The Astronomy of Milton's 'Paradise Lost'

by Thomas Nathaniel Orchard

This, when rendered correctly, reads:—

Cynthiae figuras aemulatur mater amorum.

Venus rivals the appearances of the Moon.

The phases of Venus were one of the most interesting of Galileo’s discoveries with the telescope. When observed near inferior conjunction the planet presents the appearance of a slender crescent, resembling the Moon when a few days old. Travelling from this point to superior conjunction, the illumined portion of her disc gradually increases, until it becomes circular, like the full Moon. This changing appearance of Venus afforded Galileo irresistible proof that the planet is an opaque body, which derives its light from the Sun, and that it circles round the orb—convincing evidence of the accuracy and truthfulness of the Copernican theory.

It was in this manner that Galileo announced his discovery of the phases of Venus, the peerless planet of our morning and evening skies, whose slender crescent forms such a beautiful object in the telescope, and who, as she traverses her orbit, exhibits all the varied changes of form presented by the Moon in her monthly journey round the Earth. These varying aspects of Venus were not unknown to Milton; and, indeed, he may have been informed of them by Galileo in his conversation with him at Arcetri; nor has he failed to introduce an allusion to this beautiful phenomenon in his poem. In his description of the Creation, after the Sun was formed, he adds:—

Hither, as to their fountain, other stars
Repairing, in their golden urns draw light,
And hence the morning planet gilds her horns.—vii. 364-66.

Galileo also discovered that the planet Mars does not always present the appearance of a circular disc. When near opposition the full disc of the planet is visible, but at all other times it is gibbous, and approaches nearest to that of a half-moon when at the quadratures.

In the year 1610, on directing his telescope to the Sun, Galileo detected dark spots on the solar disc. Similar spots, sufficiently large to be distinguished by the naked eye, had been observed from time to time for centuries prior to the invention of the telescope, but nothing was known of their nature. In 1609 Kepler observed a spot on the Sun, which he thought was the planet Mercury in conjunction with the orb; the short time during which it was visible, in consequence of clouds having obscured the face of the luminary, prevented him from being able to determine the accuracy of his surmise, but since then it has been ascertained that no transit of Mercury took place at that time, and Kepler afterwards acknowledged that he had arrived at an erroneous conclusion. Galileo was much puzzled in trying to find out the true nature of the spots. At first he was led to imagine that planets like Mercury and Venus revolved round the Sun at a short distance from the orb, and that their dark bodies, travelling across the solar disc, gave rise to the phenomenon of the spots. After further observation, he ascertained that the spots were in actual contact with the Sun; that they were irregular in shape and size, and continued to appear and disappear. Sometimes a large spot would break up into several smaller ones, and at other times three or four small spots would unite to form a large one. They all had a common motion, and appeared to rotate with the Sun, from which Galileo concluded that the orb rotated on his axis in about twenty-eight days. Galileo believed that the spots were clouds floating in the solar atmosphere, and that they intercepted a portion of the light of the Sun.

The Milky Way, that wondrous zone of light which encircles the heavens, remained for many ages a source of perplexity to ancient astronomers and philosophers, who, in their endeavours to ascertain its nature, had arrived at various absurd and erroneous conclusions. On directing his telescope to this luminous tract, Galileo discovered, to his inexpressible admiration, that it consists of a vast multitude of stars, too minute to be visible to the naked eye. He also discerned that its milky luminosity is created by the blended light of myriads of stars, so remote as to be incapable of definition by his telescope. In his ‘Nuncius Sidereus’ he gives an account of his observations of the Galaxy and expresses his satisfaction that he has been enabled to terminate an ancient controversy by demonstrating to the senses the stellar structure of the Milky Way. When engaged in exploring the celestial regions with his telescope, Galileo observed a marked difference in the appearance of the fixed stars, as compared with that of the planets. Each of the latter showed a rounded disc resembling that of a small moon, but the stars exhibited no disc, and shone as vivid sparkling points of light; all of them, whether of large or small magnitude, presenting the same appearance in the telescope. This led him to conclude that the fixed stars were not illumined by the Sun, because their brilliancy in all their changes of position remained unaltered. But, in the case of the planets, he found that their lustre varied according to their distance from the Sun; consequently, he believed they were opaque bodies which reflected the solar rays. On directing his telescope to the Pleiades, which, to the naked eye, appear as a group of seven stars, he succeeded in counting forty lucid points. The nebula Praesepe in Cancer, he was also able to resolve into a cluster of stars. Galileo made many other observations of the heavenly bodies with his telescope, all of which he describes as having afforded him ‘incredible delight.’

Shortly before the failure of his eyesight, Galileo discovered the Moon’s diurnal libration, a variation in the visible edges of the Moon caused by its oscillatory motion, and the diurnal rotation of the Earth on her axis.