# 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**

### Celestial pole

**North Celestial Polesouth celestial polecelestial north pole**

### 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

**starsstellarmassive 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**

### 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.

### Radian

**radiansradmicroradian**

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.