Lines of latitude, also called parallels, are imaginary lines that divide the Earth. They run east to west, but measure your distance north or south. The equator is the most well known parallel. At 0 degrees latitude, it equally divides the Earth into the Northern and Southern hemispheres. From the equator, latitude increases as you travel north or south, reaching 90 degrees at each pole.
Did you know that the National Ocean Service (NOS) has been monitoring latitude since 1899? A scientist with the U.S. Coast and Geodetic Survey (later incorporated into NOS) built an observatory near his home in Gaithersburg, Maryland, and began collecting latitude measurements as part of a global project to monitor the wobble of the Earth on its polar axis. Another observatory was built in Ukiah, California, joining international stations at locations along the 39th parallel north. Satellite data replaced human observations in the 1980s, but the data from the observatories is still being used.
There are other named lines of latitude. They’re based on the sun’s position during Earth’s orbit, and they help us understand climate, weather, and ocean currents. The Tropic of Cancer, at roughly 23 degrees north, and the Tropic of Capricorn, at roughly 23 degrees south, are the boundaries of what we consider the tropics. The Arctic Circle and the Antarctic Circle are at roughly 66 degrees north and south, respectively. They mark the boundaries of the Arctic and Antarctic regions.
Each degree of latitude covers about 111 kilometers on the Earth’s surface. One degree of latitude can be further divided into 60 minutes, and one minute can be further divided into 60 seconds. A second of latitude covers only about 30.7 meters. Unlike longitude lines, which get closer to each other at the poles, latitude lines are parallel. No matter where you are on Earth, latitude lines are the same distance apart.
Latitude and longitude have been used in astronomy and navigation since ancient times. By calculating the angle between the horizon and a celestial object (usually the sun or the North Star), navigators could determine their approximate latitude using basic tools. The calculations were simple, so measuring latitude at sea was reliable hundreds of years before accurate longitude measurements could be calculated during a voyage. If the North Star was 60 degrees above the horizon, the observer was at 60 degrees latitude (north). This process was more complex in the southern hemisphere, where the North Star is not visible.
We still look to the sky to determine our position, but the equipment is a bit more sophisticated. A constellation of over 30 global positioning satellites orbit the Earth, transmitting signals to receivers on land. NOS’s National Geodetic Survey manages a network of stationary global positioning satellite receivers called Continuously Operating Reference Stations (CORS). When combined with other positioning data in the National Spatial Reference System, processed CORS data can provide latitude, longitude, and height positions accurate to within a few centimeters.
If we assume the Earth is a perfect sphere, lines of latitude are relatively simple to calculate. The latitude of a certain point on the surface of the Earth is the angle between two lines: a line from that location to the center of the Earth and a line from the center of the Earth to the Equator. This method of calculation gives us geocentric lines of latitude. However, this method is not accurate enough for astronomy, global positioning, and other real-world applications. Instead, we use what is known as geodetic latitude. This method of calculating latitude for any point on Earth accounts for the fact that the Earth is actually squished at the poles due to the centrifugal force created by the planet's rotation. Measuring latitude lines with high degrees of accuracy is but one component of geodesy, the science of accurately measuring and understanding the Earth's geometric shape, orientation in space, and gravity field.