Celestial coordinates

Astronomy Unit I celestial coordinates detail is present

1. ASTRONOMY Prepared By Ms. R. Umadevi Assistant Professor, Department of Mathematics, Bon Secours College For Women, Thanjavur . CELESTIAL COORDINATES

2. D efinition Celestial coordinates are a reference system used to define the positions of objects on the celestial sphere. There are two main coordinate systems Declination (green) is measured in degrees north and south of the celestial equator. ... Anything south of the equator has a negative declination written with a negative sign. For instance, Vega's declination is +38° 47′ 1″, while Alpha Centauri's is –60° 50′

3. Horizontal Coordinate System Alternatively known as ‘Alt/ Az coordinates’, this system of celestial coordinates is dependent on the observer’s latitude and longitude. Using the observer’s local horizon as a reference plane, the position of an object on the celestial sphere at a particular time is given by its: Altitude - the angular distance above the horizon Azimuth - the angular distance measured east from north and parallel to the horizon

4. Equatorial system The Equatorial Coordinate System is generally the preferred way astronomers use to keep track of the positions of objects in the sky. Astronomers imagine that the Earth is surrounded by a large sphere called the celestial sphere. The Earth's equator and the plane of the Earth's orbit are projected onto this sphere.

5. LATITUDE & LONGITUDE Each city has a unique latitude and longitude. Take Tuscaloosa, Alabama, for example, which is located at latitude +33.2° north, longitude 87.6° west. Or Wonglepong , Australia, situated along that continent's east coast at –27.0° south, 153.2° east. A negative sign in front of the latitude indicates south and a positive sign north. Every location, whether it be a city, airport, or even your own home or apartment building lies somewhere on the worldwide coordinate grid (below), its location fixed by two numbers.

6. How are celestial coordinates measured ? The celestial equivalent of latitude is called declination and is measured in degrees North (positive numbers) or South (negative numbers) of the Celestial Equator. The celestial equivalent of longitude is called right ascension.

7. Declination Declination is measured in degrees north (+) or south (-) of an imaginary line called the Celestial Equator (CE). The Celestial Equator is the projection of the Earth's Equator onto the Celestial Sphere. The CE has a declination of 0 degrees, by definition. At dec = +90 degrees (90 degrees N) is the North Celestial Pole (NCP), the projection of the Earth's North Pole onto the Celestial Sphere. The South Celestial Pole (SCP) is at dec = -90 degrees.

8. The zero point for celestial longitude (that is, for right ascension) is the Vernal Equinox, which is that intersection of the ecliptic and the celestial equator near where the Sun is located in the Northern Hemisphere Spring. The other intersection of the Celestial Equator and the Ecliptic is termed the Autumnal Equinox. When the Sun is at one of the equinoxes the lengths of day and night are equivalent (equinox derives from a root meaning "equal night"). The time of the Vernal Equinox is typically about March 21 and of the Autumnal Equinox about September 22. Equinoxes and Solstices

9. Ecliptic coordinate system The ecliptic coordinate system is a celestial coordinate system commonly used for representing the apparent positions, orbits, and pole orientations of Solar System objects. Because most planets (except Mercury) and many small Solar System bodies have orbits with only slight inclinations to the ecliptic, using it as the fundamental plane is convenient. The system's origin can be the center of either the Sun or Earth, its primary direction is towards the vernal (March) equinox, and it has a right-hand convention. It may be implemented in spherical or rectangular coordinates.

10. Precession The advantage of the equatorial coordinate system is that it expresses the position of a star or galaxy in a way that is independent of the observer's position on Earth. However, the right ascension and declination of a given object change slowly over time, mainly due to a phenomenon called precession.

11. Each degree of latitude is equal to about 111 kilometers on Earth's surface. For precision we break down degrees into either fractions of a degree or divisions called minutes and seconds of arc. There are 60 minutes in one degree and 60 seconds in one minute. Tuscaloosa's precise location is +33° 12′ (minutes) 35″ (seconds) north, 87° 34′ 9″ west. You can convert fractions of degrees into minutes and seconds here.

12. Example Celestial Coordinates T he distance around the celestial equator is equal to 24 hours. ... The equator is 0° 0' 0". The position of an object is stated with the right ascension first, then the declination. For example, the bright star Sirius' position is RA: 6h45m8.

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