[The Month's Sky]
[History of Astronomy]
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A Brief History of AstronomyThe Solar System Unravelled(1671 to 1814) |
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His discovery of four moons around Saturn destroys Huygens' view of Solar System perfection.
Using the best distance measurements available, Roemer calculates the speed of light. His figure is 75% of the correct value, an excellent value for the times. Aristotle's idea of an infinite speed for light is shown to be wrong. The fact that light has a finite (though very large) speed means that the further we look into space, the further back in time we can see.
Assuming that stars were moving at the same rate as planets, it is possible to make an estimate of stellar distances. At the estimated distances, the stars had to be sun-like in their real brilliance (luminosity). This is the first hint that the Sun is an ordinary star rather than the light at the centre of the Universe.
Halley also works out the orbit of the comet that bears his name. It is a highly elliptical orbit. Up to then, comets were thought to come and go at random. Halley shows that even comets follow Newton's laws of gravity.
They do, but not in the way expected. Bradley discovers a phenomenon called the Aberration of Light. This is the first direct proof that the Earth is in motion but does not yield stellar distances. It is caused by the fact that light has a finite speed. Bradley's observations give a value for the speed of light which is close to the correct value.
Bradley also measures the diameter of Jupiter and finds that it is much larger than the Earth. Not only is the Earth not the centre of the Solar System but it isn't even the largest of the planets.
He thinks that the Milky Way is an "Island Universe" of stars arranged as a flat disk, and that some of the nebulous objects in the sky may be other similar systems outside the Milky Way. This idea would not be accepted for 170 years.
He attempts to measure stellar parallax by looking at stars that are close together in the sky. He assumes that one star may be closer than the other so that the parallax movement will be easier to observe and measure. In many cases, he finds movement but this is independent of the Earth's motion around the Sun. The stars are actually in orbit around each other. These are called Binary Stars. This demonstrates that the stars are not fixed to a crystal sphere and that Newton's law of gravity also operates amongst the stars.
Herschel also discovers many stars that change their brightness. These are called Variable Stars. Stars can no longer thought of as unchanging and uninteresting.
By counting stars, measuring their motions and applying statistics, Herschel makes the first estimate of the size of the region occupied by the stars. This region is now called the Galaxy. The observations indicate that the Solar System is a tiny speck within the Galaxy. It is apparently situated close to the galactic centre because the Milky Way appears symmetrical in the sky. Herschel's estimate of the diameter of the Galaxy is enormous (9000 Light Years) but is actually less than 10% of the true value.
The Sun is shown to have a motion of its own relative to other stars. This motion is towards the constellation of Hercules.
Herschel and others continue to speculate about the existence of other galaxies.
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