In astronomy, the hour circle, which together with declination and distance (from the planet's centre of mass) determines the location of any celestial object, is the great circle through the object and the two celestial poles.[1] As such, it is a higher concept than the meridian as defined in astronomy, which takes account of the terrain and depth to the centre of Earth at a ground observer's location. The hour circles, specifically, are perfect circles perpendicular (at right angles) to the celestial equator. By contrast, the declination of an object viewed on the celestial sphere is the angle of that object to/from the celestial equator (thus ranging from +90° to −90°).
Diagram illustrating the definition of the hour circle of a star
The location of stars, planets, and other similarly distant objects is usually expressed in the following parameters, one for each of the three spatial dimensions: their declination, right ascension (epoch-fixed hour angle), and distance. These are as located at the vernal equinox for the epoch (e.g. J2000) stated.[2]
A meridian on the celestial sphere matches an hour circle at any time. 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 hour circles (meridians) are measured in hours (or hours, minutes, and seconds); one rotation (360°) is equivalent to 24 hours; 1 hour is equivalent to 15°.
An astronomical meridian follows the same concept and, almost precisely, the orientation of a meridian (also known as longitude) on a globe.
hour, circle, astronomy, hour, circle, which, together, with, declination, distance, from, planet, centre, mass, determines, location, celestial, object, great, circle, through, object, celestial, poles, such, higher, concept, than, meridian, defined, astronom. In astronomy the hour circle which together with declination and distance from the planet s centre of mass determines the location of any celestial object is the great circle through the object and the two celestial poles 1 As such it is a higher concept than the meridian as defined in astronomy which takes account of the terrain and depth to the centre of Earth at a ground observer s location The hour circles specifically are perfect circles perpendicular at right angles to the celestial equator By contrast the declination of an object viewed on the celestial sphere is the angle of that object to from the celestial equator thus ranging from 90 to 90 Diagram illustrating the definition of the hour circle of a star The location of stars planets and other similarly distant objects is usually expressed in the following parameters one for each of the three spatial dimensions their declination right ascension epoch fixed hour angle and distance These are as located at the vernal equinox for the epoch e g J2000 stated 2 A meridian on the celestial sphere matches an hour circle at any time 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 hour circles meridians are measured in hours or hours minutes and seconds one rotation 360 is equivalent to 24 hours 1 hour is equivalent to 15 An astronomical meridian follows the same concept and almost precisely the orientation of a meridian also known as longitude on a globe See also EditMeridian astronomy DeclinationNotes Edit Introduction to Spherical Astronomy p 8 www physics csbsju edu Retrieved 2019 02 01 Wakker K F March 2010 AE4874 I Astrodynamics Part I Delft University of Technology References Edithour circle astronomy Britannica Online Encyclopedia This astronomy related article is a stub You can help Wikipedia by expanding it vte Retrieved from https en wikipedia org w index php title Hour circle amp oldid 1052624930, wikipedia, wiki, book, books, library,
