Right ascension

• Right Ascension is the Longitude of a celestial object, measured in Hours, Minutes and Seconds, from a zero point on the Celestial Equator, known as the First Point of Aries.
• The Abbreviation for Right ascension is Ra.

Declination and Right Ascension

• On Earth, a terrestrial position is obtained by its Latitude and Longitude.
• In the Sky, a Celestial object is measured by the equivalent of Latitude and Longitude, called Declination and Right Ascension.

Declination

• Declination is a star’s Latitude measured from the Celestial Equator.

First Point of Aries

• This is where the Sun crosses the Celestial Equator at the Vernal Equinox.
• Right ascension increases Eastwards from the First Point of Aries, as it is measured in a full circle of 360° always moving towards the East.
• Sidereal Hour Angle (SHA), is also a full circle of 360° like Right ascension, but instead increases Westwards from the First Point of Aries.

Equatorial Coordinate System

• The position of Stars and Planets in the Night Sky is determined by their angle relative to the Celestial Equator. This is known as the Equatorial Coordinate System.

Ecliptic Coordinate System

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