# Hour circle

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.wikipedia
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### Declination

DecDec.declinations
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. 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.
Declination's angle is measured north or south of the celestial equator, along the hour circle passing through the point in question.

### Right ascension

RAR.A.α
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.
Right ascension (abbreviated RA; symbol α) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the (hour circle of the) point above the earth in question.

### Hour angle

Greenwich hour anglelocal hour angleSidereal hour angle
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.
The hour angle of a point is the angle between two planes: one containing Earth's axis and the zenith (the meridian plane), and the other containing Earth's axis and the given point (the hour circle passing through the point).

### Astronomy

astronomicalastronomerastronomers
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.

### Center of mass

center of gravitycentre of gravitycentre of mass
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.

### Astronomical object

celestial bodiescelestial bodycelestial object
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.

### Great circle

Great Circle Routegreat circlesarcs of great circle
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.

### Celestial pole

North Celestial Polesouth celestial polecelestial north pole
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.

### Meridian (astronomy)

meridiancelestial meridianastronomical meridian
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.

### Perpendicular

perpendicularlyPerpendicularitynormal
The hour circles, specifically, are perfect circles perpendicular (at right angles) to the celestial equator.

### Right angle

right anglesright-angle90 degrees
The hour circles, specifically, are perfect circles perpendicular (at right angles) to the celestial equator.

### Celestial equator

equatorialequatorial planeequatorial sky
The hour circles, specifically, are perfect circles perpendicular (at right angles) to the celestial equator.

### Celestial sphere

celestialcelestial hemispherehemisphere
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°).

### 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.

### Planet

planetsFormer classification of planetsplanemo
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.

### Equinox

autumnal equinoxautumn equinoxequinoxes
These are as located at the vernal equinox for the epoch (e.g. J2000) stated.

### Epoch (astronomy)

J2000J2000.0epoch
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.

The hour circle is a subtype whereby it is expressed in hours as opposed to degrees, radians, or other units of angle.

### Earth's rotation

rotation of the EarthrotationEarth rotates
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.

### Longitude

WestlongitudinalE
An astronomical meridian follows the same concept and, almost precisely, the orientation of a meridian (also known as longitude) on a globe.

### Globe

terrestrial globeglobalcelestial globe
An astronomical meridian follows the same concept and, almost precisely, the orientation of a meridian (also known as longitude) on a globe.

### Parallactic angle

In spherical astronomy, the parallactic angle is the angle between the great circle through a celestial object and the zenith, and the hour circle of the object.

### Plane of reference

reference planeplaneplanes
This is usually defined as the point on the celestial sphere where the plane crosses the prime hour circle (the hour circle occupied by the First Point of Aries), also known as the equinox.

### Equatorial coordinate system

equatorial coordinatesequatorialCoordinates
, (lower case "alpha", abbreviated RA) measures the angular distance of an object eastward along the celestial equator from the vernal equinox to the hour circle passing through the object.