Moon: Lunar and Solar Eclipses

Abstract

The Moon appears to us as a disk with average diameter 31′and so with just about the same apparent size as the Sun. Its distance from the Earth can be got by triangulation from two observatories sufficiently far apart (say along the same meridian). Astronomers call the angle subtended at the Moon by the Earth’s equatorial radius the equatorial horizontal parallax of the Moon. Its value in the mean is 3422″.6. Since the Earth’s radius is known to be 6378 km, we derive for the mean distance of the Moon from the centre of the Earth

$$ 60.3\text{ Earth radii =384 400 km} $$

and hence for the radius of the Moon

$$ 0.272\text{ Earth radius =1738 km} $$

.