# Orbital Period

## Homework Statement

Halley’s Comet passed perihelion on 9 February 1986. It has a semi-major axis
a = 17.96 AU and eccentricity e = 0.9673. (One astronomical unit (AU) is the distance
between the Earth and the Sun.) Calculate the period of Halley’s Comet and the
approximate date on which you might next see it.

## Homework Equations

Period = T = 2pi(sqrt(a^3/u)

## The Attempt at a Solution

I realize all this information is available over the internet but I need to work it out by hand.
I attempted this question first using the value for 'a' given and the standard gravitational parameter (u) of earth 3.986E14 but realised this is wrong and that halley's comet would have a different value of (u). I assume the equation for the value of (u) of halley's comet will be related to its eccentricity but can't seem to work it out. Any ideas?

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Janus
Staff Emeritus
Gold Member
In determining (u), what does Halley's comet orbit?

In determining (u), what does Halley's comet orbit?
i just assumed from the question that it was orbiting earth.

Janus
Staff Emeritus
Gold Member
What do comets typically orbit?

What do comets typically orbit?
thats a very bad assumption. they orbit the sun and i can just look that value up. but using what ive been given and only those values is it possible to work this question out?

DaveC426913
Gold Member It is an excellent assumption.

carney, I give you my blessing to assume your calculations are meant to have Halley's comet orbitng the Sun, not Earth.