Jupiter's Galilean satellites have provided viewing interest for
astronomers since the 1600's. Sky and Telescope magazine regularly
publishes their positions v. time for each month. This only gives the
x-coordinate however. Using a published algorithm, both the x- and
y-coordinates are calculated and presented in three forms.
The top window gives the classical display seen in Sky and Telescope.,
giving x-coordinate v. time graphically. The middle window gives a
telescope eye view of Jupiter and the satellites updated for each time
interval in the manner of a slide show. Each satellite is displayed on
the disk when transiting Jupiter and not drawn when it is occulted.
Eclipses are not displayed due to lack of phase angle calculations.
(The phase angle is the Sun-Jupiter-Earth angle.) The bottom window
gives the graphical data for each time interval. The positions given
are accurate to within 0.1 Jupiter radii.
Once executed, the month, day and year are input (ex.: 1,1,1988.)
Decimals of a day are accepted and the time scale used is Universal
Time. The second input is the time interval between updates. The
default value is 0.5 hours, which gives the best graphical display in
the upper window. Once the simulation is running, keystroke commands
control it. Typing "Q" quits the simulation; "R" allows another
date/interval to be entered; and any other key pauses the simulation
(a second keystroke restarts the simulation.)
This simulation of Jupiter's Galilean satellites is based on the
algorithm found in Jean Meeus' book, Astronomical Formulae for
Calculators (pp. 175-183), was written using the ZBasic compiler, and
was written and debugged on a Macintosh SE.
