By the Kepler's first law, all such orbital motion is elliptical. However, in the case shown here, that of the gas giant planet Jupiter and it's satellite Ganymede, the ratio of primary mass to satellite mass is so high that Ganymede's orbital eccentricity is almost zero, i.e. a circle. Jupiter and Ganymede are not shown to scale here; if they were, Ganymede would be an invisible dot.
As Ganymede travels in its orbit, it's position is plotted onto the graph at the right. Notice how the values that make up this graph form the locus of a sin curve. The graphs vertical axis is calibrated in units of Ganymede's orbital radius. The horizontal axis represents angle of rotation in degrees.
Note as Ganymede orbits the planet, the angle and it's sin is displayed at the bottom right. In the first two quadrants of the orbit, the value of sin is positive; in the last two quadrants it is negative.
Ganymede is the largest planetary satellite in the solar system, much larger than Earth's Moon, and larger than the planet Mercury.
Copyright©2012 Koala Publishing Ltd, Bristol BS 16 5SE, United Kingdom. www.koalapub.co.uk