In astronomy, declination (abbrev. Astronomy (from the Greek words astron (ἄστρον "star" and nomos (νόμος "law" is the scientific study dec or δ) is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. The equatorial coordinate system is probably the most widely used Celestial coordinate system, whose equatorial coordinates are Declination (\delta Right ascension (abbrev RA; symbol α) is the Astronomical term for one of the two Coordinates of a point on the Celestial sphere In Astronomy, the hour angle is one of the coordinates used in the Equatorial coordinate system for describing the position of a point on the Celestial sphere Dec is comparable to latitude, projected onto the celestial sphere, and is measured in degrees north and south of the celestial equator. Latitude, usually denoted symbolically by the Greek letter phi ( Φ) gives the location of a place on Earth (or other planetary body north or south of the In Astronomy and Navigation, the celestial sphere is an imaginary rotating Sphere of "gigantic Radius " The celestial equator is a Great circle on the imaginary Celestial sphere, in the same plane as the Earth 's Equator. Therefore, points north of the celestial equator have positive declinations, while those to the south have negative declinations.

• An object on the celestial equator has a dec of 0°. The celestial equator is a Great circle on the imaginary Celestial sphere, in the same plane as the Earth 's Equator.
• An object above the north pole has a dec of +90°. The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is subject to the caveats explained below defined as the point in the northern
• An object above the south pole has a dec of −90°. The South Pole, also known as the Geographic South Pole or Terrestrial South Pole, is the southernmost point on the surface of the Earth.

The sign is customarily included even if it is positive. Any unit of angle can be used for declination, but it is often expressed in degrees, minutes, and seconds of arc. A minute of arc, arcminute, or MOA is a unit of angular measurement, equal to one sixtieth (1/60 of one degree.

A celestial object that passes over zenith, has a declination equal to the observer's latitude, with northern latitudes yielding positive declinations. In broad terms the zenith is the direction pointing directly above a particular location ( Perpendicular, Orthogonal) A pole star therefore has the declination +90° or -90°. A pole star is a visible star especially a prominent one that is approximately aligned with the Earth 's Axis of rotation; that is a star whose apparent position Conversely, at northern latitudes φ > 0, celestial objects with a declination greater than 90° - φ, are always visible. Such stars are called circumpolar stars, while the phenomenon of a sun not setting is called midnight sun. A circumpolar star is a star that as viewed from a given latitude on Earth never sets (that is never disappears below the horizon due to its proximity to one of the Celestial The midnight sun is a phenomenon occurring in Latitudes north and nearby to the south of the Arctic Circle and south and nearby to the north of the

If instead of measuring from and along the equator the angles are measured from and along the horizon, the angles are called azimuth and altitude (elevation).

Stars

Because a star lies in a nearly constant direction as viewed from earth, its declination is approximately constant from year to year. A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth However, both the right ascension and declination do change gradually due to the effects of precession of the equinoxes and proper motion. Right ascension (abbrev RA; symbol α) is the Astronomical term for one of the two Coordinates of a point on the Celestial sphere In Astronomy, Precession refers to the movement of the rotational axis of a body such as a planet with respect to Inertial space. The proper motion of a Star is the measurement of its change in position in the sky over time after Improper motions are accounted for

Varying declination

The declinations of all solar system objects change much more quickly than those of stars. The Solar System consists of the Sun and those celestial objects bound to it by Gravity.

Sun

The declination of the Sun (δ) is the angle between the rays of the sun and the plane of the earth's equator. The Sun (Sol is the Star at the center of the Solar System. Since the angle between the earth axis and the plane of the earth orbit is nearly constant, δ varies with the seasons and its period is one year, that is the time needed by the earth to complete its revolution around the sun. A season is one of the major divisions of the Year, generally based on yearly periodic changes in Weather. A year (from Old English gēr) is the time between two recurrences of an event related to the Orbit of the Earth around the Sun

When the projection of the earth axis on the plane of the earth orbit is on the same line linking the earth and the sun, the angle between the rays of the sun and the plane of the earth equator is maximum and its value is 23°27'. This happens at the solstices. Solstices occur twice a year when the tilt of the Earth's axis is most oriented toward or away from the Sun, causing the Sun to reach its northernmost and southernmost extremes Therefore δ = +23°27' at the northern hemisphere summer solstice and δ = -23°27' at the northern hemisphere winter solstice. Due to the changes in the tilt of the Earth's axis, the angle between the rays of the sun and the plane of the earth equator is slightly decreasing.

When the projection of the earth axis on the plane of the earth orbit is perpendicular to the line linking the earth and the sun, the angle between the rays of the sun and the plane of the earth equator is null. This happens at the equinoxes. An equinox is the event of the Sun passing over the Earth's equator in its annual cycle Therefore δ is 0° at the equinoxes.

Since the eccentricity of the earth orbit is quite low, it can be approximated to a circle, and δ is approximately given by the following expression:

where cos operates on degrees; if cos operates on radians, 360° in the equation needs to be replaced with 2π and will still output δ in degree; N is Day of the Year, that is the number of days spent since January 1. This article describes the unit of angle For other meanings see Degree. The radian is a unit of plane Angle, equal to 180/ π degrees, or about 57 New Year See also New Year The Ancient Romans began their consular year on January 1st since 153 BC

An alternative form is given as:^[1]

A more precise formula is given by:^[2]

where

is the fractional year in radians.

More accurate daily values from averaging the four years of a leap-year cycle are given in the Table of the Declination of the Sun. A leap year (or intercalary year) is a year containing one or more extra days (or in the case of Lunisolar calendars an extra month in order to keep the

A diagram demonstrating how the Sun's path over the celestial sphere changes with the varying declination during the year, marking the Azimuths in °N where the sun rises and sets at summer and winter solstice at a place of 56°N latitude. The Sun (Sol is the Star at the center of the Solar System. Azimuth ( is a mathematical concept defined as the angle usually measured in degrees (° between a reference plane and a point. Solstices occur twice a year when the tilt of the Earth's axis is most oriented toward or away from the Sun, causing the Sun to reach its northernmost and southernmost extremes

Moon

The Moon also has an annual cycle, with maximum declination at northern hemisphere midwinter and minimum at midsummer. There is also an approximately 19 year long cycle, varying the maximum declination from +28°35' to +18°18' and the minimum from -18°18' to -28°35'.

See also

• right ascension, celestial coordinate system
• geographic coordinates, ecliptic
• Setting circles

Declination is used in some contexts that rule out astronomical declination, to mean the same as magnetic declination. Right ascension (abbrev RA; symbol α) is the Astronomical term for one of the two Coordinates of a point on the Celestial sphere In Astronomy, a celestial coordinate system is a Coordinate system for mapping positions in the sky A geographic coordinate system enables every location on the Earth to be specified in three coordinates using mainly a spherical coordinate system. The ecliptic is the apparent path that the Sun traces out in the sky during the year Setting circles are used on Telescopes equipped with an Equatorial mount to find astronomical objects in the sky by their equatorial coordinates often used The magnetic declination (also known as grid magnetic angle in military circles at any point on the Earth is the angle between the local magnetic field -- the direction

Declination is occasionally and erroneously used to refer to the linguistic term declension. In Linguistics, declension (or declination) is the occurrence of Inflection in Nouns Pronouns and Adjectives indicating

References

1. ^ Solar Declination
2. ^ Spencer, J. W. 1971: Fourier series representation of the position of the Sun. Search, 2(5), 172.

External links

• Table of the Declination of the Sun: Mean Value for the Four Years of a Leap-Year Cycle
• Declination function for Excel, CAD or your other programs. The Sun API is free and extremely accurate. For Windows computers.