Celestial coordinates are used to define positions of objects in the sky and there are two main systems - equatorial and horizontal coordinates. The equatorial system uses right ascension and declination, where declination is measured in degrees north or south of the celestial equator. Right ascension is measured in hours, minutes, and seconds along the celestial equator. The horizontal coordinate system uses altitude and azimuth which depend on the observer's location and measure the angular distances above the horizon and east from north. Celestial coordinates can change slightly over time due to the phenomenon of precession as the equinoxes and poles very slowly change position.

1. ASTRONOMY
Prepared By
Ms. R. Umadevi
Assistant
Professor,
Department of
Mathematics,
CELESTIAL COORDINATES

2. Definition
 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.

12. Example
Celestial Coordinates
 The 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.

13. THANK YOU