Journal of Astronomical History and Heritage, 11(2), 97-105 (2008).
CASSINI, RØMER AND THE VELOCITY OF LIGHT
Bibliothèque, Observatoire de Paris, 61 avenue de l’Observatoire, 75014 Paris, France.
LERMA, Observatoire de Paris, 61 avenue de l’Observatoire, 75014 Paris, France.
Abstract: The discovery of the finite nature of the velocity of light is usually attributed to Rømer. However, a text at
the Paris Observatory confirms the minority opinion according to which Cassini was first to propose the ‘successive
motion’ of light, while giving a rather correct order of magnitude for the duration of its propagation from the Sun to the
Earth. We examine this question, and discuss why, in spite of the criticisms of Halley, Cassini abandoned this
hypothesis while leaving Rømer free to publish it.
Keywords: velocity of light, satellites of Jupiter, longitude, Jean-Dominique Cassini, Jean Picard, Ole Rømer,
Edmond Halley, James Bradley, Christiaan Huygens.
“The Danish astronomer Olaus Römer (1644-1710) discovered the velocity of propagation of light at
the Paris Observatory in 1676.” Inscription on the north frontage of the Paris Observatory.
The discovery of the finite nature of the velocity of
light has been abundantly commented on by many
authors. The general opinion is that it is due to Ole (or
Olaus) Rømer (Figure 1),
who published it on 7
December 1676 in the Journal des Sçavans. The paper
(1676), well-written and very clear, shows
that the discovery was made while studying the motion
of the first Galilean satellite of Jupiter, Io (Figure 2).
There is, however, some doubt about this discovery,
which we will now try to dissipate. Before this, let us
examine why the satellites of Jupiter were so actively
observed during the seventeenth century.
Figure 1: Ole Rømer, engraving by J.G. Wolffgang (1735).
Rømer appears here in full glory. After his return to Denmark,
around 1681, he became Mayor and head of the police of
Copenhagen, and also head of the State Council of the Realm
(Library of the Paris Observatory).
Figure 2: Rømer’s drawing in his article of the Journal des
Sçavans. The Sun is in A, Jupiter in B with its shadow cone,
and the drawing is in the reference system Sun-Jupiter. Two
positions of the Earth, L and K, are represented at the times of
two emersions of the first satellite out of Jupiter’s shadow; in
D, the Earth moved away from Jupiter between these two
observations, and the second one seems late because of the
extra time required for the light to propagate. Conversely,
immersions of the satellite in the shadow, in C, seem
increasingly early when the Earth moves from a non-labelled
point to G (Library of the Paris Observatory).
Immediately after he discovered the four main
satellites of Jupiter, Galileo proposed that their motion
could be used as a natural clock. In 1692 Jean-
Dominique Cassini (Figure 3) wrote:
It is not by curiosity alone that the most famous
astronomers of the present century have observed with
so much care the planet Jupiter; they mainly did it in
order to obtain an exact knowledge of longitudes, on