Page:Astronomy for Everybody.djvu/293

tends, when one shall appear. Of what are called great comets, there were, five or six during the nineteenth century. The most remarkable and brilliant of all appeared in 1858, and bears the name of Donati, its discoverer, an astronomer of Florence, Italy. Its history will show the changes through which such a body goes. It was first seen on June second, but was then only a faint nebulosity, visible in the telescope like a minute white cloud in the heavens. No tail was then visible, nor was there the slightest indication of what the little cloud would grow into until the middle of August. Then a small tail gradually began to form. Early in September the object became visible to the naked eye. From that time it increased at an extraordinary rate, growing larger and more conspicuous night after night. Its motions were such that it seemed to move but little for the period of a whole month, floating in the western sky night after night. It attained its greatest brilliancy about October tenth. Careful drawings of it were made from time to time by George P. Bond, of the Harvard Observatory. We give two of these, one a naked eye view, the other a telescopic one showing what the head of the comet looked like. After October tenth it rapidly faded away. It soon travelled toward the south, and passed below our horizon, but was followed by observers in the southern hemisphere until March, 1859.

Before the comet had passed out of sight, computers began to calculate its orbit. It was soon found not to move in an exact parabola, but in a very elongated ellipse. The period was not far from nineteen hundred years, but