Celestial Coordinates

Celestial Coordinates are very important for learning and enjoying astronomy.
Every position of an object in the sky is located using this coordinate system. It's as important to an astronomer as latitude and longitude is to a navigator on a ship. Chaisson's explanation is very concise and needs supplementing.

Study Figures 1.7, 1.10 and 1.13 with their corresponding text.

Contents

1. Celestial Sphere - figure 1.7
2. Ecliptic -figure 1.10
3. Celestial Coordinates -figure 1.13
4. Star Atlases
5. Examples

Figure 1.7 - This shows the concept of the celestial sphere and the basis of the celestial coordinate system referenced to the Earth's rotational axis and equator. Study this figure carefully while you read 'The Celestial Sphere' on page 9.

Important Implications:
• North Polar axis --> North Celestial Pole - NCP in the sky in Ursa Minor
• South Polar axis --> South Celestial Pole SCP in the sky in Octans
• Earth Equator --> Celestial Equator
• Earth rotation from West to East -->Motion of the Celestial Sphere from East to West.

Figure 1.10 - This adds the motion of the Sun on the Celestial Sphere due to the motion of the Earth in its orbit. As The Earth moves in its orbit in a counter-clockwise direction as seen from the north, the Sun appears to move across the celestial sphere from west to east. This path in the sky is called the ecliptic. It is a great circle on the celestial sphere and crosses the celestial equator at two points.

• Ascending Node (where the Sun moves from south to north): This intersection defines the reference mark to define celestial coordinates measured parallel to the celestial equator. The sun is here (Pisces)at the Vernal Equinox (~March 21).
• Decending Node (where the Sun moves from north to south): 180 degrees from the ascending node and the sun is here (Virgo) on the Autumnal Equinox (~September 21).

Figure 1.13 - This defines the celestial coordinates,

• Right Ascension, R.A. measured along the celestial equator from the Vernal Equinox in a eastward direction. (analagous to longitude on the Earth) If measured in degrees of angle, right ascension would go from 0 to 360 degrees of angle. However, astronomers uses units of hours, minutes and seconds and since the Celestial Sphere makes one rotation in 24 hours, 24 h R.A. = 360 degrees.
  Units of R.A.
  • 24 Hours of R.A. = 360 degrees arc
  • 1 hr R.A. = 15 degrees arc = 60 minute R.A.
  • 1 degree arc = 4 minutes R.A. (This is very useful to remember)
  • 1 minute R.A. = 15' arc
  • 1 second R.A. = 15" arc
• Declination, dec measured north or south from the celestial equator. The value of declination can vary from south 90 degrees (SCP) to north 90 degrees at the north celestial pole (NCP). + is used for north and - is used for south. (Read the box on Angular Measurement on page 14)

Star Charts

• Simple Charts in Chaisson: There are 4 star charts at the end of Chaisson showing the sky from -60 to +60 declination. Each chart covers 6 hours of R.A. The lines on these charts make the celestial coordinate grid. These will be useful for quickly finding constellation and bright stars. Unfortunately, the celestial polar regions do not have charts.
• Sky Program, Earth Centred Universe. This program is available as a shareware program and runs under Windows 3.x and Win95. It is available on the AXE network at the path x:\public\apps\ecu\ecu.exe. You can download a shareware version of this program from ftp://hercules.stmarys.ca/ecu/. Several exercises and projects will be done with the program througout the course. If you download the program and use it on your computer, you are encourage to register the program with its author, Dave Lane.

Examine the origin of the celestial coordinate presently in the constellation of Pisces (the fish). Above Pisces is the Great Square of Pegasus. Find the celestial coordinates of the star in the south-east (lower left in the northern hemisphere) corner of the square (gamma in Pegasus). Use the Star Chart at zero of R.A. and declination or the star chart in Chaisson on page S-4. You will have to estimate the values from the given coordinate lines.
..gamma is 15 degrees north of the celestial equator and hence has a declination of +15 degrees or 15 degrees north.
..it is east of the zero of right ascension by 13 minutes of right ascension. Remember 1 degree = 4 min R.A. so this is 2-1/4 degrees east.
..The coordinates of gamma are 13 m R.A. and +15 degrees dec.

Name the Star and Constellation at 16h 30m and -26 degrees 18'. Notice, usually right ascension is given first; hour is abbreviated as h and minutes as m. You will find that the Chaisson chart to use is S-3 and that the on-line atlas is south 45 dec and 16 hours R.A..
..The star is alpha in Scorpius and has the name Antares.