The angle a body subtends in the sky. The Sun is about 0.53°, the Moon about 0.50–0.56° — almost the same.
Symbols
α
angular size (viewing angle)
[rad / °]
d
diameter of the body
[m]
D
distance to the body
[m]
Where it comes from
This coincidental match of angular sizes makes both total eclipses possible (the Moon nearer, covering the Sun) and annular ones (the Moon farther, leaving a ring).
When to use it
When you compare the sizes of the Sun and Moon in the sky, or explain the types of solar eclipse.