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5.5 The OuterPlanetChart Function

OuterPlanetChart produces a graphic plate that is typically glued to the back of a planisphere created with Planisphere. It lets you determine the position of the outer planets month by month for the next few years (15 years for Mars; 11 years for Jupiter; and 28 years for Saturn).

Charting the future positions of the outer planets.

The general procedure for using OuterPlanetChart is to generate it once and then glue it to the back of the Planisphere plate. The existing curves will last for 11 years or more before you need to generate another OuterPlanetChart graphic.

Once the OuterPlanetChart graphic is printed, you can easily read off the position of the outer planets for any given month. To find the position of a planet, look for its curve line and locate the year mark you are interested in. Then count dots along the curve to reach the month you want, remembering that the initial year mark is January, the immediate dot after that is February, and so on. After finding the dot, move radially out to the rim to read off the right ascension.

Here is the OuterPlanetChart starting at year 1994. From this chart you can see that in March 1996, Jupiter has a right ascension of about 19h. If you paste this graphic to the back of a Planisphere plate, then you can turn over the plate and locate the 19h point on the ecliptic line to find where Jupiter is in relation to the stars during that month.


Month names are written around the rim of the graphic, as well as right ascension hours. The position of a month name indicates the direction of the Sun at the start of that month.

One final point to note in the graphic is that you can clearly see the retrograde motion of Mars nearly every couple of years. Jupiter and Saturn also have retrograde motion about once a year, but the angle that these planets move backward in the sky is much less than for Mars. In the case of Mars, the distance of the curve from the center, which represents the Earth, gives an indication of true distance and hence the apparent brightness.

The graphic produced by OuterPlanetChart is only suitable for printing on, say, a laser-quality printer. It is not intended to be readable on the screen.