The first evidence.

In 1671 Römer went to Hven, an island community near Copenhagen, to help re-determine the longitude of the observatory located there. With others, he began observing a series of eclipses of Io, Jupiter's largest moon. In the end they had eight months of observations or, since Io makes one revolution of Jupiter in 42 hours, timings on about 140 eclipses over 2/3 of the year. The time intervals between these eclipses were not regular but appeared related to where the Earth was in its orbit. The length of the interval became shorter as the Earth approached Jupiter and longer as it moved away; the mathematically predicted time of an eclipse was too early if the Earth was near Jupiter and too late if the Earth was far from Jupiter. This systematic lack of fit allowed Römer to announce in Paris in September 1676 that the eclipse predicted for November 9 that year would actually occur 10 minutes later. The observation bore him out and Römer argued that the discrepancy was due to the finite speed of light. The light takes longer to reach us the farther we are from its source.

From his observations, Römer estimated that light takes about twenty-two minutes to cross the full diameter of Earth's orbit or about eleven minutes for light from the sun to reach us on Earth. On this basis, he estimated light's speed to be about 214,000 kilometres per second.³⁸

Römer's ``proof'' was not immediately accepted by all. Alternative explanations were provided by Gian Domenico Cassini (1625-1712) then an astronomer at the newly formed Academie des Sciences in Paris. In 1666 Cassini had published tables on the eclipses of the satellites of Jupiter from which work he also noticed inequalities in time intervals of eclipses that depended on the location of Jupiter in its own elliptical orbit. He had briefly considered a finite speed of light in 1675 but soon rejected it for a more traditional explanation. Cassini, and later his nephew Giacomo Filippo Maraldi (1665-1729), suggested that Jupiter's orbit and the motion of its satellites might explain the observed inequalities ([50], [42] and [32]). Many astronomers continued to hold the view that light's movement was instantaneous.

It was not until a study by James Bradley (1693-1762)³⁹ was reported in 1729 that nearly all agreed that the speed is finite. Bradley had been studying the parallax of the stars and discovered an annual variation in the position of stars that could not be explained by the parallax effect. However, it could be explained by the motion of the Earth if light's speed were finite. Based on careful observations, Bradley estimated that light took eight minutes and twelve seconds to reach the Earth from the sun resulting in a value for light's speed of 301,000 km/sec.

In 1809, based on observations on the eclipses of Jupiter's moons for 150 years, Jean-Baptiste Joseph Delambre (1749-1822) estimated the time taken by light to travel from the sun to Earth to be eight minutes and 13.2 seconds resulting in a speed of about 300,267.64 $\approx$ 300,300 km/sec.⁴⁰

The results of these early astronomical estimates are summarized in Table 1.

 

Table: Studies based on astronomical observation.

Year	Authors	Observational Source	Speed (km/sec)
1676	Römer	Jupiter satellites	214 000
1726	Bradley	Aberration of stars	301 000
1809	Delambre	Jupiter satellites	300 300

 

Unfortunately, measurements of the speed made in this way depended on the astronomical theory and observations used. Simon Newcomb (1835-1909) tells of an inaugural dissertation in 1875 by Glasenapp whereby observations of the eclipses of Io from 1848 to 1870 show that widely ranging values for the speed ``could be obtained from different classes of these observations by different hypotheses'' ([42] page 114). It was shown that values for the sun to Earth time could be produced between 496 and 501 seconds resulting in speeds between 295,592.8 $\approx$ 295,600 and 298,572.6 $\approx$ 298,600 km/s. ⁴¹

Better determinations of the speed might be made if both source and observer were terrestrial. Because all would then be accessible, greater control could be exerted over the study and hence the observations. But this brings us back to the age old problem: how could the speed of light be measured terrestrially?