Coordinate Systems

Celestial Sphere - points to know:


- Both Equinoxes
- Both Solstices
Celestial north
Celestial south
Ecliptic
Celestial equator


Coordinates:

On Earth:

Longitude
half-circle lines from North to South pole
Zero longitude runs through the site of the Royal Greenwich Observatory in England - the Prime Meridian (0 degrees long.)
Number of degrees east or west of the PM

Latitude
Full circle lines parallel to the equator (0 degrees latitude)
+ or - 90 degrees corresponds to the poles

International Date Line (IDL)
Near or along 180 degrees longitude line, through the Pacific Ocean
As we travel eastward around the globe, the hours get later roughly each 15 degrees (a time zone). When we cross the IDL, we go BACK one day. This keeps only 24 hours on the Earth at a time.

In the Sky:

Celestial Equator - imaginary line above the Earth's equator

Right Ascension (RA)
Celestial analog of longitude (both measure east-west)
Measured in hours (each hour of RA equals 15 degrees) along the celestial equator

Declination (dec)
Celestial analog of latitude (both measure north-south)
Measured perpendicularly above (+) or below (-) the celestial equator

RA and dec form a coordinate system fixed to the stars. To observers on Earth, the stars appear to revolve every 23 h 56 min. So, the coordinate system appears to revolve at the same rate. Of course, it is the Earth which is really moving (most noticeably).

Ecliptic
Although the stars are fixed in their positions in the sky, the Sun's position varies through the whole range of RA throughout the year. This path (the "apparent" path of the Sun) is called the ecliptic and is inclined 23.5 degrees with respect to the celestial equator (CE), since the Earth's axis is tipped by that amount. (The "ecliptic plane" is the plane that the Earth and Sun make.)

The ecliptic and CE cross at two points:

Vernal equinox
March 21 (approx)
the first day of Northern Hemisphere spring
the zero-point of RA
Sun's declination is 0 degrees
Nearly equal amounts of day and night

Autumnal equinox
Sep 23 (approx)
the first day of autumn
Sun's declination at 0 degrees
Nearly equal amounts of day and night

Two other noteworthy days:
Winter solstice
Dec 22 (approx)
Shortest day of the year in Northern hemisphere
9.5 h of daylight (in the DC area)
As you travel farther north, the days are even shorter
- in Anchorage, Alaska, the day will be 5 h long
- in Barrow, Alaska, the sun will not "come out" at all; noontime is like deep twilight
the North pole is angled most steeply away from the Sun

Summer solstice
June 21 (approx)
longest day of the year in the Northern hemisphere
amount of tipping toward Sun is greatest for N. hemisphere
Sun highest in sky (dec is 23.5 degrees)

Length of daylight depends on latitude, calendar date, but not longitude
Each point on the globe receives an average of 12 hours of light each day. So, students in Barrow, Alaska have several days of endless sunshine as well.

Since the Moon goes around the Earth, its RA changes through the entire range of values each month. Since its orbit is inclined to the CE, its dec also changes.