"For longitude on the Earth, we arbitrarily assigned Greenwich, England as the Prime Meridian," Palma says. "The Prime Meridian for the right ascension system is called 'The First Point of Aries,' and it is defined as the position of the Sun on the sky as it moves from south to north along the Ecliptic" — an imaginary line that denotes the path of the Sun — "and passes through the celestial equator. When the Sun is at that location, it is the vernal (or March) equinox on Earth. Right ascension increases to the East from there. So, a star on the sky that is exactly halfway around the sky from the Sun on the vernal equinox would have a right ascension of 180 degrees." … [Read more...] about How Do We Find Things in the Blackness of Space?