Lines of latitude are imaginary reference lines that describe how far north or south a location on the Earth is from the Equator. Latitude is measured in degrees, minutes, and seconds north or south with the Equator valued at zero degrees and the north and south poles as 90 degrees north and south, respectively. Latitude combined with longitude gives a coordinate for any location on the Earth.
The Earth is nearly spherical, though not truly a sphere as it bulges slightly in the middle. A sphere cut in half forms a circle along the cut line. Circles are divided into 360 degrees. This allows the surface of a sphere to be divided into 360 degrees as well. Unlike a circle, a sphere is a three-dimensional object. Thus a sphere needs perpendicular reference lines each with 360 degrees to describe a location on the sphere.
Lines of Latitude
The 360 degree reference lines on the Earth are designated latitude for the horizontal lines and longitude for the vertical lines. This allows lines of latitude to determine how far up or down on the Earth a location is, and lines of longitude to describe how far left or right a location is from a standard reference point. In geographical terms, the up, down, left, and right, are replaced with the cardinal directions north, south, west and east.
Describing the up, down, left, or right of a location is incomplete without giving a reference point or line. To make the lines of latitude and longitude useful, reference lines were established on the Earth allowing longitude and latitude to determine how far up, down, left, or right a location is from an accepted reference. For latitude, the Equator was designated as the zero degree reference line which is equidistant from the poles. The poles then became 90 degrees north and south. Longitude uses the Prime Meridian or Greenwich Line as zero degrees with other lines marked as east or west of this line.
Arctic/Antarctic Circle and Tropics of Cancer and Capricorn
The Earth is tilted on its axis, giving rise to seasonal climate patterns on the Earth. This tilt has also led to several special latitudes to be given names. The Arctic and Antarctic circles lie at 66.5 degrees north and south. Between these latitudes and their respective poles the sun stays in the sky at least one full day each year. Between the Tropic of Cancer at 23.5 degrees north and the Tropic of Capricorn at 23.5 degrees south, the sun reaches zenith (directly overhead) during the course of the year.
Using the equator as the reference line for latitude also allows celestial navigation to be performed very simply. The North Star, Polaris, is located almost directly over the North Pole. Measuring the angle of the North Star above the horizon when standing on the North Pole gives an angle of nearly 90 degrees, the same latitude north as the North Pole. On the Equator, assuming a clear line of sight, the North Star rests near the horizon, an angle of about zero degrees--the same as the latitude of the Equator. Latitudes north of the Equator will likewise measure an angle to the North Star nearly the same as their degrees latitude. The development of watches and star tables allowed other stars to likewise be used as reference points for geographic location.