U.S. Corps of Topographical Engineers

Checking Greenwich Time Using Jupiter's Moons
Callisto, Europa, Io, and Ganymede

Approximate Sidereal Times of the Occultations of Jupiter's Satellites by Jupiter, and of the Transit of the Satellites and their Shadows over the Disc of the Planet.

These phenomena are inserted in order to apprise Astronomers when they are about to happen, as observations of them may tend to improve the Tables of the Satellites.  The instruments required to observe them with anything like precision will preclude the possibility of their ever becoming available at sea.  The times are given in days, hours, and minutes;  the day being supposed to commence at mean noon, and the hours and minutes representing sidereal time, such as will be shown by a sidereal clock on that day.


(Key to animated image:  Jupiter is large body in center;  C is Callisto;  E is Europa;  G is Ganymede;  I is Io)

     The Phenomena for each Satellite are arranged under three distinct heads, and each in the order of the days of the month, so that an inspection of the columns opposite to each Satellite is necessary to determine what phenomena will happen on a given day.
     An asterisk annexed to the day of the month, signifies that the phenomena is visible at Greenwich, and a dagger, that the phenomenon may be visible under favourable circumstances, the limits in either case being the same as those adopted for the eclipses.
     In the month of September, 1843, under the general heading "Occultations," opposite to Satellite I., and under Immersion, the first quantity recorded is 1d 5h 26m, which signifies that at 5h 26m sidereal time on September the 1st an Immersion of the 1st satellite takes place, but that it is invisible at Greenwich.  Under Emersion we find, for the whole of the month, "In the shadow," which signifies that the Emersion of the Satellite cannot be seen, because, although it ceases to be occulted by the body of the Planet, it is still involved in its shadow, from which it does not indeed escape until 8h 10m 12s.4 sidereal time.  (See Eclipses of the Satellites of Jupiter on the preceding page of the month.)  Again, in the column of Occultations opposite to Satellite III., it appears that the 3rd Satellite is occulted on the 23rd day of the month;  that it disappears behind the disc of the Planet at 10h 9m, and reappears on the 24th day at 13h 51m, Sidereal time;  but that both Immersion and Emersion are invisible at Greenwich.
     In the column headed "Transits of Satellites,' the first transit of Satellite I. at Greenwich appears to be on the 2nd day, when the ingress takes place at 2h 43m, and the egress at 5h 3m, Sidereal time; that is, it comes in contact with Jupiter's disc at 2h 43m, remains on the disc 2h 20m, and quits it again at 5h 3m, sidereal time; both ingress and egress are invisible at Greenwich.
     The Transits of Shadows are to be interpreted in a similar manner.



Lords Commissioners of the Admiralty,  The Nautical Almanac and Astronomical Ephemeris for the Year 1843.  William Clowes and Sons, London, 1839.  (text and image)

Animated Galilean Moons of Jupiter adapted from Gif created by Ed Stephan, Western Washington University.  Created from observations  made April 19, 1997, every 10 minutes from 10 pm to 4 am, from Bellingham, Washington,.

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