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gopanda13 wrote:Do you guys know of a good website that has which constellations the planets will be in on April 17th?

http://www.astronomy.com/asy/default.aspx?c=a&id=2297
This has seasonal star charts, which may be more useful than daily constellations, and I have found that they include most of the constellations specified

I just recently took a practice test, and one of the questions on it was:

A faint object with a perhelion at the Sun has been discovred in the solar system with an aphelion at 50,000 astronomical units. How many years will it take to reach the Sun?

I don't just want the answer, but how you solve it so I can do the same later. I understand that it has to do with Kepler's 3rd law () but how? How do you solve this?

Thanks!

robodude wrote:I just recently took a practice test, and one of the questions on it was:

A faint object with a perhelion at the Sun has been discovred in the solar system with an aphelion at 50,000 astronomical units. How many years will it take to reach the Sun?

I don't just want the answer, but how you solve it so I can do the same later. I understand that it has to do with Kepler's 3rd law () but how? How do you solve this?

Thanks!

Kepler's third law.
the length of the semimajor axis in AU ^3= years of orbit ^2
50000^3=years of orbit^2
simplify and it's 11180339.89 years.

someone please correct me if I'm wrong, i haven't done this event since regionals and this is all off my memory...

new_horizon wrote:

robodude wrote:I just recently took a practice test, and one of the questions on it was:

A faint object with a perhelion at the Sun has been discovred in the solar system with an aphelion at 50,000 astronomical units. How many years will it take to reach the Sun?

I don't just want the answer, but how you solve it so I can do the same later. I understand that it has to do with Kepler's 3rd law () but how? How do you solve this?

Thanks!

Kepler's third law.
the length of the semimajor axis in AU ^3= years of orbit ^2
50000^3=years of orbit^2
simplify and it's 11180339.89 years.

someone please correct me if I'm wrong, i haven't done this event since regionals and this is all off my memory...

Well, the answer packet says that its or about 4 million years, but then you have to divide by two... so its about 2 million years.

I'm still confused, though...

that's 1.98 million years, not 4 million years.
I think I know where I went wrong, they give the perihelion while you need the semimajor axis to use kepler's law. Unfortunately, they're not the same.
Is that all they give you or do they also give you the aphelion? another foci? I don't think it's solvable without some other point.

If you do know something else, an axis or the aphelion, you could use what you know about ellipses (this is taught in alg. II), and use the equations to figure out what your semimajor axis is, and then go from there.

does semi-major axis mean the perihelion or the aphelion?

The semi major axis is 1/2 (aphelion + perihelion)

go the the 2010 test exchange. I have created a test (by smarticle 13) to help prepare for the solar system event. It is 50 questions long, and I did the best I could to cover the objectives. Most of the questions are on the characteristics of the planets. There are some true/false questions and short answer. I would recommend you take it if you are doing this event for state.
http://scioly.org/wiki/2010_Test_Exchange#Solar_System

Also, if there is a grading error or mistake in the test, let me know (send me a personal message) and I will fix it as soon as I can.

rfscoach wrote:The semi major axis is 1/2 (aphelion + perihelion)

oh yeah, right!
is it possible for you to assume that the perihelion is 2/3 of the major axis?
if you can do that,
so it is 75000 for the major axis, half that and it's 37500, plug it into kepler's and it's 7261843.774 years.
I don't know what's with the answer key, but that seems to be the right answer...

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