In astronomy, the hour circle is the great circle through a given object and the two celestial poles.[1] Together with declination and distance (from the planet's centre of mass), it determines the location of any celestial object. 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°).
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.
^"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.
In astronomy, the hourcircle is the great circle through a given object and the two celestial poles. Together with declination and distance (from the...
navigation, the hour angle is the dihedral angle between the meridian plane (containing Earth's axis and the zenith) and the hourcircle (containing Earth's...
along the celestial equator from the Sun at the March equinox to the (hourcircle of the) point in question above the Earth. When paired with declination...
The CirclingHour is a progressive rock album by Iona, released in 2006. It marked a return to the studio for the band after a break of nearly six years...
other being hour angle. The declination angle is measured north (positive) or south (negative) of the celestial equator, along the hourcircle passing through...
horsepower-hour (hph), a measure of energy used in the railroad industry hour angle, a measure of the angle between the meridian plane and the hourcircle passing...
point on the celestial sphere where the plane crosses the prime hourcircle (the hourcircle occupied by the First Point of Aries), also known as the equinox...
Southern Temperate Zone. South of the Antarctic Circle, the Sun is above the horizon for 24 continuous hours at least once per year (and therefore visible...
south pole of the sphere is a circle of 25 hours, fixed to the rings, and on the axis is an index which goes round that circle, if the globe be turned round...
meridian to the hourcircle passing through the object. Unlike right ascension, hour angle is always increasing with the rotation of Earth. Hour angle may be...
parallactic angle is the angle between the great circle through a celestial object and the zenith, and the hourcircle of the object. It is usually denoted q....
refers to the consecutive 24-hour periods of sunlight experienced north of the Arctic Circle and south of the Antarctic Circle. Other phenomena are sometimes...
circle that passes through the March equinox (the northern hemisphere's vernal equinox) and both celestial poles, and is usually expressed in hours,...
The Arctic Circle is one of the two polar circles, and the most northerly of the five major circles of latitude as shown on maps of Earth at about 66°...
". Circle Chart (in Korean). Archived from the original on April 20, 2023. Retrieved May 5, 2023. "All non-single tracks from Magic Hour". Circle Chart...
A circle of latitude or line of latitude on Earth is an abstract east–west small circle connecting all locations around Earth (ignoring elevation) at...
Circle K Stores, Inc. is an American chain of convenience stores that is headquartered in Tempe, Arizona, and owned by Alimentation Couche-Tard, Inc.,...
A magic circle is a circle of space marked out by practitioners of some branches of ritual magic, which they generally believe will contain energy and...
heist sequence which is about half an hour in length and has almost no dialogue. The film's title means "The Red Circle" and refers to the film's epigraph...
41°56′N 87°38′W / 41.93°N 87.64°W / 41.93; -87.64 The Wieners Circle is a hot dog stand on Clark Street in the Lincoln Park neighborhood of Chicago...
life. A quality circle is a small group of workers that work in the same area or do similar sorts of work and meet once a week for an hour to identify, analyse...
coordinate systems), hours (in the medical field), degrees of temperature or alcohol proof. The symbol consists of a small superscript circle. The word degree...
Armstrong Circle Theatre is an American anthology drama television series which ran from June 6, 1950, to June 25, 1957, on NBC, and from October 2, 1957...
The Circle Line (CCL) is a medium-capacity Mass Rapid Transit line in Singapore. It runs in a loop from Dhoby Ghaut station in the city-centre to HarbourFront...