The Science Book

59

From position 1 on Earth’s orbit, the predicted

eclipse of Jupitier’s moon Io appears to occur

later than from position 2. Rømer reasoned that

this was due to the extra distance light from

Io had to travel to reach Earth in position 1.

See also: Galileo Galilei 42–43 ■ John Michell 88–89 ■ Léon Foucault 136–37

SCIENTIFIC REVOLUTION

of one of Jupiter’s moons and
comparing it to the predicted
time of the eclipse in Paris.
It was not possible to hold a
telescope steadily enough onboard
ship to observe the eclipses,
and measuring longitude at sea
remained impossible until John
Harrison built the first marine
chronometers—clocks that could
keep time at sea—in the 1730s.

Finite or infinite speed?
Rømer studied observations of
the eclipses of the moon Io taken
over a period of two years and
compared these to the times
predicted by Cassini’s tables.
He found a discrepancy of
11 minutes between observations
taken when Earth was closest
to Jupiter and those taken
when it was farthest away. This
discrepancy could not be explained
by any of the known irregularities
in the orbits of Earth, Jupiter, or
Io. It had to be the time it took
for light to travel the diameter of
Earth’s orbit. Knowing the diameter
of Earth’s orbit, Rømer could now

calculate the speed of light. He

produced a figure of 133,000 miles/s

(214,000 km/s). The current value

is 186,282 miles/s (299,792 km/s),

so Rømer’s calculation was off by

about 25 percent. Nevertheless,

this was an excellent first

approximation, and it solved the

previously open question as to

whether light had a finite speed.

In England, Isaac Newton

readily accepted Rømer’s

hypothesis that light did not

For the distance of

about 3,000 leagues,

which is nearly equal

to the diameter of the

Earth, light needs not

one second of time.

Ole Rømer

travel instantaneously. However,

not everyone agreed with

Rømer’s reasoning. Cassini

pointed out that discrepancies

in the observations of the other

moons were still not accounted

for. Rømer’s findings were not

universally accepted until

English astronomer James Bradley

produced his more accurate

figure for the speed of light in

1729 by measuring the parallax

of stars (p.39). ■

Ole Rømer

Born in the Danish city of

Aarhus in 1644, Ole Rømer

studied at the University of

Copenhagen. On leaving the

university, he helped to prepare

the astronomical observations

of Tycho Brahe for publication.

Rømer also made his own

observations, recording

the times of the eclipses of

Jupiter’s moons from Brahe’s

old observatory at Uraniborg,

near Copenhagen. From there,

he moved to Paris, where he

worked at the Royal Observatory

under Giovanni Cassini. In 1679,

he visited England and met

Isaac Newton.

Returning to the University

of Copenhagan in 1681, Rømer

became professor of astronomy.

He was involved in modernizing

weights and measures, the

calendar, and building codes,

and even the water supplies.

Unfortunately, his astronomical

observations were destroyed in

a fire in in 1728.

Key work

1677 On the Motion of Light

Io

Jupiter

Sun

Earth

1

2