Esfera celeste

S Y S T E M   O F

C O O R D I N A T E S

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  Introduction

        Like a point located on the terrestrial sphere can be determined exactly by its coordinates (geographic length l & latitude j), thus also a star on the celestial sphere. According to why aim, systems from coordinates are used in astronomy different.

 

System of horizontal coordinates

        Height h is the angular distance of a star on the horizon. The height is of 0º when the star is on our horizon, & 90º when it is in the zenith, that is to say, on our head. It is clear that when the value is negative it refers to positions below our horizon. It is possible to emphasize that the value of (90º-h) denominates zenith distance .

        The circle that, parallel to the horizon, happens through the star denominates horizontal circle or azimuth circle . The arc that unites the zenith with the star & the horizon calls vertical circle .

        The azimuth a gives the angular distance between the foot of the vertical circle that passes through the star & the South point. In astronomy usually it is counted in the direction south-west-north-est, that would represent 0º, 90º, 180º & 270º.

        But also, in some books the system is used to count from the North point. There is no a consensus on it.

        The azimuth & the height vary constantly, due to the movement of stars in the sky, therefore, we have a height h & a azimuth to in a determined fixed time varying with the coordinate of the time.

 

System of equatorial coordinates

        Declination d   gives to the angular distance of a star from the call celestial equator us.   She is positive if the star is located to the north of the celestial equator, that is to say, in the North hemisphere, & refusal when the star is below the equator and, therefore, in the austral hemisphere. The North & South celestial poles have, therefore, a declination of ±90º respectively.

        Clock angle t , gives to the angular distance between the star & the meridian in the south. One tells either of 0º 360º in direction the west, north, this & South ones, is to say in the sense of the apparent turn of the celestial vault, or, positive from the meridian towards the west & negative towards the east. The clock angle usually is not moderate in angular units but in units of time.

        A great circle course that happens through the star & the pole denominates hour circle . The circle that happening through the star is parallel to the celestial equator denominates parallel circle.

        Since the clock angle varies with time, it is possible to be introduced & the A.R is introduced in its place or right ascension a .

        The right ascension is also told   of 0º to 360º from the first point of Aries towards the east or of 0h. 24h. The first point of Aries is the point where is the Sun in the spring equinox. One is in the constellation of Piscis & is the point whose equatorial coordinates are of A.R.:0h.00m. & declination d :00º00 '.

        The North Pole is in the constellation of Ursa Minor or Little Bear, whose main star is the denominated polestar or Polaris. One is at the moment to 50 ' of arc of the true celestial North Pole, & one will come near until year 2154 to only 42 ' of arc.

 

North Pole

ecunorte.GIF (9501 bytes)

 

        And the celestial South Pole is in the constellation of Octans or Octans. Peculiarly those that live in the South hemisphere, do not have an austral polestar next to the South Pole as you can see down.

South Pole

ecusur.GIF (11944 bytes)

 

        The star catalogues or any object of the Solar System are catalogued with these coordinates & are the used ones for the location of the object in individual. Most of the telescopes they have equatorial mount & therefore, have circles of right ascension & declination for the location of any star, accumulations, galaxies, etc., of the firmament.

        In addition this system to coordinates is unique since doing without variations to more long term such as aberration, precession, nutation & own movement, always remain fixed, the same happens with the positions of the Sun, the Moon & planets & so on.

 

System of ecliptic coordinates

        Ecliptic latitude b gives the angular distance of a star with the ecliptic, therefore, she will be positive when it is over the ecliptic & refusal when it is below the same one. The ecliptic is the imaginary line that crosses the Sun during a year on the celestial vault. At the moment it happens through 13 constellations that are those of the zodiac along with the constellation of Ophiuchus. They exist, therefore, two poles, the North Pole of the ecliptic that is in the constellation of Draco & the South Pole that is in Doradus .

North Pole

eclnorte.GIF (8655 bytes)

 

South Pole

eclsur.GIF (11011 bytes)

        Ecliptic length l is moderate from the first point of Aries or point g towards the east, in arc units.

 

System of galactic coordinates

        Galactic latitude b is the angular distance of a star from or great circle course defined like mean line of the Milky Channel or galactic equator; the positive values indicate positions to the north of the galactic equator, the negative positions to the south of the same one. & there are, logically two poles galactic, the one north located in the constellation of Coma Berenice & the south located in Scultor .

 

North Pole

galnorte.GIF (8825 bytes)

 

 

South Pole

galsur.GIF (6326 bytes)

        Galactic length l used to before mediate from the point intersection between the galactic equator & the celestial equator that is located in the constellation of Aquila, in the direction of the constellation of Cygnus. Recently it has been agreed upon taking like point a zero from the galactic length point from the constellation from Sagittarius of which one assumes with great security agrees with the direction of the center of the Milky Way.

 

Celestial charts

        It exists in the Web celestial charts centered in the following equatorial coordinates to along with locate to any constellation of the celestial vault the name of the most important stars & the limits between present constellations it is accepted internationally by the International Astronomical Union or I.A.U .  

 Masm © (Last update 01.01.2004)