Galilean satellites can cast shadows on each other and transit or occult each other . ( The correct word to use depends on the sizes of the involved satellites and who is doing what to whom as seen from Earth .) The durations of these events are typically a few minutes but can , occasionally be much longer if the geometry is right . It is worth noting that , when the geometry is right , similar interactions are also possible among all the other outer planets and their satellites , though the frequency of the seasons is generally less . For each planet , the event seasons are twice per orbit around the Sun . For long orbits , there is a long wait until the next season .
Sky ’ s Up 19