MEASURING THE SPEED OF LIGHTEARTHSUNJUPITERIO DISTANCE LIGHT TRAVELSIn the top diagram, Earth’s orbit around the Sun
puts it far from Jupiter. Later in its orbit, Earth is
closer (below) so light has less distance to travel,
shortening the interval between eclipses of IoDISTANCE LIGHT TRAVELSEARTH
SUNIOJUPITERDiagram not to scaleHow observing the movement of Jupiter’s moons provided 17th-century astronomers
Giovanni Cassini and Ole Rømer with an early indication of the speed of light
Io, the innermost moon of Jupiter, orbits
that planet every 42.5 hours. Viewed from
Earth, Io periodically disappears behind
Jupiter and reappears later. It was thought
that the time between eclipses would
be the same.
However, when Giovanni Cassini made
measurements around the year 1671, the
results kept changing. He realised that
this could be due to light taking time to
travel from Jupiter to Earth, during whichperiod the Earth had moved. Therefore, the
distance travelled from Jupiter to Cassini’s
telescope would vary from one eclipse to
another, depending on whether Earth was
moving towards or away from Jupiter.
Cassini seems not to have trusted his
intuition, but his assistant, Ole Rømer,
performed his own measurements. When
these were combined with Cassini’s, Rømer
realised that the variations correlated with
the relative motion of Earth and Jupiter.Rømer made a long series of
measurements that established this, as
well as leading to an estimate of light’s
speed to be in excess of 220,000km/s. For
many, this was so unimaginably fast as to
be regarded as infinite and Rømer’s ideas
were not universally believed.
It was not until 1729 when Astronomer
Royal James Bradley measured the speed
of light by means of stellar aberration that
Rømer’s theory was finally accepted.“In 1690, Dutch mathematician Christiaan Huygens
used this to estimate a speed for light of around
220,000km/s, about 70 per cent of the modern value”
THE KEY EXPERIMENT