This document discusses coordinate systems used to describe the positions of objects in the sky. It describes two main coordinate systems: the horizon system which uses altitude and azimuth, and the equatorial system which uses right ascension and declination based on the celestial sphere. The celestial sphere is an imaginary sphere where stars appear to reside, with the same poles and equator as the Earth. Right ascension is similar to longitude, measured in hours, minutes, and seconds along the celestial equator. Declination is similar to latitude, measured in degrees north or south from the celestial equator. These equatorial coordinates slowly change over long periods of time due to changes in the equinoxes and Earth's rotation.

1. Astronomy & Space Science Association
in
University of Kelaniya
Astro Night 2014
Lecture 2
Coordination Systems in the
Sky & Celestial Sphere
Eranga Jayashantha
Hon. mentd Dip in Astronomy & Astrophysics ( IOAA – China )
National Representative at Space Generation Advisory Council ( UN )
Sri Lanka national Astrophysics Olympiad Team Trainer at Institute of Physics Sri Lanka

2. Latitude and Longitude
 Latitude (f) meas from
equator
 Longitude (l) point of
reference – Greenwich
UK
 Units of measure:
 Deg, arcmin, arcsec
 O ‘ “

5. Two types of Coordination Systems are used to
describe the position of an object in the Sky
1) Horizon System
2) Equator System

6. The Horizon System
 Altitude (h) – angle
measured from the
horizon to Zenith (Z)
 Azimuth – the angle
measured from NE
along horizon
 Problem as a celestial
system?

9. What is an Equator System?
Based on
Right Ascension
&
Declination

10. Celestial Sphere
 Imaginary sphere where stars
reside
 Extension of Earth’s equator,
poles
 Celestial Equator
 Celestial poles
 Zenith & Nadir

14. • Celestial equator
divides the sky into
northern and southern
hemispheres
•Celestial poles are
where the Earth’s axis
of rotation would
intersect the celestial
sphere
• Polaris is less than 1°
away from the north
celestial pole, which is
why it is called the
North Star or the Pole
Star.
•Point in the sky directly
overhead an observer
anywhere on Earth is
called that observer’s
zenith.
– Nadir is directly
opposite the zenith

15. Declination & Right Ascension
 Declination = Latitude
 Celestial Equator d = 0
 Latitude, NCP elevation
 Units: deg, arcmin, ‘’
 The Celestial Meridian
 Great circle going through
zenith & NCP
 Right Ascension =
Longitude
 Units: hh:mm:ss
 360o = 24 hr (1 hr = 15o)
 Where to start RA?

17. How the Equatorial Coordinates are changed over a long time?
 Due to two main reasons
1) Precision of Equinoxes
2) Lunar orbit is getting far from Earth by ~3cm per year
( Earth’s rotational angle is getting changed )

19. Thank you