58
MEASURING THE
SPEED OF LIGHT
OLE RØMER (1644–1710)
J
upiter has many moons,
but only the four largest
(Io, Europa, Ganymede,
and Callisto) were visible through
a telescope at the time that Ole
Rømer was observing the skies
of northern Europe, in the late
17th century. These moons are
eclipsed as they pass through
the shadow cast by Jupiter and
at certain times they can be
observed either entering or leaving
the shadow, depending on the
relative positions of Earth and
Jupiter around the Sun. For nearly
half of the year, the eclipses of
the moons cannot be observed
at all, because the Sun is between
Earth and Jupiter.
Giovanni Cassini, the director
of the Royal Observatory in Paris
when Rømer started work there in
the late 1660s, published a set
of tables predicting the moons’
eclipses. Knowing the times of
these eclipses provided a new
way to figure out longitude. The
measurement of longitude depends
on knowing the difference between
the time at a given location and the
time at a reference meridian (in this
case, Paris). On land at least, it was
now possible to calculate longitude
by observing the time of an eclipseIN CONTEXT
BRANCH
Astronomy and physicsBEFORE
1610 Galileo Galilei
discovers the four largest
moons of Jupiter.1668 Giovanni Cassini
publishes the first accurate
tables predicting eclipses
of the moons of Jupiter.AFTER
1729 James Bradley calculates
a speed of light of 185,000
miles/s (301,000 km/s) based
on variations in the positions
of stars.1809 Jean-Baptiste
Delambre uses 150 years’
worth of observations of
Jupiter’s moons to calculate
a speed of light of 186,600
miles/s (300,300 km/s).1849 Hippolyte Fizeau
measures the speed of light
in a laboratory, rather than
using astronomical data.Eclipses of Jupiter’s
moons do not always
match predictions.If light does not
propagate instantaneously,
this explains the
discrepancies.The distance between
Earth and Jupiter
changes as the planets
orbit the Sun.The speed of
light can be
calculated from the
time differences
and distances in the
solar system.