# 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

## 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
- 360Ëš can easily just be 24 hours
- 15Ëš 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 90Ëš
- 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 0Ëš

### Relative to Viewer

- Zenith
- Point directly above your head

- Horizon
- Where the dome meets the Earth

Space Astronomy