Astronomy & Space Science Association

in

University of Kelaniya

Astro Night 2014

Lecture 2

Coordination Systems in the

Sky & Celestial Sphere

Eranga JayashanthaHon. 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

Latitude and Longitude

Latitude (f) meas from

equator

Longitude (l) point of

reference – Greenwich

UK

Units of measure:

Deg, arcmin, arcsec

O ‘ “

Two types of Coordination Systems are used to

describe the position of an object in the Sky

1) Horizon System

2) Equator System

The Horizon System

Altitude (h) – angle

measured from the

horizon to Zenith (Z)

Azimuth – the angle

measured from NE

along horizon

Problem as a celestial

system?

What is an Equator System?

Based on

Right Ascension

&

Declination

Celestial Sphere

Imaginary sphere where stars

reside

Extension of Earth’s equator,

poles

Celestial Equator

Celestial poles

Zenith & Nadir

•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

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?

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 )

Thank you