Astronomical Reference Points

Astronomical Reference Points

Celestial Sphere

  • The celestial sphere is an imaginary sphere with the earth at its centre. All objects in the observer’s sky can be thought of as projected upon this sphere.
  • The celestial sphere is divided into 88 constellations.

Celestial Equator and Ecliptic

  • The celestial equator is an imaginary circle directly above the Earth’s equator.
  • The ecliptic is the Sun’s apparent path across the celestial sphere over the course of a year.

North and South Celestial Poles

  • The north and south celestial poles are the points in the sky directly above the Earth’s North and South Poles, respectively. The North Celestial Pole is close to the bright star Polaris.
  • Stars seem to rotate around the celestial poles because of Earth’s rotation.

Zenith and Nadir

  • The zenith is the point in the sky directly above an observer.
  • The nadir is the point in the sky directly below the observer, or directly opposite the zenith.

Horizon

  • The horizon is the boundary where the sky seems to meet the ground. It divides the observer’s celestial sphere into two halves - the visible half (above the horizon) and the invisible half (below the horizon).

Meridian

  • The meridian is an imaginary line that runs from the north point on the horizon, up to the observer’s zenith, and down to the south point on the horizon.
  • It divides the celestial sphere into the eastern and western halves.

Azimuth and Altitude

  • Azimuth and altitude are coordinate systems used to locate objects in the sky.
  • Azimuth is the direction to an object measured along the horizon in degrees, from the north, through the east, south, west and back to the north.
  • Altitude is how high an object is above the horizon, measured in degrees.

Right Ascension and Declination

  • Right Ascension and Declination are another set of coordinates used by astronomers to locate objects in the sky.
  • Right ascension is the celestial equivalent of terrestrial longitude, and declination is comparable to latitude.

Vernal and Autumnal Equinoxes

  • An equinox is a time when the Sun appears directly above the equator and day and night around the world are of equal length.
  • The vernal equinox occurs around March 21stand the autumnal equinox around September 22nd.

Solstices

  • A solstice is either of the times in the year when the Sun is farthest from the celestial equator, resulting in the longest and shortest days.
  • The summer solstice occurs around June 21st and the winter solstice around December 21st.