# Inner Solar System Model

The model inner solar system that you see running above is intended to give you a feel for the relationships of the orbits of these planets (and the Earth's Moon). It also allows you to change your viewpoint on the solar system in order that you may see what a planetary system (or a planet and its moons) looks like from other viewpoints. The text entry box below the animation allows you to enter new values for your viewing angle. "pushing the button" then redraws the animation at the chosen viewing angle. An angle of 90° shows the system looking down from the top; an angle of 0° shows the system edge on with the orbits reduced to a line through the center of the Sun. An angle of 45°, halfway between these two extremes is used initially.

There are several important points that must be made about the drawing of this model. The scaling of the objects in the model is very difficult to do. This is because, while the individual objects are large, the orbital sizes dwarf them completely. Therefore the objects and the orbits must be scaled separately, as detailed below. The scaling of the Sun also presents a problem because, while it is small relative to the orbits, it is very large relative to the planets. In the animation, size scales have been adjested to allow easy identification of the objects.

First, the relative sizes (radii/diameters) of the planets are correct within the limitations of integer arithmetic.
Object Model True
Mercury 0.38 0.38
Venus 0.95 1.00
Earth 1.00 1.00
Moon 0.33 0.27
Mars 0.50 0.54

Second, the relative orbital sizes are also correct for the planetary orbits.
Object Model True
Mercury 0.40 0.39
Venus 0.75 0. 72
Earth 1.00 1. 00
Mars 1.50 1. 52

Third, The relative sizes of the planets to the sizes of their orbits is not correct. If the planets were scaled properly to the orbit sizes, they would not be visible. They have been arbitrarily scaled upward by a factor of 1000. Therefore the planets appear 1000 times larger than they should appear if scaled properly with respect to their orbital radii.

Fourth, the Sun is not properly scaled in size. Its radius is only 2.2 times that of the Earth, but is still 500 times oversized with respect to the size of the orbits.

Fifth, the size of the Moon is properly scaled with respect to the Earth and the planets, but the orbital size is not. If the orbital radius were to be properly scaled, the orbit would lie well inside the "Earth" as drawn here. The orbital size was arbitrarily scaled to avoid collision with the Earth and collision with Venus. The inclination of the Moon's orbit to the ecliptic plane is correct. What cannot be seen is the inclination of the Earth's axis to that plane, making the inclination of the Moon's orbit appear extreme.

Sixth, the orbital periods are properly scaled. The Earth makes one orbit (a year) in 40 seconds, in the animation. The Moon orbits the Earth 13 times during that "year." The other planetary orbital periods obey Kepler's Third Law:

P2       is proportional to       a3

or

P       is proportional to       a1.5

Seventh, the orbits are all circular. This is not true in reality but the eccentricities of the orbits of the inner planets are very small and can be ignored here.

Eighth, the use of integer arithmetic to draw the planets makes the planets appear to lie off their orbital paths at times. This is simply a computational effect and is not real.

The images in this animation are calculated in real time. They are not precalculated and then downloaded. It will run for a very long time without repeating.

Credits I was first inspired to write this applet when I saw the applet of a single planet and its moon from the book On To Java by Patrich H. Winston and Sundar Narasimhan. This applet was used as a demonstration of a good use of double buffering in the Java tutorial offered at the ADASS '96 meeting in Charlottesville, VA.

© 1997 Karen M. Strom and the Five College Astronomy Department.