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robodude wrote:So my only question is this:
How do you find the semi-major axis if only given the perihelion, aphelion, or major axis?

it's all math. I'd suggest you to go to your high school and borrow the alg. 2 textbooks. they're really useful for stuff like this, but i'll try to explain.

so the equation for an ellipse is
x^2/a +y^2/b = 1
so a is your semimajor axis, and b is 1/2 your minor axis
there is a "c" which is the distance from the center to your foci. using kepler's laws, you know that the sun is one of the foci in the solar system.
so another equation is a^2+b^2=c^2. You can determine your A, B, or C values using this.

sorry i'm really bad at explaining, but an algebra II textbook would help lots for this type of stuff.

Actually, I just figured out! Here's what I figured out:

The major axis is the longest axis that runs through the ellipse.
The minor axis is the shorter axis that runs through the shortest part of the ellipse.
The semi-major axis is half of the major axis, and
the semi-minor axis is half of the minor axis.

Kepler's 3rd law uses the semi-major axis, which is half of the major axis, which is 50,000 AU.

Therefore:

So...

So p then equals 3952847, or about 4 million years, which is the correct answer!
Sorry for the confusion everyone!

that's a wrong question then. they should've given you the major axis, but they gave you the perihelion.

No, wait. The question said that the perihelion was at the Sun, and aphelion was 50,000 AU, so the major axis is 50,000 AU and the semi-major axis is 25,000 AU, so you can use Kepler's Third Law from there.

Yes, this little graphic should make it simpiler to understand:

Just giving us the major/semimajor axis would be too easy.

my only question is how do you determine the major axis if you only have the aphelion?

*sorry aphelion

perihillion = at sun
aphelion = 50,000AU
Therefore, the major axis is 50,000 AU
Refer to my picture.

wow i am stupid....
haha i saw the sun was the perihelion but I was like "how is that possible" so i disregarded that part
oh well, yeah you're right, but they pretty much gave you a major axis, just the question was worded weirdly.

so you do refer to kepler's third law in this question?

Does anyone know where you can get all the practice tests for solar system online?
can you get the questions from state and regionals so you can prepare?
sry for the many questions....