Longitude

The Longitude of your location denotes how far east or west you are of an imaginary circle that runs from the north pole to the south pole through the Royal Observatory, Greenwich in England. Astronomers use a coordinate called the right ascension to describe the position of the planet, or any celestial object for that matter, using the Celestial Sphere.

Right Ascension
The right ascension of an object is the angular distance from the vernal equinox eastward along the celestial equator to the circle used in measuring its declination. Right ascension can be measured in time units such as hours, minutes and seconds.

More specifically, the right ascension is measured from a line that runs between the north and south celestial poles and passes through a point on the celestial equator known as the vernal equinox This point is one of two locations where the Sun crosses the celestial equator during its apparent annual motion. Simply, in the Earth's northern hemisphere, spring officially begins when the Sun reaches the vernal equinox in late March.