The Celestial Sphere

The Celestial Sphere

Celestial Sphere It is very large imaginary sphere surroundings the Earth and to which the Sun, the Moon, the Planets, and the Stars are considered to be attached

HOW DOES ASTRONOMERS KEEP TRACK OF THE SKY

WHEN THE EARTH IS CONSTANTLY MOVING?

In order to locate an object in the sky, it is necessary to

set up a reference system of coordinates.

SYSTEM OF COORDINATES

1. TERRESTRIAL COORDINATE SYSTEM

Latitude and Longitude

2. CELESTIAL COORDINATE SYSTEM

This coordinate system is an extension of the terrestrial system on

to the celestial sphere.

3. THE ECLIPTIC SYSTEM

Based on the Earth's orbit around the Sun.

4. THE HORIZON SYSTEM

A completely local system that is based on an observers latitude

and longitude and the time. 

TERRESTRIAL COORDINATE SYSTEM

Latitude = north/south measurement (degrees).

Latitude is based on a scale of 90o at poles and 0o at the equator.

Longitude = east/west measurement (degrees).

Longitude is based on 0o at Greenwich and 180o at International Date Line. 

THE ECLIPTIC SYSTEM

Cardinal Points on the Ecliptic

•Based on the Earth's orbit around the Sun.

•Ecliptic is the plane of the Earth's orbit projected on to the celestial sphere.

–The 12 zodiac constellations are located in a band following

   the ecliptic.

–The Sun, Moon, and planets are found on or near the ecliptic.

–The ecliptic is tilted 23.5o with respect to  the celestial equator. 

CELESTIAL COORDINATE SYSTEM

 •This coordinate system is an extension of the terrestrial system on to the celestial sphere.

–Declination = north/south measurement

•Units of degrees.

•Based on 90o at poles and 0o at the equator.

–Right Ascension = east/west measurement

•Units of time.

•Based on 0h at March Equinox point in the sky.

•Measured continuously from 0h to 24h. 

THE HORIZON SYSTEM

The horizon system is a completely local system that is based on an observer’s latitude and longitude and the time.

–Altitude = vertical measurement.

•Altitude is based on, 0o at horizon and 90o at the zenith.

–Zenith is the point in the sky directly over the observer.

–Azimuth = direction measurement.

•Azimuth is based on:                                                      

  north = 0o, east = 90o, south = 180o, west = 270o.

–The meridian is a vertical circle that runs due N/S.

•When celestial objects transit the meridian,   they are highest in the sky.

Horizon System