Longitude Explained.

Let me start off by saying that this is a top-notch app! In 1884 it was decided that the Prime Meridian (the place where East meets West on a map) would be in line with the Royal Greenwich Observatory in the United Kingdom so the Earth would have a standardized method of measuring longitude as opposed to having several Prime Meridians all over the globe as there had been before. Since the Earth is round and there are 360 degrees in a circle, it was best to count up to 180 degrees half-way around the world and then have another Prime Meridian that began to subtract numbers to avoid having a longitude of both 360 and 0 at the Greenwich Observatory. Usually if you are, say, 85 degrees West of 0, to find the place exactly half-way around the world you would start at 180 and then subtract 85 from it to get the Eastern measure. However, this map only measures in one direction, so if you find yourself in the Western Hemisphere (in or around the Americas) then simply add your, say, 85 degrees to 180 for a grand total of 265 degrees. This will flip Ursa Major in the right direction and raise Orion above the horizon, or account for the fact that the sun is shining. If you live to the West across the field of the Greenwich Observatory, you will find that your longitude will be very close to 360 degrees indeed. Enjoy!

Melram Lesabre about Starry - Stars, Astronomy and Planets Guide