The Probert Encyclopaedia of Science & Technology

ASTRONOMY

Astronomy is that science which investigates the motions, distances, magnitudes, and various phenomena of the heavenly bodies. That part of the science which gives a description of the motions, figures, periods of revolution, and other phenomena of the heavenly bodies is called descriptive astronomy; that part which teaches how to observe the motions, figures, periodical revolutions, distances, etc, of the heavenly bodies, and how to use the necessary instruments, is called practical astronomy; and that part which explains the causes of their motions, and demonstrates the laws by which those causes operate, is termed physical astronomy. In the 19th century new fields of investigation developed. The first of these - celestial photography - furnished us with invaluable light-pictures of the sun, moon, and other bodies, and recorded the existence of myriads of stars invisible even by the then best telescopes; while the second, spectrum analysis, revealed a knowledge of the physical constituents of the universe, revealing for the first time for instance that in the sun there exist many of the elements familiar to us on the earth. It has also been applied to the determination of the velocity with which stars are approaching to, or receding from, our system; and to the measurement of movements taking place within the solar atmospheric envelopes. From analysis of some of the unresolved nebulae the inference was drawn that they are not star-swarms but simply cosmical vapour; whence a second inference results favourable to the hypothesis of the gradual condensation of nebulae, and the successive evolutions of suns and systems.

The most remote period to which we can go back in tracing the history of astronomy refers us to a time about 2500 BC, when the Chinese are said to have recorded the simultaneous conjunction of Saturn, Jupiter, Mars, and Mercury with the moon. This remarkable phenomenon is found, by calculating backward, to have taken place 2460 BC Astronomy has also an undoubtedly high antiquity in India. The mean annual motion of Jupiter and Saturn was observed so early as 3062 years BC; tables of the sun, moon, and planets were formed, and eclipses calculated. In the time of Alexander the Great, the Chaldeans or Babylonians had carried on astronomical observations for 1900 years. They regarded comets as bodies travelling in extended orbits, and predicted their return; and there is reason to believe that they were acquainted with the true system of the universe. The priests of Egypt gave astronomy a religious character; but their knowledge of the science is testified to only by their ancient zodiacs and the position of their pyramids with relation to the cardinal points.

It was among the Greeks that astronomy took a more scientific form. Thales of Miletus (born in 639 BC) predicted a solar eclipse, and his successors held opinions which are in many respects wonderfully in accordance with modern ideas. Pythagoras (about 500 BC) promulgated the theory that the sun is the centre of the planetary system. Great progress was made in astronomy under the Ptolemies, and we find Timochares and Aristyllus employed about 300 BC in making useful planetary observations. But Aristarchus of Samos (born in 267 BC) is said, on the authority of Archimedes, to have far surpassed them, by teaching the double motion of the earth around its axis and around the sun. A hundred years later Hipparchus determined more exactly the length of the solar year, the eccentricity of the ecliptic, the precession of the equinoxes, and even undertook a catalogue of the stars. It was in the second century after Christ that Claudius Ptolemy, a famous mathematician of Pelusium in Egypt, propounded the system that bears his name, viz that the earth was the centre of the universe, and that the sun, moon, and planets revolved around it in the following order: nearest to the earth was the sphere of the moon; then followed the spheres of Mercury, Venus, the Sun, Mars, Jupiter, and Saturn; then came the sphere of the fixed stars; these were succeeded by two crystalline spheres and an outer sphere named the primum mobile or first motion, which last was again circumscribed by the coelum empyreum, of a cubic shape, wherein happy souls found their abode.

