Astronomy
Astronomy Calculators
Moon phases, planetary weight, telescopes, orbits, magnitudes, and distances.
8 tools
Distance Modulus Calculator
Distance modulus calculator: convert magnitude difference m minus M into distance in parsecs or light years. Free, editable units, runs in your browser.
Light Year Distance Calculator
Light year distance calculator: convert light years to miles, km, and AU, and see travel times at light speed instantly. Free, runs in your browser, no signup.
Moon Phase Calculator
Moon phase calculator: current moon phase from the synodic cycle. Free moon phase calculator with the formula shown - runs entirely in your browser, no signup.
Orbital Period Calculator
Orbital period calculator: derive a satellite or planet's orbital period from semi-major axis and body mass using Kepler's third law. Free, no signup.
Planetary Weight Calculator
Planetary weight calculator: convert your Earth weight to what you would weigh on every planet and major moon by surface gravity. Free, browser, no signup.
Star Magnitude Calculator
Star magnitude calculator: convert between apparent and absolute magnitude using distance, or compare two stars' brightness ratio instantly. Free, no signup.
Stellar Temperature Calculator
Stellar temperature calculator: estimate a star's surface temperature and spectral class from its color index. Free, private, runs locally, no signup.
Telescope Magnification Calculator
Telescope magnification calculator: find eyepiece magnification and true field of view from telescope and eyepiece focal lengths. Free, runs locally, no signup.
How these astronomy tools work
The calculations come straight from published physics: weight on another planet scales by surface gravity, moon phases follow the 29.53-day synodic cycle, telescope magnification is focal length divided by eyepiece focal length, and magnitude differences follow the 2.512 luminosity ratio defined by the magnitude scale.
Each page shows its constants and formula, so a result can be verified by hand in seconds. Where the tool uses an approximation (such as treating planetary orbits as circular for average distance), the assumption is stated on the page.
Choosing a tool
Weight questions use the planetary weight tool, observing questions use the moon-phase and telescope tools, and brightness comparisons use the magnitude tool. For distance questions, note the difference between average orbital distance (AU) and current distance, which changes daily.
Honest limits
Moon-phase results are averaged; the actual phase boundary can shift by hours. Planetary gravity values assume mean surface gravity and ignore rotation effects. These are display-level approximations, fine for planning an observation session, not for ephemeris-grade work.