What Is an Angular Diameter Calculator?
Angular diameter is the apparent size of an object as seen from a given distance, measured as an angle rather than a physical length. This calculator converts an object's real (physical) diameter and its distance into that apparent angular size, expressed in degrees, arcminutes, and arcseconds — the units astronomers use to describe how large something looks in the sky or through a telescope.
The same physical object looks smaller the farther away it is, which is why the exact same math applies equally to the full Moon, a distant galaxy, or a satellite passing overhead.
How to Read Your Results
Angular Diameter
The primary result, automatically shown in whichever unit (degrees, arcminutes, or arcseconds) best fits the size of the answer.
In Degrees / Arcminutes / Arcseconds
The same angle expressed in all three standard units side by side. One degree equals 60 arcminutes, and one arcminute equals 60 arcseconds — useful for comparing against telescope resolving power or field-of-view specifications, which are usually quoted in arcseconds or arcminutes.
Real-World Example
The Moon has a diameter of about 3,474.8 km and orbits at an average distance of 384,400 km.
| Input | Value |
|---|---|
| Physical diameter | 3,474.8 km |
| Distance | 384,400 km |
| Angular diameter | ≈ 0.518° (≈ 31.1 arcminutes) |
That matches the Moon's well-known apparent size of roughly half a degree — the same rough angular size as the Sun, which is why total solar eclipses are possible.
Tips for Using This Calculator
- Match your diameter and distance to the same scale of object — use AU for planets, light-years for stars.
- The human eye can resolve details down to about 1 arcminute; a good amateur telescope can resolve down to around 1 arcsecond.
- This formula treats the object as a simple sphere or disk viewed face-on; irregular or elongated objects will have different angular sizes along different axes.
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Last updated: August 2026 · Reviewed by: Simple Calculator Tools Editorial Team