First Point of Aries

• The First Point of Aries is where the Sun crosses the Celestial Equator at the Vernal Equinox.
• The Celestial Longitude of Stars and Planets in the Night Sky is measured by their angle relative to the Celestial Equator, and this is known as the Equatorial Coordinate System.

Latitude and Longitude in the Sky

• On Earth, a terrestrial position is obtained by its Latitude from the Equator (zero,) and its Longitude from the Prime Meridian (zero).
• In the Sky, a celestial object is measured by the celestial equivalent which is Declination (Latitude) from the Ecliptic (Equator) and its Right Ascension (Longitude) from the First Point of Aries (Prime Meridian).

• Declination

  • Its Latitude is called Declination, which is its Latitude measured from a zero point, the Celestial Equator.

• Right ascension

  • Its Longitude is called Right ascension, measured in Hours, Minutes and Seconds, from a zero point on the Celestial Equator, known as the First Point of Aries.
  • This is where the Sun crosses the Celestial Equator at the Vernal Equinox.
  • Right ascension increases Eastwards, as it is measured in a full circle always moving towards the East.

• Sidereal Hour Angle

  • Sidereal Hour Angle (SHA), which is similar to Right ascension, is used in Celestial Navigation but increases Westwards.

Hipparchus (c.190-c.120 BCE)

• In Antiquity, Hipparchus determined the position of the Stars relative to the Suns Path or Ecliptic. This is known as the Ecliptic Coordinate System.