Celestial Sphere

The celestial sphere

An imaginary, hollow dome surrounding the Earth
- It's essentially the sky we see when we look up
- Earth's coordinate system has been extended to the sphere
It's what you see when you look up at the sky; you are at the center
The path the Sun takes on the celestial sphere is called the ecliptic
- Apparent yearly path of the Sun
- Projection of the Earth's orbit onto the sky
- Tilted by 23.5 degrees with respect to the celestial equator

Celestial tilt

Coordinate System

Declination
- Vertical position of an object
- Celestial equivalent of latitude
- Degrees, arcmin, arcsec
Right ascension
- Horizontal position of an object
- Celestial equivalent of longitude
- Expressed as either degrees or units of time
  - Celestial sphere turns around the Earth once a day
    - As Earth turns on its axis
    - 360˚ can easily just be 24 hours
    - 15˚ arc is equal to 1 hour
  - Hours, mins, sec
- Starts getting counted at the vernal equinox
Altitude
- Vertical position of an object
- Angle from the horizon to a point on the sphere
- Zenith is 90˚
- Altitude of Polaris with give you the latitude of where you are on Earth
Azimuth
- Horizontal position of an object
- Angle measured clockwise around observer's horizon from north
- Object due north has azimuth of 0˚