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Other Findings of General Relativity

A further success of general relativity was the clearing up of a long-standing puzzle in

astronomy. The perihelion of a planetary orbit is the point in the orbit nearest the sun.

Mercury’s orbit has the peculiarity that its perihelion shifts (precesses) about 1.6°per

century (Fig. 1.21). All but 43 (11 arc second 36100 of a degree) of this shift is

due to the attractions of other planets, and for a while the discrepancy was used as

evidence for an undiscovered planet called Vulcan whose orbit was supposed to lie

Relativity 35

Star

Apparent

position

of star

Sun

Starlight

Sun

Figure 1.19Starlight passing near the sun is deflected by its strong gravitational field. The deflection

can be measured during a solar eclipse when the sun’s disk is obscured by the moon.

Earth

Massive

object

Apparent

position

of source

Source

Light and radio waves from source

Apparent

position

of source

Figure 1.20A gravitational lens. Light and radio waves from a source such as a quasar are deviated by a massive object such as a

galaxy so that they seem to come from two or more identical sources. A number of such gravitational lenses have been identified.

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