Understanding of the location of physical and cultural features of Earth is based on knowledge of the geographic grid, otherwise known as latitude and longitude. This knowledge is vital. For instance, the changing angle of the sun in the sky is responsible for seasons and is dictated by latitude. Time, with all its implications for life, is kept by the relative longitudes of locations. The world system of latitude and longitude can be used to precisely specify absolute location on the planet and, so, is essential for mapping and navigation.
Latitude and longitude is a spherical grid, and Earth’s surface is not spherical because of its oblate shape and topographic features. However, the nonspherical properties are ignored in the use of the grid. Latitude and longitude are based on the classical Greek system of angular measurement. A circle is divided into 360°.
Staring clockwise from point A, one-quarter of the way around represents 90° (B), one half is 180° (C), three-quarters is 270° (D), and a return to the start at point A is 360°. Degrees are divided into minutes and seconds. One degree includes 60 minutes and one minute is composed of 60 seconds. The notation for degrees, minutes, and seconds are, respectively °, 0, and 00. An example of a location on Earth’s surface is 37°0701300N, 97°0402000W. Such a notation will direct one to within a couple ofmeters from any point on the planet.
The system of latitude and longitude is universally accepted. Latitude has been certain since the time of the classical Greeks. They appreciated the roundness of the Earth and that there was an equator. The equator is key to the numbering of latitude because it provides a physically based “0” latitude. The equator is in the center of the rhythmic migration of the solar declination between the Tropic of Cancer and Tropic of Capricorn. There is 90° of angular arc between the equator and the north end of the axis of Earth’s rotation and between the equator and the south end of the Earth’s axis of rotation. So, latitudes are numbered from 0° to 90° in the Northern Hemisphere and the Southern Hemisphere, designated by their positions relative to the equator. Any individual line of latitude represents a circle around the globe oriented west and east. The equator has the largest circumference possible and other lines of latitude have progressively shorter circumferences with increasing angular distance from the equator.
Lines of latitude are always parallel with each other and, therefore, are also known as parallels. Any number of parallels can be designated using degrees, minutes, and seconds. Neglecting Earth’s oblate shape, one degree of latitude is anywhere equivalent to approximately 111 km. Important lines of latitude usually shown on maps or globes are the Tropic of Cancer, Tropic of Capricorn, Arctic Circle, Antarctic Circle, and the equator . The tropic lines delimit the poleward boundary of the tropics, 23.5° north and south of the equator. The Arctic and Antarctic circles denote the equatorward limits of polar latitudes and are the limits for the regions in which the sun can stay above the horizon for more than 24 hours in the summer and be below the horizon for 24 hours or more in the winter.
Lines of longitude are also known as meridians. As is the case with lines of latitude, any number of lines of longitude can be drawn on a map or globe. Yet, there are substantial differences between meridians and parallels. Meridians cross parallels at right angles and as such, are north-south lines. Importantly, meridians converge to the North and South poles. That is, the Earth distance associated with one degree of longitude approximates 111 km at the equator and becomes progressively less by latitude. At 45° the distance lessens to 79 km and at 90° the distance is 0 because of the convergence of meridians.
Time is reckoned from the daily appearance of the sun overhead of the local meridian. A day, apart from a few minutes’ variation caused by the elliptical Earth orbit around the sun, is 24 hours long. This is because the sun appears to pass westward over 15° of longitude per hour. It is not a coincidence that 24 hours multiplied times 15° per hour yields a circle of 360°. Indeed, the configuration of clocks with hands for hours, minutes, and seconds is an analog to Earth/sun relations.
Where is 0° longitude? Unlike latitude, there is no physical argument for any meridian to garner this label. Originally, most cities reckoned their time using their local meridians. As human society became more time aware and time dependent the need for global timekeeping became vital. By the 19th century, matters of increasingly rapid communication and navigation were confused by the plethora of central meridians. A passenger traveling the transcontinental railroads of North America was subject to a new time at every major stop! In 1884, an international convention adopted a universal time scheme with time zones. The prime meridian of the world was set as the longitude of the Royal Observatory at Greenwich, United Kingdom. This was a result of the United Kingdom’s long-standing astronomical observations at this location and the fact that the International Dateline, at which point calendar days change any time it is crossed, was relegated to the population-sparse Pacific Ocean.
Longitude is numbered as the angular difference east and west of the prime meridian. Eastward, longitude is numbered from 0° to 180°, and this is the Eastern Hemisphere, which contains Asia. Westward, the numbering is also from 0° to 180°, and this is the Western Hemisphere, which contains the Americas. The International Dateline nominally follows 180° longitude but deviates around islands and countries so as to not be a nuisance to population centers.
Accurate measurement of latitude and longitude are essential to location and navigation. Historically, latitude was measured by use of a sextant, a device that determines the angle of the sun above the horizon. Using this device when the sun is overhead at the local meridian allows a ready determination of latitude from knowledge of time of year and solar declination. This device was limited to use on days without overcast and was difficult to employ when a ship’s deck was vigorously pitching. Alternatively, the angles of stars above the horizon in the night sky could be checked against astronomical tables. Longitude was impossible to measure with precision until the invention of the first portable chronometer in 1722. The chronometer is a clock that keeps solar-based time for a specified meridian (now universally the prime meridian). At local noon when the sun is directly over the meridian, the chronometer is checked. The hours and minutes of time difference between local noon and the chronometer reading is converted into longitude at the rate of 15° of longitude per hour. Sextants and chronometers can provide accurate measurements down to seconds of the geographic grid. There has been a revolution in latitude/longitude determination. This was enabled by the 1993 launch of the last of a constellation of 24 geopositioning satellites. Ground units, hand held and inexpensive enough to be owned by virtually anyone, can be used to determine latitude, longitude, and altitude by geometrically determining the angles between the receiver and—usually four or more—satellites.