Eclipses and the speed of light

Eclipses explained by the straight-line propagation of light

Eclipses are a direct consequence of the fact that light travels in a straight line and that the Earth, the Moon and the Sun can align.

• Solar eclipse: the Moon passes between the Sun and the Earth. The Moon blocks the Sun’s rays and casts its shadow over part of the Earth.
• Lunar eclipse: the Earth passes between the Sun and the Moon. The Earth casts its shadow onto the Moon, which then appears darkened or reddish.

The speed of light

Light does not travel instantaneously: it has a very high but finite speed, denoted by c:

c ~ 300,000 km/s (in a vacuum)

At this speed, light circles the Earth about 7.5 times in one second!

A surprising consequence

The Sun is about 150,000,000 km from Earth. Light therefore takes:

t = distance / speed = 150,000,000 / 300,000 = 500 s, or just over 8 minutes

This means that when you look at the Sun, you are seeing it as it was 8 minutes ago! For very distant stars, this delay can be as long as several years (we use ‘light-years’ to measure such distances).

A pitfall to avoid

Don’t assume that light travels instantly from one point to another: even though it is extremely fast, it still takes a non-zero amount of time to cover a distance, which explains the time lag between ‘seeing’ a celestial body and its actual state at that very moment.