5.6 The PtolemyChart Function
PtolemyChart produces a graphic plate that can be glued to the back of a planisphere created with Planisphere. This function is not as useful as OuterPlanetChart, which produces a more accurate graphic. The PtolomyChart function lets you determine the position of the planets month by month for the next few years (5 years for Mercury; 8 years for Venus; 15 years for Mars; 11 years for Jupiter; and 28 years for Saturn).
Charting the future positions of the planets.
Mercury and Venus are always quite close to the Sun so it is relatively easy to determine their approximate positions for many years in advance. In the graphic, the Sun's position is shown in the outer ring, even though Ptolemy would have placed it in the third ring.
An important point to remember is that the position of Venus repeats almost exactly after eight years. Similarly, the position of Mars repeats roughly every fifteen years.
Big dots represent the start of a new year, that is, January 1, and smaller dots represent months within that year. The smaller dots can sometimes clump together when the planet is undergoing retrograde motion as seen from Earth.
This graphic shows, for example, that Jupiter is in the constellation of Capricornus throughout the years 1997 and 1998. Mars is in Virgo from January to August, 1997.