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Longitude Defined
Longitude tells where you are in the world as you travel from east to west. Latitude, by the way, tells you where are going north to south. Lines of longitude, also called meridians, extend from the North to South poles. Greenwich, England, is at zero longitude, also known as the prime meridian. Due to the earth's shape, meridians grow farther apart as you approach the equator.
Sun Time Defined
Sun time, or solar time, is based on movement of the sun across the sky and is separated into mean sun time and apparent sun time. Mean sun time involves computations based on the average location of the sun. Apparent sun time, measured by sundials, tracks time as it relates to how long it takes the sun to return to its highest point. This varies throughout the year because of the earth's elliptical orbit and tilt on its axis.
Longitude and Time
The earth turns 360 degrees on its axis every 24 hours. Dividing this movement into equal one-hour portions, you get 15; therefore, each line of longitude is 15 degrees from the next one. This makes up the world's time zones. When it is 1 p.m. at your location, it is 2 p.m. if you go 15 degrees east and noon if you go 15 degrees west. Greenwich, England, is the beginning point for measuring time. This is why for official purposes time is sometimes stated as "Greenwich mean time plus X hours."
Calculating Time
Suppose your watch stopped, and no one is around to ask the time. Luckily, there is a sundial and you just happen to know your longitude. Check the time on the sundial and add four minutes for each longitudinal degree west you are from Greenwich. If you are east of Greenwich, subtract four minutes for each degree. This is because it takes four minutes for the sun to move one longitudinal degree.