114 The Poetry of Physics and The Physics of Poetry
Using the value of the diameter of the Earth’s orbit about the sun,
Roemer calculated the velocity of light, c, to be 2.14 × 10^8 m/sec. This
value is fairly close to the presently known value of 3 × 10^8 m/sec. The
chief source of error in his calculation was due to the poor estimate in his
day of the distance between the Earth and the sun. Although Roemer’s
determination of c was not very precise, one cannot help but be
impressed by the ingenuity of his approach.
A more accurate determination of c was made in 1725 by James
Bradley, who also made use of astronomical observations. He observed
that the stars that lie above the plane of the Earth’s orbit seem to move
about a very tiny circle in the sky in the course of a year. This
phenomenon, known as the aberration of starlight, is due to the fact that
the angle at which the star is observed changes as the Earth moves about
its orbit and hence, the star appears to also move in a circle. The angle at
which the star is observed depends on the velocity of the Earth. As
illustrated in Fig. 13.1, in order for the starlight coming directly from
overhead to enter the telescope and subsequently be observed, the
telescope must be tipped by an angle α = v/c where v is the velocity
of the Earth. From his knowledge of the Earth’s velocity, and his
measurement of the angle of aberration, Bradley was able to determine
the velocity of light. He found the value 3.1 × 10^8 cm/sec, a value
extremely close to the modern value.
Fig. 13.1