Measurements for Navigation

Parallels of Latitude

On the Earth

Measurement is fundamental to navigation. To find our position on the Earth, we need a frame of reference to fixFix

The determination of a ship's position by celestial observations or by terrestrial observation of landmarks or aids to navigation, or some combination of each. The term is used only when the position is not subject to doubt.
our position. In a classroom, we can describe our location using tiles on the floor: so many tiles in one direction from the door and so many tiles in another direction from the wall. On the Earth, we locate ourselves using latitudeLatitude

Latitude is the measure of how far north or south one is from the equator. This angular measurement is given in degrees, minutes (1/60th of a degree), and seconds (1/60th of a minute) of arc.
Read More
and longitudeLongitude

Longitude lines around the globe run north-south, and measure position east or west of a reference line.
Read More

Meridians of Longitude

Latitude measures location north or south of the equator. Latitude lines are parallel to the equator starting at 0°. The north and south poles are at 90° north or south. Each minute of latitude equals one nautical mileNautical Mile

One minute (or 1/60th of a degree) of latitude. Because the earth is not a perfect sphere, the length of a nautical mile varies somewhat according to latitude.
Read More
, 6080 feet. Sixty nautical miles are in a degree.

Longitude is measured in degrees east and west of an arbitrary reference line. Since 1884 the longitude line running through Greenwich, England near London has been that reference, called the prime meridianPrime meridian

The meridian from which longitude is measured eastwards or westward. The longitude of the prime meridian is 0 degrees.
Read More
. Like the slices of an orange, longitude lines converge at the poles.

Navigators use time to measure speed and to determine longitude. As there are 360° in a circle, and the earth rotates once every 24 hours, the earth turns 15° every hour and 1° every four minutes.

Modern Compass Rose

Direction is indicated by compass bearing, measured in degrees. The direction to the North Pole is zero. Magnetic compasses point towards the Magnetic North PoleMagnetic North and Variation

Magnetic north is one of two points on the globe where the line of total magnetic force is vertical and towards which the magnetic needle points in all adjoining regions. Variation (also called magnetic declination) is the difference in direction between true north as determined by the earth's axis of rotation and magnetic north as determined by the earth's magnetism.
Read More
, located in the Canadian Arctic islands. This was the only type of compass available from medieval times to the mid 20th century. The magnetic North Pole moves a small amount annually, changing the angle between the magnetic North Pole and the true North PoleTrue North

The direction of the North Pole from any place on the earth's surface, through which all meridians of longitude pass on maps and charts.
Read More
. This angle is called variation. In Penobscot Bay, the variation, or difference between true north and the magnetic north pole is 18° West. This information is found on the compass roseCompass rose

Outer and two inner circles engraved on a nautical chart, used for laying off courses or bearings.
Read More
on most nautical charts.

Map of Magnetic Variation

Large ships have gyroscopic compassesGyroscopic compass gyro compass, gyrocompass

An instrument receiving its directive force from a gyroscope operated by electric motors. Its directive action is based on the mechanical laws governing the dynamics of rotating bodies.
Read More
compasses that use the earth’s rotation around its north-south axis to determine true north. A GPSGlobal Positioning System GPS

Satellite-based navigation system developed and operated by the United States Department of Defense becoming operational in 1993. It uses 24 satellites, and users can determine position, speed and time.
system can also show directions as true north. Magnetic compasses are used to back up gyro compasses and are still the main type used on smaller boats.

In the Heavens

Astronomers have a grid similar to latitude and longitude to identify the positions of the sun, moon, planets, and stars. The celestial sphereCelestial sphere

Imaginary sphere on the inside surface of which celestial bodies (Sun, Moon, stars and planets) appear to be located. The sphere is so large that the position of the observer on the earth can be taken as the center. It is used to locate the position of celestial bodies on the earth's surface using celestial coordinates: declination, right ascension or sidereal hour angle.
is cut with latitude-like parallels called declinationDeclination

Term used in celestial navigation to indicate the angular distance of a celestial body north or south of the celestial equator, which is a projection of the earth's equator.
Read More
, which locate a star’s position north or south of the celestial equator. The celestial equivalent to longitude is called right ascensionRight Ascension

Right ascension is the celestial or astronomical equivalent of longitude. It is expressed in hours and takes its reference base from the First Point of Aries, the point where the Sun crosses the Equator in the spring, measured east.
. The celestial poles are directly over the earth’s poles, and the celestial equator runs over the earth’s equator.

The Celestial Sphere

The eclipticEcliptic

Great circle on the celestial sphere apparently traced out by the sun in the course of a year. It is so called because for an eclipse of the sun or moon to occur, the moon must lie on or near the ecliptic.
Read More
is the line the sun follows through the year, ranging from 23 1/2° North declination to 23 1/2° South declination, in keeping with the angle of the earth’s axis to the plane of the solar system.

Astronomical measurement corrects for small time errors caused by seasonal variation in the earth’s rotational speed around the sun, using the equation of timeEquation of time

The amount of time that the real or apparent sun is ahead or behind the mean (clock time) sun.
Read More