Artificial Horizon A form of mirror (for example a trough of mercury) to represent the horizon. The altitude measured will then be double the actual altitude.   Circumpolar Circumpolar means to move in a circle around a fixed point. This term usually refers to the fact that the stars, while remaining fixed relative to one another, appear to rotate about the Polaris. The Polaris appears to be fixed because it lies very close the Earth's axis of rotation. As the other stars appear to revolve, their position at any moment indicates the time.   Dead Reckoning A piece of flotsam is thrown over the side of the ship. There are two marks on the ship's rail a measured distance apart. When the flotsam passed the forward mark, the pilot would start a quick chant, and when it passed the aft mark, the pilot would stop chanting. The pilot would note the last syllable reached in the chant, and use a mnemonic that would convert the syllable into speed in miles per hour.   Ecliptic The plane on which the path of revolution of Earth lies.   Eclipse Timing Method The local time of the start of an eclipse at the current location and a  reference location are noted. The difference between the two timings can be used to find longitude.   Escapement The heart of a clock.   Feng Shui A Taoist method of environment organization.   Lunar Distance Method  From an accurate catalogue of the positions of the stars and the position of the Moon measured accurately relative to the stars, the Moon's motion could be used as a natural clock to calculate Greenwich time. Sailors at sea could measure the Moon's position relative to bright stars and use tables of the Moon's position, compiled at the Royal Observatory (King Charles II founded the Royal Observatory Greenwich in 1675 to solve the problem of finding longitude at sea.), to calculate the time at Greenwich.   Middle Ages A period in European history from about 1100 AD to 1500 AD.   Obliquity The angle between Earth’s axis of rotation and the Ecliptic. The value is approximately 23.5°.   Ocular Parallax In simple terms, Ocular Parallax means error made by the eye. The human eye is unable to focus on 2 objects at the same time (in this context, they would be a celestial body and the horizon), and as a result, would not be able to judge whether the two objects lie at the two tips of the vane accurately.   Precession The change in the direction of Earth’s axis of rotation, due to gravity of the Moon and the Sun acting on Earth. The axis returns to its initial direction every 26,000 years.   Polaris  A star that is also known as North Star or Northern Celestial Pole Star because of its close proximity to the Northern Celestial Pole. This proximity changes, however, due to the precession of Earth.   Prime Meridian The meridian with longitude 0°. Prime Meridian was first located in the Portuguese Madeira Islands (indicated by the red-and-blue-flag above). For the next three centuries the world calculated longitude from Portuguese islands. Even as their naval power waned, Portugal's prime meridian remained the world standard until the English achieved dominance during the nineteenth century. As nautical charts increasingly became produced in England sailors began to use both Portuguese and English standards. In 1884 a European conference narrowly ratified the placement of the prime meridian in Greenwich England.   Remontoire A device in a timepiece whereby the main source of power periodically winds up a spring or lifts the weights to drive the actual mechanism, so that it can provide a constant force. For example a spring-driven clock winds up a weight which transmits the power to the heart of the timekeeper or clock.   Sidereal Time Time taken by a star to travel from meridian to meridian . A sidereal day is 23 hours, 56 minutes and 4.09 seconds long.   Solar Time Time taken by the Sun to travel from meridian to meridian. A Solar day is 24 hours long.   Southern Cross The Southern Cross consists of four stars which form a cross. to Find South, the Southern Cross and the Pointer Stars have to be used. Extend an imaginary line (from the top towards the bottom) out of the bottom of the cross. In addition, draw an imaginary line perpendicular to the center of the Pointer stars. At the point where the lines intersect, draw a third line straight down towards the ground and this line would represent the southern direction.   Trilateration Trilateration is a basic geometric principle that allows you to find a location using relative positions of three or more known locations. In this case, it makes use of signals transmitted by artificial celestial bodies - satellites. In two-dimensional space, at least three circles are needed to pinpoint the location, as shown below. In three-dimensional space, at least four spheres are needed. Due to the fact that satellites transmits signals radially, the space the signal cover forms a sphere.   Ursa Major  A constellation, also commonly known as Great Bear or Big Dipper (due to its shape), which lies relatively close to Ursa Minor. It is commonly used to locate the Polaris.   Ursa Minor A constellation, also commonly known as Little Bear or Little Dipper (due to its shape), which lies relatively close to Ursa Major. The star located at the tip of the ‘handle’ of the ‘dipper’ is the Polaris.