Useful Formulae in Astronomy

circumference C of a circle/sphere of radius r: C = 2 pi r
(pi=3.1415927 should be good enough)

surface area A of a circle: A = pi r2

surface area A of a sphere:  A = 4 pi r2

volume V of a sphere: V = (4/3) pi r3

volume V of a cylinder with radius r and length l:  V = pi r2 l

Force:

Newton's second law:  F = ma where F is force, m is mass, a is acceleration

Gravitational force:  F = GMm/r2 where G is the gravitational constant,
M and m are the masses of the two bodies (for example the Sun and Earth) and
r is the distance between the two

Centripetal force (which keeps an object moving in a circle): F =
mv2/r where m is the mass of the object moving in a circle,
v is its velocity and r is the radius of the circle.

Momentum:

ordinary momentum for motion in a straight line: mv where m is the
mass of the body and v is its velocity

angular momentum L (for example for motion in an orbit): L = mvr where
m is the mass of the body and v is its velocity and r is the distance
from the body to the point at which the momentum is being measured
(e.g. usually the Sun in the case of planets)

Kepler's Third Law: D3=(M1+M2)P2