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In astronomy and celestial navigational, the hour angle is the dihedral angle between the meridian plane (containing Earth's axis and the zenith) and the hour circle (containing Earth's axis and a given point of interest).
A culminating star on the observer's meridian is said to have a zero hour angle (0 h). One sidereal hour (approximately 0.9973 solar hours) later, Earth's rotation will carry the star to the west of the meridian, and its hour angle will be 1 h. When calculating topocentric phenomena, right ascension may be converted into hour angle as an ...
The hour circle is a subtype whereby it is expressed in hours as opposed to degrees, radians, or other units of angle. The hour circles make for easy prediction of the angle (and time due to Earth's fairly regular rotation , approximately equal to the time) between the observation of two objects at the same, or similar declination.
The Right ascension of the star is about 18 h. 18 h means it is a March early-hours star and in blue sky in the morning. If 12 h RA, the star would be a March all-night star as opposite the March equinox. If 6 h RA the star would be a March late-hours star, at its high (meridian) at dusk.
Angles in the hours ( h), minutes ( m), and seconds ( s) of time measure must be converted to decimal degrees or radians before calculations are performed. 1 h = 15°; 1 m = 15′; 1 s = 15″ Angles greater than 360° (2 π ) or less than 0° may need to be reduced to the range 0°−360° (0–2 π ) depending upon the particular calculating ...
Beginning during the 1970s, the radio astronomy methods very-long-baseline interferometry (VLBI) and pulsar timing overtook optical instruments for the most precise astrometry. This resulted in the determination of UT1 (mean solar time at 0° longitude) using VLBI, a new measure of the Earth Rotation Angle, and new definitions of sidereal time ...
The angle between an object's rotational axis and its orbital axis, or, equivalently, the angle between its equatorial plane and orbital plane. Axial tilt usually does not change considerably during a single orbital period; Earth's axial tilt is the cause of the seasons. Axial tilt is distinct from orbital inclination. axis of rotation azimuth
In astronomy, declination (abbreviated dec; symbol δ) is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. The declination angle is measured north (positive) or south (negative) of the celestial equator, along the hour circle passing through the point in question. [1]