The large satellites are very interesting objects for the telescope. When they come between the sun and Jupiter their round black shadows can be plainly seen moving across his disk, and when they pass round into his shadow they are suddenly eclipsed, emerging after a time out of the other side of the shadow. These phenomena are known as transits and eclipses, and their times of occurrence are carefully predicted in the _American Ephemeris and Nautical Almanac_, published at Washington for the benefit of astronomers and navigators, because these eclipses can be employed in comparing local time with standard meridian time. They were formerly utilised to determine the velocity of light, in this way: