Fig. 497.—Radii Vectores.
It was about this time—viz., the beginning of the seventeenth century—that the telescope was invented, and logarithms came into use. The actual discoverer of this now almost perfected instrument is uncertain. Borelli, who wrote in the seventeenth century, ascribes the discovery to Zachariah Jansen and Hans Lippersheim, spectacle makers of Middleburg. Baptista Porta, also a spectacle maker, has had the credit of discovering the magnifying power of the lens, and, so far, the originator of the telescope.
Fig. 498.—Ecliptic and Equator.
But whoever invented it, the telescope did not penetrate into southern Europe till 1608-9. Galileo then made inquiries concerning the new instrument, and Kepler made some propositions for their construction. But Harriot had used the instrument so far back as 1611 or 1612, and had observed spots upon the sun’s disc. Galileo, in 1610, had also made observations with the telescope, and discovered the satellites of Jupiter. He thereby confirmed the Copernican theory;[25] and when Newton promulgated his immortal discovery of gravitation, after Picard’s researches, the relations of the sun and planets became more evident. His researches were published in the Principia, and then one-half the scientific world began to question the principle of gravitation, which was supported by Newton and his adherents. Subsequently the researches of Lagrange and Laplace, Adams and Leverrier, Sir J. Herschel, etc., brought astronomy into prominence more and more; and the innumerable stars have been indicated as new planets have been discovered. The spectroscope, which gives us the analyses of the sun and other heavenly bodies, has, in the able hands of living astronomers, revealed to us elements existing in the vapours and composition of the sun, etc. Stars are now known to be suns, some bearing a great resemblance to our sun, others differing materially. The nebulæ have been analysed, and found to be stars, or gas, burning in space—hydrogen and nitrogen being the chief constituents of this glowing matter. Instruments for astronomical observation have now been brought to a pitch of perfection scarcely ever dreamed of, and month by month discoveries are made and recorded, while calculations as to certain combinations can be made with almost miraculous accuracy. The transit of Venus, the approaches of comets, eclipses, and the movements of stars, are now known accurately, and commented upon long before the event can take place.
We will close this chapter by giving a brief explanation of the various definitions most usually employed in astronomy.
1. The Axis of the earth is an imaginary line passing through the centre (north and south); the poles are the extremities of this line.
2. The Equator is an imaginary circle passing round the globe, dividing it into northern and southern hemispheres. The equinoctial is the plane of the former circle extended to the heavens, and when the sun appears in that line the days and nights are of equal duration—twelve hours each.
3. The Ecliptic is the sun’s path through the heavens—though, of course, the sun does not actually move, and therefore the track, or supposed circle, is really the earth’s motion observable from the sun. When the moon is near this circle eclipses happen. The ecliptic cuts the equinoctial at an angle of about 23°. One half is to the north and the other to the south of the equinoctial.