Orbits and the wider universe

Kepler’s first law of motion

Each planet moves in an ellipse with the sun at one focus

Kepler’s second law of notion

The line joining the planet to the centre of the sun sweeps out equal areas in equal times

Kepler’s third law of motion

T², the square of the period of the planet’s motion is proportional to r³, the cube of the semi major axis

Dark matter

Matter which we can’t see or detect by any sort of radiation, but whose existence we infer from its gravitational effects

Radial velcocity of a star

The component of a star’s velocity along the line joining it and an observer on earth

Galactic radial velocity

This is the mean component of a galaxy’s velocity along the line joining it and an observer on earth

Hubble’s law

Each object has a superimposed radial velocity given by v=H(nought)D