The semidiameter of a celestial body such as the Sun, Moon, or a planet is its apparent equatorial radius expressed as an angle.
The general formula is S = S0 / Δ
With:
S — semidiameter
S0 — the body's semidiameter at unit distance (1 AU)
Δ — the body's distance to the Earth in AU.
Current values
Time:
Sun
S0 = 15′59″.63 = 959″.63
Current Sun distance: UA
Semidiameter: ″
Moon
S0 = 3583″.9 at 1 AU (equatorial radius)
Current Moon distance: UA
Semidiameter: ″