Ask an expert
Dr Fred Watson, astronomer at the Australian Astronomical Observatory
Why, during an eclipse, is the shadow of the moon the same size as the Sun?
In fact, the Moon’s shadow on the Earth’s surface during a total solar eclipse varies in size depending on the circumstances of the eclipse and the observer’s location. As it moves across the Earth’s surface, the shadow traces out a ‘path of totality’, within which anyone watching will see the Sun’s disc blotted out by the disc of the Moon. Typically (as was the case in the August 2017 eclipse in the USA), the path of totality is about 100km wide.
But I think what this questioner is really asking is why does the Moon exactly cover the Sun during an eclipse – in other words, why are they the same apparent size in the sky? The Sun is roughly 400 times bigger than the Moon but, on average, is 400 times further away. This extraordinary coincidence means the two bodies appear to be the same diameter, and has no known cause.
The ‘path of totality’ is what we see as the disc of the Moon passes over the disc of the Sun during a total lunar eclipse.