📏Angular Diameter Calculator
δ = 2·arctan(d/2D) — the relationship between true size, distance, and apparent size.
Free Angular Diameter Calculator
Why do the Moon and the Sun look almost exactly the same size in our sky, even though one is 400 times smaller? Our free angular diameter calculator answers questions like this using δ = 2·arctan(d/2D) — the formula relating an object's true size, its distance, and how big it appears. Enter any two of true diameter, distance, and angular size, and the calculator solves for the third instantly.
Try the built-in presets for the Moon, the Sun, and Jupiter to see real numbers, with results shown in degrees, arcminutes, or arcseconds depending on scale. It's the same math behind why solar eclipses are possible and how astronomers measure the apparent size of distant stars and planets.
No sign-up, no downloads, no limits — just clear answers whenever you're curious.