Eclipses of the Moon, though more often and more widely visible than eclipses of the Sun, do not offer by any means the same variety of interesting or striking phenomena to the mere star-gazer, and it was long thought that they were in a certain sense of no use to science. Now, however, astronomers are inclined to utilise them for determining the diameter of the Moon by noting occultations[116] of stars by the Moon, the duration of a star’s invisibility behind an eclipsed Moon being a measure of the lunar diameter when such an observation is properly transformed and “reduced.” Observations of the heat radiated (or rather reflected) by an eclipsed Moon have also been made with the interesting result of showing that during an eclipse the Moon’s power to reflect solar heat to the Earth sensibly declines. The duration of an eclipse of the Moon is dependent on its magnitude. Where the eclipse is total the darkness, or what counts for such, may last for nearly 4 hours, though this is an extreme limit rarely attained. An eclipse of from 6 to 12 digits (to use the old-fashioned nomenclature which has been already explained) will continue from 2½ to 3½ hours. An eclipse of 3 to 6 digits will last 2 or 3 hours, and a smaller eclipse only 1 or 2 hours. The visual observations to be made in connection with partial or total eclipses of the Moon chiefly relate to the appearances presented by our satellite when immersed in the Earth’s shadow. On such occasions, as has been already stated, it frequently happens that the Moon does not wholly disappear, but may be detected either with a telescope or even without one. It may exhibit either a dull grey appearance, or more commonly a pinkish-red hue to which the designation “coppery” is generally applied. Perhaps the most remarkable instance of this was the eclipse of March 19, 1848. Mr. Forster who observed the phenomenon at Bruges thus describes[117] what he saw:—“I wish to call your attention to the fact which I have clearly ascertained, that during the whole of the late eclipse of March 19 the shaded surface presented a luminosity quite unusual, probably about three times the intensity of the mean illumination of the eclipsed lunar disc. The light was of a deep red colour. During the totality of the eclipse the light and dark places on the face of the Moon could be almost as well made out as on an ordinary dull moonlight night, and the deep red colour where the sky was clearer was very remarkable from the contrasted whiteness of the stars. My observations were made with different telescopes, but all presented the same appearance, and the remarkable luminosity struck everyone. The British Consul at Ghent, _who did not know there was an eclipse_, wrote to me for an explanation of the blood-red colour[118] of the Moon at 9 o’clock.”