eastward of magnetic north, the deviation is east; if it points westward of magnetic
north, the deviation is west.
The sum of variation and deviation is termed compass error.
Distance is the spatial separation between two points without regard to direction. In
navigation, it is measured by the length of a line on the surface of the earth from one
point to the other.
Units of Measurement
Obviously, there must be some way to accurately describe the distance traveled. The
customary units are yards, miles, or kilometers. The "mile" used in navigation is the
international nm, 6,076 feet, which is longer than the statute mile used in land travel
(5,280 feet). Also, 1 minute of arc on the equator is equal to 1 nm, and 1 minute of arc
on a meridian (1 minute of latitude) is equal to 1 nm.
The consideration of time is always of major importance in the flight planning process.
Almost every planning action is concerned in some way with timely arrival at the
destination and intermediate fixes enroute.
Understand the concept of time requires a basic knowledge of how time is derived. In
the late 1800s, the development of comparatively rapid transit systems, such as the
railroad and the steamship, made the development of an accurate method of keeping
time a necessity. The concept that a mean solar day was equal to a theoretical mean
sun passing completely around earth at the equator once every 24 hours was
developed. This concept came to be widely used for marking the passage of time. One
mean solar day is 24 hours in length, with each hour consisting of 60 minutes, and each
minute, 60 seconds. Since the mean sun completes one circuit of earth (360°) every 24
hours, it follows that it moves at the rate of 15° of arc as measured at the equator, or
15° of longitude, per hour (360° ÷ 24 = 15°).