Angular size describes how big an object appears to an observer, rather than how big it actually is — the Moon and the Sun look almost the same size in Earth's sky despite the Sun being vastly larger, simply because it's also vastly farther away.
Angular size describes how big an object appears to an observer, rather than how big it actually is — the Moon and the Sun look almost the same size in Earth's sky despite the Sun being vastly larger, simply because it's also vastly farther away. This calculator finds angular size from true size and distance (using the small-angle approximation).
The Moon has a diameter of about 3,474 km and is about 384,400 km away. Angular Size = 3474/384400 ≈ 0.00904 rad ≈ 0.518° ≈ 31.1 arcminutes — matching the Moon's familiar apparent size in our sky.
It's how large an object appears from a given distance, measured as an angle (usually in degrees or arcminutes) rather than physical size.
Angular size ≈ 2 × arctan(object size ÷ (2 × distance)), which works well for objects that are small compared to their distance.
Despite being vastly different in actual size, their angular sizes from Earth are nearly identical because the Sun is both much larger and much farther away.