Shape and Mean Diameter of the Moon

The Moon appears as a disc shape, not a sphere, to observers on Earth.
This is because we are only able to see one side (the near side) of the Moon from Earth since it is tidally locked.
The Moon’s apparent shape changes throughout the lunar month due to the varying amount of the illuminated part of the Moon becoming visible. This is known as lunar phases.
The Moon goes through a sequence of shapes (phases) from new moon, first quarter, full moon, last quarter back to new moon.

The mean diameter of the Moon is about 3,476 kilometres.
This equates to about 27% the diameter of the Earth.
Measuring the Moon’s diameter from different locations on Earth and comparing the data can provide a more accurate measurement, this is due to libration.
Libration in longitude allows us to see around the edge of the Moon at different times.
Libration in latitude allows us to see a little over the top and bottom of the Moon due to the axis of the Moon being tilted relative to its plane of orbit.
Librations cause the visible diameter to vary slightly during the month (between 29.3 and 34.1 arc minutes).
The Moon’s distance from Earth varies due to its elliptical orbit around Earth, this also causes the apparent size of the Moon to fluctuate.

From Earth, the Sun and the Moon appear approximately the same size in the sky. This is due to a coincidence - the Sun is 400 times larger but also 400 times further away from Earth than the Moon.
The coincidence allows for lunar and solar eclipses to occur. During a total solar eclipse, the Moon fully blocks the Sun from view.