1. Coordinate Systems

2. Purpose To locate stars/galaxies/other objects
To locate an object in the space, how many quantities do we need?
x, y, z
Direction (2 quantities), distance

3. How do we locate a spot on the earth?
Maps, mapquest, Google Map, GPS
If we ignore how high it is above the sea
To describe a spot on the surface of the earth, we use a set of numbers (degrees), called Coordinates
Longitude
Latitude 0º

4. Position in Degrees 0º 90º N Amherst 42°22′49″N, 72°31′25″W 0º 90º S
Longitude – connecting the poles, 360 degree, or 180 degree east degree West
Latitude – parallel to the equator, 0 – 90 N and 0 – 90 S
A location is the intersect of a (virtue) longitude line and a latitude line 0º 0º
90º N
90º S
Amherst
42°22′49″N, 72°31′25″W

5. Same idea when we describe the position of a celestial object
Celestial Sphere
An imaginary giant sphere, centered on the earth
All objects seem to be on the surface of this imaginary sphere
Earth’s poles extends and intersect with the celestial sphere as the North Celestial Pole and the South Celestial Pole
Earth’s equator extends and intersects with the celestial sphere as Celestial Equator
To locate an object, two numbers (in degrees) like the longitude and latitude are enough!

6. The horizon coordinate system
Altitude
Angle above the horizon
0° - 90°
The altitude of the north celestial pole equals the observer’s latitude on the earth
Azimuth
Angle measured eastward along the horizon, starting from the north
0° - 360°
Zenith
The extended vertical line intersects with the celestial sphere
Meridian
The great circle passing through the celestial poles and the zenith
Meridian
Horizon
The great circle whose pole is the zenith

8. Pros and Cons of the horizon system
Easy to tell and understand
Cons
At different position on the earth, the same object has different coordinates
At different time, the same object has different coordinates
The Coordinates of an object Change in the horizon system!

9. Equatorial Coordinate System
A system in which the coordinates of an object does not change
Like the longitude and latitude on the earth, we have Right Ascension and Declination in the Equatorial system
The equatorial coordinate system rotates with stars and galaxies

10. Equatorial Coordinate System
Declination (DEC)
A set of imaginary lines parallel to the Celestial Equator
0 ° at the celestial equator, increases from south to north
negative in the southern hemisphere
Dec of the north celestial pole is 90 °
Dec of the south celestial pole is -90 °
0 °
90 °
-90 °

11. Equatorial Coordinate System
0 °
90 °
-90 °
Right Ascension (RA)
imaginary lines that connect the celestial poles
The origin of the longitude of the earth is the Greenwich Observatory
The origin of the RA is Vernal Equinox
What is Vernal Equinox?

12. The equatorial system Ecliptic
The earth revolves annually around the Sun
The Sun appears to moves from west to east on the celestial sphere
The path of the sun is called ecliptic

13. The equatorial system
The earth’s axis is titled – line through the celestial poles is NOT perpendicular to the plane of ecliptic
23.5 degree angle between the celestial equator and the ecliptic
The ecliptic and the celestial equator intersect at vernal equinox and autumnal equinox

14. The equatorial system RA 360 degrees
Historically, use HOURS:MINS:SECS as unit – 24 hours
Starts from Vernal equinox (0 h)
increases from west to east
Stars w/ larger RA rise later
0 h
6 h
Andromeda:
RA: 00h 42m 44.3s
DEC: +41° 16′ 9″

15. Vernal Equinox: RA: 0h DEC: 0º Summer Solstice: RA: 6h DEC: 23.5º
Autumnal Equinox: RA: 12h DEC: 0º
Winter Solstice: RA: 18h DEC: -23.5º
Vernal Equinox: RA DEC
Summer Solstice: RA DEC
Autumnal Equinox: RA DEC
Winter Solstice: RA DEC

16. Seasons

17. Why do we have seasons?
The path of the earth moves around the sun is not a circle, but an eclipse
The distance of the earth to the sun is not fixed
Summer is hot, winter is cold, is it because we are closer to the sun in the summer time, and more distant from the sun in the winter time?

18. The axis of the earth is tilted!
23.5 degree angle between the earth’s axis and line that perpendicular to the earth’s orbit plane

19. Seasons More hours of daylight in the summer than in the winter
Angle of sunlight: Earth’s tilt constant at 23.5 degrees, in the summer time of the Northern Hemisphere, the sun light falls more directly on the Northern Hemisphere

20. Seasons
Vernal Equinox, around March 21, day hour = night hour at any place on the earth
Summer Solstice, around June 22, for the northern hemisphere, day hour > night hour; longest day; opposite in the southern hemisphere
Winter Equinox, around Sept. 23, day hour = night hour at any place on the earth
Winter Solstice, around Dec 22, for the northern hemisphere, day hour < night hour; longest night; opposite in the southern hemisphere

22. Vernal Equinox: RA: 0h DEC: 0º
Summer Solstice: RA DEC: 23.5º
Autumnal Equinox: RA DEC: 0º
Winter Solstice: RA DEC: -23.5º