An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian
فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک
M. Heydari-Malayeri - Paris Observatory
Homepageapparent sidereal day ruz-e axtari-ye padidâr Fr.: jour sidéral apparent The time interval between two successive → upper transits of the → true equinox of date. |
Greenwich sidereal day number šomâre-ye ruz-e axtari-ye Greenwich Fr.: nombre du jour sidéral de Greenwich The integral part of the → Greenwich sidereal date. |
mean sidereal day ruz-e axtari-ye miyângin Fr.: jour sidéral moyen The average time interval between two successive → upper transits of the → mean equinox. |
sidereal day ruz-e axtari (#) Fr.: jour sidéral The period of → Earth's rotation around its axis, the mean value of which is about 23h 56m 04.092s (23.934 470 hours). In other words, the interval between two successive passages of a star across a given → meridian. The sidereal day is 3 minutes 56 seconds shorter than the → mean solar day. The reason is that Earth moves a little less than a degree around the Sun during the time it takes for one full axial rotation. So, for the Sun to appear on the same meridian in the sky again after one full axial rotation, the Earth has to rotate one extra degree to bring the Sun into the same apparent meridian in the sky. This is also why the stars rise and set about 4 min earlier each day. |
