Free astronomy calculators for students, stargazers and anyone curious about space. Find your age and weight on other planets, see how far away each planet is today, look up the Moon phase on any date or work out what your telescope and eyepieces can show. Every calculator runs in your browser, with no sign-up.
Space and astronomy calculators
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Enter your birthday and weight to see your age, weight and next birthday on every planet, the Moon and the Sun.
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Live distances from Earth to every planet today, and how long the trip takes by car, jet, rocket or at light speed.
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Pick any date and see what the Moon looked like that night, from your birthday to a wedding or anniversary.
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Work out the power of every eyepiece, with or without a Barlow, and the highest magnification your telescope can use.
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See how much sky each eyepiece shows and whether the Moon, a planet or a nebula fits in the view.
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Find the finest detail your telescope can split with the Dawes limit and Rayleigh criterion, and which double stars it can resolve.
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Get the times the Great Red Spot crosses the middle of Jupiter for your location, on the nights Jupiter is up.
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Turn a redshift into light-travel, comoving and luminosity distance, with the lookback time and the age of the universe when the light left.
Pick a calculator by topic
- Planets: the age and weight on other planets calculator covers every planet at once, and How Far Away Is Each Planet? shows today’s distances and travel times.
- The Moon: Moon Phase on My Birthday works for any date. For tonight, see Moon Phase Today.
- Telescopes and eyepieces: start with the magnification calculator, then check the field of view and the resolution of your scope.
- Observing: the Jupiter Red Spot calculator tells you when to look for the Great Red Spot from where you live.
- Galaxies and cosmology: the redshift to distance calculator converts a galaxy’s or quasar’s redshift into how far away it is and how long its light has traveled.
Weight on each planet
How much would you weigh on Mercury, Venus, the Moon, Mars, Jupiter, Saturn, Uranus, Neptune or the Sun? Each page has its own calculator and a worked example.
Age on each planet
How old would you be on Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune or Pluto? A year on each one is the time it takes to go once around the Sun, so your age changes a lot from planet to planet.
More space tools
Plan a night out with Meteor Shower Tonight, the next eclipse finder or the rocket launch schedule. The Telescope View Simulator shows what planets and galaxies look like through different telescopes. For games, try the space quizzes.