Si·de·re·al a.
 1. Relating to the stars; starry; astral; as, sidereal astronomy.
 2. Astron. Measuring by the apparent motion of the stars; designated, marked out, or accompanied, by a return to the same position in respect to the stars; as, the sidereal revolution of a planet; a sidereal day.
 Sidereal clock, day, month, year. See under Clock, Day, etc.
 Sideral time, time as reckoned by sideral days, or, taking the sidereal day as the unit, the time elapsed since a transit of the vernal equinox, reckoned in parts of a sidereal day. This is, strictly, apparent sidereal time, mean sidereal time being reckoned from the transit, not of the true, but of the mean, equinoctial point.
 ◄ ►
 Clock n.
 1. A machine for measuring time, indicating the hour and other divisions; in ordinary mechanical clocks for domestic or office use the time is indicated on a typically circular face or dial plate containing two hands, pointing to numbers engraved on the periphery of the face, thus showing the hours and minutes. The works of a mechanical clock are moved by a weight or a spring, and it is often so constructed as to tell the hour by the stroke of a hammer on a bell. In electrical or electronic clocks, the time may be indicated, as on a mechanical clock, by hands, but may also be indicated by direct digital readout, with the hours and minutes in normal Arabic numerals.  The readout using hands is often called analog to distinguish it from the digital readout.  Some clocks also indicate the seconds. Clocks are not adapted, like the watch, to be carried on the person.  Specialized clocks, such as atomic clocks, may be constructed on different principles, and may have a very high precision for use in scientific observations.
 2. A watch, esp. one that strikes. [Obs.]
 3. The striking of a clock. [Obs.]
 4. A figure or figured work on the ankle or side of a stocking.
 Note: ☞ The phrases what o'clock? it is nine o'clock, etc., are contracted from what of the clock? it is nine of the clock, etc.
 Alarm clock. See under Alarm.
 Astronomical clock. (a) A clock of superior construction, with a compensating pendulum, etc., to measure time with great accuracy, for use in astronomical observatories; -- called a regulator when used by watchmakers as a standard for regulating timepieces. (b) A clock with mechanism for indicating certain astronomical phenomena, as the phases of the moon, position of the sun in the ecliptic, equation of time, etc.
 Electric clock. (a) A clock moved or regulated by electricity or electro-magnetism. (b) A clock connected with an electro-magnetic recording apparatus.
 Ship's clock Naut., a clock arranged to strike from one to eight strokes, at half hourly intervals, marking the divisions of the ship's watches.
 Sidereal clock, an astronomical clock regulated to keep sidereal time.