Magnitude Determines brightness, luminosity, apparent and
absolute magnitude.

Advanced Magnitude Calculator Distance, Luminosity, Apparent and
Absolute Magnitude  if you know two, this calculates the other two.

Kepler's 3rd Law Calculator for rudimentary T²=R³ computations.

Kepler's 3rd Law Advanced Calculator This allows mass to be
brought into the T²=R³ equation.

Escape Velocity Ultra Calculator Calculates escape velocity and surface gravity.

Synodic Period Calculator Calculates synodic period.

A tutorial about opposition, conjunction, quadrature and synodic period is located here.

Angular Size Calculator
Accurate for angles up to 180 degrees.
