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Astronomy, the science which treats of the heavenly bodies. Spherical astronomy is purely mathematical, and treats of the apparent and real positions of the heavenly bodies in the celestial sphere, including the calculation of their past or future positions, their distances and magnitudes.

Nautical astronomy is an application to the needs of navigators for the determination of position on the earth by means of the configurations in the celestial sphere. Physical astronomy discusses the forces which cause motion, the analysis and composition, early history, and development of the heavenly bodies. The sun is the centre of a system of planets travelling round it, all of them very nearly in the same plane. One of these is our earth. Each planet has its year, or period of revolution round the sun, and its day, or period of rotation about its own axis. The axis of a planet does not remain fixed in direction, but "wobbles" slowly, like the axis of a spinning top. This wobbling is called precession (q.v.).

Some of these planets have satellites of their own. The earth's satellite is the moon (q.v.). Saturn (q.v.), besides having eight moons, has a very remarkable ring round it - the nature of which will be discussed separately. When the moon intervenes between the earth and the sun, part of the latter is obscured, and we have a solar eclipse. If the earth's shadow falls on the moon we have a lunar eclipse. When the space between earth and sun is traversed by another planet we have a transit. The transits of Venus are of very great importance in the accurate estimation of the sun's distance from the earth - about 92,000,000 miles being its mean value. The variation of the sun's distance, together with the obliquity of the earth's axis to its ecliptic or plane of motion round the sun, determines the seasons. The attraction of the moon and sun on the waters of the earth produce tides, which are of greater or less extent as the sun and moon act in conjunction or oppose each other's effects.

Besides our sun, there are innumerable other suns in the universe, i.e. the stars, but at such tremendous distances that they are but points in the heavens even when viewed with the largest telescopes. The stars were fancifully arranged into constellations by the ancients, and for convenience the old names are still employed in classification. Hundreds of stars have been found to be double, i.e. they exist in pairs, each pair revolving about the common mass-centre, like chain shot, but with only the immaterial link of gravity keeping them together. Finally, we have nebula:, some consisting of matter in a gaseous state, others composed of immense aggregations of stars resembling faintly luminous clouds, and requiring telescopes of high power to resolve them into individual stars. The Milky Way is an example of the latter.

Astronomy was studied by the ancients to a considerable extent. The Chinese are said to have recorded a conjunction of Saturn, Jupiter, Mars, and Mercury with the moon, which phenomenon took place B.C. 2500. The Indians were able to calculate eclipses, and certain observations of the Chaldean astronomers have been proved to be true by recent calculations. Aristarchus (260 B.C.) taught the double motion of the earth round the sun and round its own axis. Hipparchus determined the eccentricity of the earth's orbit, the length of the solar year, noted the precession of the equinoxes, and started a catalogue of stars. The Ptolemaic system (q.v.) was propounded in the second century; it regarded the earth as the centre of the universe, round which revolved the sun, moon, and planets. It held sway till the time of Copernicus in the sixteenth century, who taught that the sun was the centre of our system. Then Kepler, chiefly by means of the observations of Tycho Brahe, arrived at his three laws of planetary motion (q.v.). Newton gave the world his theory of gravitation and the laws of motion, and from that time to the present, chiefly on account of the advance in optical science and the consequent development of the telescope, astronomical discovery has progressed to an amazing extent.