Laurence Bobis and James Lequeu x C assini, Rømer and the Velocity of Light

103

phenomenon. But t his is mistaken; one can see in a

writing b y Cassini, published in August 16 75 [actually

1676], that this astronomer was the first author.

However, p erhaps Cassini wrote on behal f of his

team, which inc luded P icard, Rø m er and pe rhaps eve n

Richer and Philip pe de La Hire ( 1640 –1718). T his

becomes a most convincing hypothesis when one reads

the mi nutes of the Academy a nd consid ers the working

methods at the P aris Ob servator y : it may b e t hat the

discovery was collective, and was due to bo th Cassini

and Rø mer, as s uggested b y Fontenelle ( we should

remember that Cassini was still alive when Fontenell e

was writing his ‘history’, and that they both at tend-

ed Acade my meeti ngs ever y Saturday). In a ny case,

Cassini can not be d ismissed for this discover y, a s

proposed by so me co mmentators, and we must ac-

knowledge his emi nent contrib ution to t he solutio n “…

of one of the most bea utiful p roblems i n ph ysics.”

(Cassini, 1 693b: 46). He behaved like an op en-minded

scientist, who le ft to others the p ossibility of promoting

ideas opp osite to his o wn b eliefs; but he also sho wed

some stubb ornness when refusin g to ado pt the ide a of

the f inite velocit y of light, in spite of Halle y’s de mon-

stration— wh ich he co uld hardly ignore.

Even if the discovery o f aberratio n solved in a de-

finitive way the pro blem o f t he velocit y o f light, the

situation surrou nding the ep hemerides o f Jupiter’s

satellites re mained unsatisfact ory until the t ime of La -

grange and Laplace, in spite of the e fforts of Wargen -

tin a nd o f Maraldi I I. E mpirical ter ms were still

introduced i n o rder to account for the o bservations i n

the best possible way. T he ephemerides re mained in

use for deter mining longit udes until the end o f the

eighteenth ce ntury, because t hey were pr ecise enough

in the short-ter m to gi ve ti me, hence longitude, within

a few minute s: thi s only r equired a single eclip se

observation, without need for comparison with a

simultaneous observat ion in Pa ris. But t his was onl y

possible on la nd; observatio ns of J upiter’s satellite s

made at sea were i mpossible in practice because of the

motions o f the ships. In this case, the solution finall y

came with the constructio n of precise marine chrono-

meters b y John Harrison (169 3 –17 76) in Engla nd

between 173 7 and 1773 . Go od marine chrono meters

were a lso buil t in France by c lock-makers like Ferdin-

and Berthoud (1727 –1807), Duro y and Jean-André

Lepaute ( 1709–178 9), and were tested ashore and at

sea by astro nomers. By 180 0, longitude could be

determined within a fractio n o f a d egree o n voyages of

one or t w o months’ d uration.

7 NOTES

1. Rømer’s name is also sp elt Römer, Roe mer, Rœmer

and even Ro m er.

2. The obser vations u sed in the discover y are collected

in a manuscript by Rømer which was written t wo

years later.

3. Astronomers used to call pro stapheresis (modern

equivalent: equation of c entre) the difference be-

tween the mean a nd the true position of the S un, o f

a planet or of a satellite.

4. Du Ha mel i nserts the te xt i n que stion in pa ge 14 5 o f

his boo k, in a chapter entitled “De rebu s Astro-

nomicis anni 1675” (beginni ng on pa ge 143 ). I n

the margin of pa ge 144 we find the mention ‘Ann.

1675 ’, but at the e nd of the c hapter, on page 146, it

becomes ‘Ann. 1 675 & 76’. It is clear, when

reading t he chapter , that the te xt dated 22 August i s

from the same year as the publication b y Rø mer,

i.e. 1 676, b ut some c ommentators c onfused the

dates: for example, Mo ntucla (1 758) attrib utes t he

text to Augu st 1 675 and Fontenelle ( 1707) to

August 167 4.

5. Indeed, Cassini writes in an unpublis hed pr oject for

an ‘Abrégé d’Astronomie ’ preserved in the Lib rary

of the Paris Obser vatory:

The observations show that aside from the known in-

equalities there are oth ers which are l arger in the

second and the third satellite, and smaller in the first

and th e fourth. They clearly change their di sta nces

from Jup iter and anticip ate or delay con junctions and

eclipses.

Reason demands that there are three oth ers similar to

those of the Moon, and more d iff icult to d isantangle,

because one of them results from the equilibrium of all

satellites to gether, which is continuously changin g and

produces effects on each satellite. Experience shows

however that the su m of these in equalities i s not large

and that they do not p revent a p rediction of the

conjunctions and eclipses w ith appro xim ately the same

accuracy as for the predictio ns o f those of t he Su n an d

of the Moon. (Cassini, MS B4[2]; ou r translation).

6. On 14 Octob er 1677 Huygens ( Oeuvres Comp lètes ,

1888 -1950, t. 8: 36-37; our translation) wrote to

Colbert, the P rime Minister of Fra nce:

I have seen recently with much pleasure th e beautiful

invention [ sic] o f Mr. Romer, to d em onstrate that light

takes time to pro pagate, and even to me asure this time;

this is a very i mportant d iscove ry, worth y of a co n-

firmation by t he Royal Observatory. A s to myself,

this dem onstration suits me more especially a s, in

what I am writing ab out Dioptics, I supposed the same

thing about l ight, and d em onstrated with it th e

properties of refraction , and recently those of the

Iceland Cristal.

7. T hese ‘lie ues de 25 au degré’ measure 4 ,444 metres,

so the velocit y of light is calculated as 214 ,000

km/s, a fi gure somewhat smaller than that de rived

by Huygens and much smaller than the curren t

value of 299 ,792.458 km/s.

8. Here is what Ca ssini ob served, as d ocumented in

letters to Picard, written in Italian, and preserved in

the Librar y of the Paris Ob servatory (Ms B4[3]) .

On 24 Octob er 1671, Cassini wrote:

I already told you about the difference I found for the

largest elevation of the Pole St ar o bserved last f all,

with respect to the p resent one . .. I p lan to set up a

fixed telescope in order to see if this difference aris es

from the thi ng itself , or from th e observation . (O ur

translation of the French translation Ms A4[2]).

The “largest ele vation” was the elevatio n of the

Pole Star a bove the horiz on at culmination. If it

varied, this was beca use the Pole Star was gettin g

closer or further from the Nor th Ce lestial Pole.

Picard w rote Cas sini on 13 N ovember 1671 that he

had also seen thi s variation:

I can say that, unless th e observation s I have made last

summer d uring several following evenings are wrong,

the Pole Star must presently be at a distance from the

Pole of 2° 28 ′ 30 ″ instead of 2° 2 8 ′ 10 ″ . Whatever i t

may be, I have no t much difficulty to i magine that the

axis o f diurn al motion of the Earth, by changing its

parallelism [ sic], might experience so me periodical

agitation or libration. This would be enough to

account for these kinds o f anomalies. (Ou r trans-

lation).