148 ESSAY ON THE VELOCITY OF LIGHT. 
‘stars will be augmented, and the eclipses of the first satellite of Jupiter will 
present in their succession a smaller perturbation. 
The observations of the immersions of Jupiter’s satellites present some un- 
‘certainty, as these bodies traverse the penumbra of Jupiter before entering the 
pure umbra; so that in their movement around the planet, the latter screens them 
gradually from the sun’s rays until they are completely cut off; so that the 
light of a satellite in eclipsing, insensibly decreases to total extinetion. ‘The 
time of the satellite’s eclipse may therefore vary, according as we make use of 
‘a telescope of greater or less power, and as the atmosphere is more or less 
transparent. ‘The same remarks apply to the emersions of the satellites. On 
the other hand, in supposing that the times of immersions and of emersions 
have been exactly observed, it is further necessary, in the irregularities which 
the succession of these phenomena present, to eliminate what is due to the eccen- 
tricity of the orbit of the satellite, and to the perturbations with which it is 
really affected, from what is but an appearance arising from the progressive 
transmission of light. It is for these reasons that the first satellite is selected 
as the most proper to furnish the data for the determination of the velocity of 
light; the greater velocity of its motion and the small depth of the penumbra 
*at the point where it crosses it, causes it to lose its light more rapidly than the 
other satellites; also the almost entire absence of eccentricity in its orbit renders 
its motion more regular, and thus gives more prominence to the irregularity due 
to the time that the light takes to traverse the orbit of the earth. But we can 
never arrive at but a rough approximation of the velocity of light as deduced 
from the observations of the eclipses of this satellite, as Bradley has shown in 
the last paragraph cited from his letter; since the results arrived at by this 
means, for the time employed by the light to come from the sun to the earth, 
varies from seven to eleven minutes. 
The phenomena of aberration furnish far more precise data for the determin- 
ation of the velocity of light. As all the stars are affected by aberration, we 
ean multiply at will observations which have a great degree of precision. 
Bradley estimates that the error in hig determination of eight minutes and thir- 
teen seconds for the light to travel from the sun to the earth cannot exceed 
ten seconds. 
Employing the most exact means of modern science, Mr. W. Struve, director 
of the Central Russian Observatory, near St. Petersburg, made numerous ob- 
servations during three years (from April, 1840, to the end of 1842) to obtain 
the exact value of aberration. In discussing these observations, he found that 
the half of the greater axis of aberration, which is the same for all stars, has 
an amplitude of 20.45 seconds; whence it results that the velocity of light is 
10,089 times greater than the mean velocity of the earth in its orbit. 
The earth employing 365} days to run over its orbit, light takes 10,089 times 
less, namely 52 minutes and 8 seconds, to go over a line of the same length; 
and regarding that orbit as the cireumference of a circle, we have but to divide 
52 minutes 8 seconds by twice the ratio of the circumference to the diameter 
(3.1416,) to have the time employed in traversing its radius—that is to say, the 
mean distance of the sun from the earth. We thus find 8 minutes 18 seconds 
the number which is now adopted by all astronomers. 
The observations of the eclipses of Jupiter’s first satellite, and those of the 
phenomena of aberration, lead directly, as we have just seen, although with a 
different degree of approximation, to the determination of the time light occupies 
to run over the mean distance of the sun from the earth. ''o deduce from this 
the absolute value of the velocity of light referred to our ordinary units of 
length, the metre, (the foot or the mile,) we must know how many metres (miles) 
are contained in the distance from the sun to the earth. ‘The value of this 
distance is found by means of the parallax of the sun; we designate thus the 
angle under which, being at the sun’s centre, we would see the radius of the 
