Page:Astronomy for Everybody.djvu/250

When the orbit of the planet was determined, so that its course in former years could be mapped out, the curious fact was brought to light that it had been seen and recorded nearly a century before, as well as a few years previously. Flamsteed, Astronomer Royal of England, while engaged in cataloguing the stars, had marked it down as a star on five occasions between 1690 and 1715. What was yet more singular, Lemonnier, at the Paris Observatory, had recorded it eight times in the course of two months, December, 1768, and January, 1769. But he had never reduced and compared his observations, and not till Herschel announced the planet did Lemonnier know how great a prize had lain for ten years within his grasp.

The period of revolution of Uranus is eighty-four years, so that its position in the sky changes but slowly from year to year. During the first ten years of our century it will be in or near the region of the Milky Way, which we see in summer and autumn, low down in the southern sky. This will make it difficult of detection by the naked eye.

The distance of Uranus is about twice that of Saturn. In astronomical units it is 19.2; in our familiar measures 1,790,000,000 miles, or 2,870,000,000 kilometres.

Owing to this great distance, it is hard to see with certainty any features on its surface. In a good telescope it appears as a pale disk with a greenish hue. Some observers have fancied that they saw faintly marked features on its surface, but this is probably an illusion. We may regard it as certain that it rotates on its axis; but