These values are important in observation from Earth for calculating the apparent position of astronomical objects. The component perpendicular to the ecliptic is known as the nutation in obliquity. The component which works along the ecliptic is known as the nutation in longitude. Values of nutation are usually divided into components parallel and perpendicular to the ecliptic. However, there are other significant periodical terms which must be calculated depending on the desired accuracy of the result. The largest component of Earth's nutation has a period of 18.6 years, the same as that of the precession of the Moon's orbital nodes. In the case of Earth, the principal sources of tidal force are the Sun and Moon, which continually change location relative to each other and thus cause nutation in Earth's axis. Nutation is a slight irregular motion (etymologically a "nodding") in the axis of rotation of a planet, due to the fact that the tidal forces which cause precession of the equinoxes vary over time so that the speed of precession is not constant. Astronomers use local sidereal time because it corresponds to the coordinate right ascension of a celestial body that is presently on the local meridian.
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Local mean sidereal time is computed from the current Greenwich Mean Sidereal Time plus an input offset in longitude (converted to a sidereal offset by the ratio 1.00273790935 of the mean solar day to the mean sidereal day.) Applying the equation of equinoxes, or nutation of the mean pole of the Earth from mean to true position, yields local apparent sidereal time. The daily motion of this point provides a measure of the rotation of the Earth with respect to the stars, rather than the Sun. Sidereal time is the hour angle of the vernal equinox, the ascending node of the ecliptic on the celestial equator. For Financial dates and legal dates, days between dates, days before a date or days after a date, day of the week for a given date in history, involving the last 2 centuries, we suggest that you use our Julian to current day and date calculator. A Julian date is valid from JanuB.C., or to those that do not accept Christianity, BCE, to present, sometime after 2004 A.D., or for the non-Christians, CE. Each field MUST have a valid value in it. Then press the Calculate button to calculate the Julian Date and Sidereal times, local and Greenwich. Enter the UTC date and time in the appropriate fields also enter the desired longitude and latitude. This is an advanced special date and time calculator. This calculator requires the use of Javascript enabled and capable browsers. Calendar Date And Time To Julian Day And Sidereal Times
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