英和和英

出典:『Wikipedia』 (2011/05/30 11:54 UTC 版)

英語による解説

引用

Sidereal time ( /saɪˈdɪəri.əl/) is a time-keeping system astronomers use to keep track of the direction to point their telescopes to view a given star in the night sky. From a given observation point, a star found at one location in the sky will be found at basically the same location at another night when observed at the same sidereal time. This is similar to how the time kept by a sundial can be used to find the location of the Sun. Just as the Sun and Moon appear to rise in the east and set in the west, so do the stars. Both solar time and sidereal time make use of the regularity of the Earth's rotation about its polar axis. The basic difference between the two is that solar time maintains orientation to the Sun while sidereal time maintains orientation to the stars in the night sky. The exact definition of sidereal time fixes it to the vernal equinox. Precession and nutation, though quite small on a daily basis, prevent sidereal time from being a direct measure of the rotation of the Earth relative to inertial space. Common time on a typical clock measures a slightly longer cycle, accounting not only for the Earth's axial rotation but also for the Earth's annual revolution around the Sun of slightly less than 1 degree per day.