3.4 The JupiterCoordinates Function
JupiterCoordinates determines the precise positions of the main moons of Jupiter. It can also determine the positions of various cloud features on the surface of Jupiter, such as the Great Red Spot.
Calculating position in Jovian coordinates.
The first coordinate returned by JupiterCoordinates indicates the number of Jovian radii east (in the right ascension direction) of Jupiter. The second coordinate is the number of Jovian radii north (in the declination direction) of Jupiter. The third coordinate is the object's distance from Jupiter in Jovian radii in the direction away from Earth.
You can apply JupiterCoordinates to the four Galilean moons Io, Europa, Ganymede, and Callisto. You can also apply the function to the Great Red Spot JupiterGreatRedSpot.
Find the Jupiter coordinates of the moon Io on the given date.
JupiterGreatRedSpot gives the Jupiter coordinates of the Great Red Spot on the same date.
Remember that the position of the Great Red Spot is somewhat unpredictable. However, once its position, or Jovian longitude, is known on a particular date, it moves slowly relative to that position over time.
Specifying the position of the Great Red Spot.
The Great Red Spot stays roughly fixed in the System II coordinate system of Jupiter, but over the years it slowly drifts. Apart from that drift, the spot moves around Jupiter about once every 9 hours 55 minutes. Some values for $JupiterGreatRedSpotLongitude are -45° (June, 1990), -33° (June, 1992), -40° (June, 1994), -41° (July, 1994), -42° (July, 1995), -49° (November, 1995), -51° (September, 1996), -61° (November, 1996), and -62° (July, 1997). The default is -40°. You can sometimes obtain current values by consulting the magazine Sky & Telescope, published by Sky Publishing Corporation, Belmont, MA. Note that they use the opposite direction than the one used here for the definition of positive Jovian longitude-that is, west is positive, rather than east-so you must remember to use the negative of these Jovian longitudes in Scientific Astronomer.
The central meridian of Jupiter is the semicircular line passing through the poles of Jupiter and the center of the disk as seen from Earth. Features on Jupiter rotate with respect to the central meridian roughly every 10 hours. The rotation rate is, however, latitude dependent, but it neatly splits into two systems. System I is the equatorial belt with latitudes between +10 and -10 degrees. This system makes a full rotation in a period of about 9 hours 50 minutes 30 seconds, although System I is actually defined so that the mean motion is exactly 877.90 degrees per day. System II consists of all latitudes outside System I. All features in this region complete a full rotation in a period of about 9 hours 55 minutes 40 seconds, although System II is defined so that the mean motion is exactly 870.27 degrees per day. The Great Red Spot is near the top of the southern portion of System II.