# Perpendicular velocity of planets

## Main Question or Discussion Point

Hi

I am making a solar system emulation. I need some fairly precise velocities for the planets as a start velocity of the planets and since i calculate the gravity i need it perpendicular in a known distance from the sun.

First of all.. is it correct that if i calculate gravity i would only need the 'start'-velocity(perpendicular) of the planet to be able to simulate a correct path around the sun?

i have searched the web and i don't know where to find them any help?

Kasper

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jedishrfu
Mentor
cepheid
Staff Emeritus
Gold Member
Welcome to PF,

If you assume the orbits are circular, then you can figure out what the speed (which is constant) is just based on the radius.

Of course, the orbits aren't circular, they're elliptical. But given key numbers for each orbit, like the eccentricity and semi-major axis, you can determine what the speed is at any point in the orbit. These numbers are what you need to model things correctly.

i was missing the keyword semi-major axis ;)

You need to find all the Keplerian elements to find perpendicular velocity:

a - Semimajor
e - Eccentricity
i - Inclination
LAN - Longitude of the Ascending Node
LP - Longitude of the Perihelion
ML - Mean Longitude
The Anomalies - Mean, Eccentric and True

You need all of these just to calculate the Radii.