The Arabs began to make scientific astronomical observations about the middle of the eighth century, and for 400 years they prosecuted the science with assiduity. Ibn-Yunis (around 1000 AD) made important observations of the disturbances and eccentricities of Jupiter and Saturn. In the sixteenth century Nicholas Copernicus, born in 1473, introduced the system that bears his name, and which gives to the sun the central place in the solar system, and shows all the other bodies, the earth included, revolving around him. This arrangement of the universe came at length to be generally received on account of the simplicity it substituted for the complexities and contradictions of the theory of Ptolemy. The observations and calculations of Tycho Brahe, a Danish astronomer, born in 1546, continued over many years, were of the highest value, and claim for him the title of regenerator of practical astronomy. His assistant and pupil, Johann Kepler, born in 1571, was enabled, principally by the aid he received from his master's labours, to arrive at those laws which have made his name famous: 1. That the planets move, not in circular, but in elliptical orbits, of which the sun occupies a focus. 2. That the radius vector, or imaginary straight line joining the sun and any planet, moves over equal spaces in equal times. 3. That the squares of the times of the revolutions of the planets are as the cubes of their mean distances from the sun, Galileo, who died in 1642, advanced the science by his observations and by the new revelations he made through his telescopes, which established the truth of the Copernican theory.

Isaac Newton, born in 1642, carried physical astronomy suddenly to comparative perfection. Accepting Kepler's laws as a statement of the facts of planetary motion he deduced from them his theory of gravitation. The science was enriched towards the close of the eighteenth century by the discovery by Herschel of the planet Uranus and its satellites, the resolution of the Milky Way into myriads of stars, and the unravelling of the mystery of nebulae and of double and triple stars. The splended analytical researches of Lalande, Lagrange, Delambre, and Laplace, mark the same period. The nineteenth century opened with the discovery of the first four minor planets; and the existence of another planet (Neptune) more distant from the sun than Uranus, was, in 1845, simultaneously and independently predicted by Leverrier and Adams. Of later years the sun attracted a number of observers, the spectroscope and photography having been especially fruitful in this field of investigation. From transit observations carried out at the end of the 19th century the former calculated distance of the sun has been corrected, and is now given as 92,560,000 miles. The two satellites of Mars, and of others belonging to Jupiter were also discovered towards the end of the 19th century.

The objects with which astronomy has chiefly to deal are the earth, the sun, the moon, the planets, the fixed stars, comets, nebulae, and meteors. The stellar universe is composed of an unknown host of stars, many millions in number, the most noticeable of which have been formed into groups called constellations. The nebulae are cloud-like patches of light scattered all over the heavens. Some of them have been resolved into star-clusters, but many of them are but masses of incandescent gas. Of the so-called fixed stars, many are now known to be by no means fixed, but revolve in company with another or others. Variable stars and non-luminous stars are also known. The fixed stars preserve, at least to unaided vision, an unalterable relation to each other, because of their vast distance from the earth. Their apparent movement from east to west is the result of the earth's revolution on its axis in twenty-four hours from west to east. The planets have not only an apparent, but also a real and proper motion, since, like our earth, they revolve around the sun in their several orbits and periods.

The mid-20th century saw great leaps in astronomical research with rockets, derived from the German terror weapons of the Second World War, being used to send probes and men into space for closer examination of the heavenly bodies. A retroreflector left on the Moon's surface by Apollo astronauts during the NASA Apollo missions returns a high-power laser beam emitted from the Earth, enabling researchers to carry out regular monitoring and measure the distance between the Earth and the Moon to an accuracy of a few centimetres.

We now know something of the planets in our solar system. We know that Mercury is too hot to retain an atmosphere, and that Venus' brilliant white appearance is the result of its being completely enveloped by thick clouds of carbon dioxide. Below the upper clouds Venus has a hostile atmosphere containing clouds of sulphuric acid droplets. The cloud cover shields the planet's surface from direct sunlight, but the energy that does filter through warms the surface, the heat being trapped by the dense clouds, resulting in a very high surface temperature of almost 480 degrees Centigrade. Radar can penetrate the thick Venusian clouds which obscure the surface from telescopes, and has been used to map the planet's surface. Yet, despite advances, the origins of the universe, the stars planets, and the planets' asteroids remains a matter of conjecture, theory and debate.
Research Astronomy