Sidereal Time
Time regulated by the transit, over the meridian of a place of the first point of Aries, or the vernal equinox, and chiefly used in astronomical observations.
The sidereal day is 3 m., 56 s. shorter than the mean solar day. The pendulum of a clock, to show sidereal time, must be a trifle shorter than that of one used to show mean time, both clocks having the same train. On or about the 15th of April the two clocks would agree, but from that time on there would be a divergence of 3 m., 56 s. per day. In the absence of a transit instrument and a table giving the right ascension of particular stars, Britten advises the selection of a window having a southern aspect, from which a chimney, or a steeple, or any other fixed point, may be seen. To the side of the window attach a thin plate of brass having a small hole in it, in such a manner that by looking through the hole toward the edge of the elevated object, some of the fixed stars may be seen; the progress of one of these being watched, the instant it vanishes behind the fixed point a signal is made to a person observing the clock, who then notes the exact time at which the star disappears, and on the following night the same star will vanish behind the same object 3 m., 56 s. sooner. If a clock mark 10 h. when the observation is made, when the star vanishes the following night it should indicate 3 m., 56 s. less than 10 h. If several cloudy nights have rendered it impossible to compare the clock with the star, it will then be necessary to multiply 3 m., 56 s. by the number of days that have elapsed since the observation, and the product deducted from the hour the clock then indicates gives the time the clock should show. The same star can only be observed during a few weeks, for, as it gains nearly a half hour a week, it will, in a short time, come to the meridian in broad daylight and become invisible; to continue the observation, another star must be selected. In making the observation, care must be taken that a planet is not observed instead of a star; Mars, Jupiter, and Saturn are those most likely to occasion this error, more especially Saturn, which, from being the most distant of the three, resembles a star of the first magnitude. The planets may, however, be easily distinguished, for being comparatively near the earth, they appear larger than the stars; their light also is steady because reflected, while the fixed stars scintillate and have a twinkling light. A sure means of distinguishing between them, is to watch a star attentively for a few nights; if it change its place with regard to the other stars, it is a planet. See Transit Instrument.