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Astronomy

Astronomy calculators — redshift, light-travel distance, parallax, and surface gravity.

12 tools

Redshift to Velocity Calculator

Recession velocity from redshift z, low-z and relativistic.

Light Travel Time Calculator

How far light travels in a time, or how long it takes over a distance.

Stellar Parallax Calculator

Distance to a star from its parallax angle, in parsecs and light-years.

Schwarzschild Radius Calculator

Event-horizon radius of a black hole from its mass.

Surface Gravity Calculator

Surface gravity of a planet or star from its mass and radius.

Angular Size Calculator

Apparent angular size of an object from its size and distance.
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Planet Weight Calculator

See what you would weigh on every planet, the Moon, and the Sun.
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Moon Phase Calculator

Find the Moon phase and illumination for any date.
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Wien’s Law Calculator

Convert between a blackbody’s temperature and its peak wavelength.
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Hubble’s Law Calculator

Relate a galaxy’s recession velocity and distance with v = H₀d.
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Telescope Magnification Calculator

Magnification, focal ratio, and exit pupil from your telescope and eyepiece.
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Distance Modulus Calculator

Solve for distance, apparent, or absolute magnitude from the distance modulus.

About astronomy tools

The measurements of the night sky — redshift to recession velocity, light-travel distances in light-years and parsecs, stellar parallax, the Schwarzschild radius of a black hole, surface gravity, and angular size. Each tool shows the formula and constants (c, G) with the working, for astronomy and astrophysics coursework.

When to use these tools

Common astronomy tasks and the tool that handles each one.

Frequently asked questions

Common questions about astronomy — the concepts behind the tools above.

Is a light-year a measure of time or distance?
A light-year is a measure of distance, not time, despite the word “year” in its name. It is the distance light travels in one year in a vacuum, roughly 9.46 trillion kilometres. Because light has a finite speed, seeing an object 100 light-years away means seeing it as it was 100 years ago.
What is a parsec?
A parsec is a unit of distance used by astronomers, equal to about 3.26 light-years. It is defined using parallax: the distance at which one astronomical unit subtends an angle of one arcsecond. Astronomers favour it because it comes directly from parallax measurements of nearby stars.
Why does redshift indicate that a galaxy is moving away?
Redshift is the stretching of light to longer, redder wavelengths as a source recedes, an effect analogous to the Doppler shift. When a galaxy moves away, its light waves are stretched, shifting spectral lines toward the red end. The greater the redshift, the faster the galaxy is receding, which underpins the expansion of the universe.
What is an astronomical unit (AU)?
An astronomical unit is the average distance between the Earth and the Sun, about 149.6 million kilometres. It is a convenient yardstick for distances within the solar system. For example, Jupiter orbits at roughly 5.2 AU from the Sun.

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