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
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 ||
| Venus || 0.75 || 0.
| Earth || 1.00 || 1.
| Mars || 1.50 || 1.
- 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
P2 is proportional to a3
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
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.