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Determining the Shape of the Solar System

INTRODUCTION. This project is in many ways similar to that for determining the shape of the earth-moon-sun system. In fact, you should start out the very same way - finding the orientation of the earth's orbit around the sun. We shall be using the terrestrial orbital plane as our "reference plane," and making a valiant attempt to orient the planes of the other planets to this reference. In particular, we shall be interested in the orbits of Venus, Mars, Jupiter and Saturn.

Because not all of these planets will be visible at one time, this project could be a long one - even as long as 9 months. But remember that the length of the project does not mean that it is very difficult to do. It is a "catch-as-catch-can" type of project. About the only trouble the extended time will impose on you is that you will have to re-establish your earth-orbit plane (i.e.: your stakes and cardboard) on the days immediately before you do your night-viewing of the planets. But, welcome to astronomy! You are asking: how do I know when to set up the cardboard BEFORE the night of viewing? This is not too difficult: if one night you see some planets, and the weather report is for fair weather for the next 36 hours, you know that you can set up your stakes and cardboard the next day, and do your viewing the next night. (If you are observant, you will notice that "your" selected planet has moved a little among the background of true stars between your two nights. Afterall, the word planet derives from "wanderer.")

Placement of your cardboard plane for planetary observations is a bit more tricky than for watching the sun. Planets, such as Venus particularly, are often low to the horizon. Thus your set-up should have a fairly good view of the eastern and western horizons.

Because you will want to know whether a given planet is on one side or the other of your earth orbital plane, you might want to make a slight modification of your stakes and cardboard. First, where the stakes go into the ground should be a foot or more apart. Second, you should put a small nail into each stake about a foot above the ground. Let these nails support the cardboard. Now you can place an old pillow on the ground between the stakes. Lie down and put your head on the pillow. Your eyes will be able to sight directly up one side and the other so that you can see exactly whether a planet is sighted on one side or the other. You might also want to keep a record in your diary of how far away from the plane the planet is. If you see, for example, that jupiter is on one side of the plane one month, and several months later it is on the other side, you will know that jupiter's orbital plane intersects with the earth's orbital plane. In other words, the two orbits are not exactly coplanar.

If you are so lucky as to see a comet, which lasts for many nights before it "sets" as it approaches the sun, you might want to determine its orbital plane relative to that of the earth's plane. You know enough now to be able to figure out how to do that. How to find the planets in the first place. Probably by this time you have read that the solar system is rather "flat" - all the planets have orbits that a moderately close to being coplanar - except for Pluto, which is "way out there". But you cannot see Pluto anyway, so we will ignore it until you advance far enough to have access to a multimillion dollar telescope! Thus you can expect to find the planets somewhere close to the ecliptic. Happily, they are rather easy to find because they are usually among the brightest objects in the nighttime sky. (Of course, if you see something bright that you can see perceptibly moving across the sky, it might be a high flying airplane - OR an orbiting space lab that is still reflecting the sun's rays.) There are several bright true stars - Sirius, Regulus, etc., but, fortunately, none of these lie on the ecliptic. For some of the latest information of where and when to find the planets, click .

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