✦Astronomy Calculators
Free astronomy calculators for planets, stars, and cosmic distances — explore the universe from your browser. No signup required.
See how much you'd weigh across two planetary systems — our Solar System and the real TRAPPIST-1 exoplanet system — by entering your weight just once.
Find out how old you'd be on Mars, Jupiter, and even planets in the TRAPPIST-1 system — enter your birth date once and compare your age across worlds.
Convert light years to km, AU and parsecs with cosmic comparisons.
See the Moon's phase for any date — phase name, illumination, and the dates of the next new and full moon, with a visual moon.
Calculate the escape velocity for any planet or custom body.
Find the event horizon radius of any mass.
Apply Kepler's third law to find the orbital period of any body.
Convert between absolute and apparent magnitude using the distance modulus.
Calculate recession velocity or distance using Hubble's law with an adjustable Hubble constant.
Find the angular size of any object at any distance.
Distinguish between sidereal and synodic orbital periods.
Estimate your sky quality and what you can observe from your location.
About these astronomy calculators
These astronomy tools put cosmic scales in human terms. Find how your weight changes as surface gravity varies across the planets — from the crushing pull of Jupiter to the gentle tug of Mars. Convert vast distances between light years, astronomical units, and parsecs with context about what each unit represents in practice.
Every tool runs entirely in your browser with no signup required. Whether you're a student exploring orbital mechanics, an amateur astronomer planning a night's observation, or simply curious about the universe — these calculators are ready immediately.