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A clock position, or clock bearing, is the direction of an object observed from a vehicle, typically a vessel or an aircraft, relative to the orientation of the vehicle to the observer. The vehicle must be considered to have a front, a back, a left side and a right side. These quarters may have specialized names, such as bow and stern for a vessel, or nose and tail for an aircraft. The observer then measures or observes the angle made by the intersection of the line of sight to the longitudinal axis, the dimension of length, of the vessel, using the clock analogy.
In this analogy, the observer imagines the vessel located on a horizontal clock face with the front at 12:00. Neglecting the length of the vessel, and presuming that he is at the bow, he observes the time number lying on the line of sight. For example, 12 o'clock means directly ahead, 3 o'clock means directly to the right, 6 o'clock means directly behind, and 9 o'clock means directly to the left.
The clock system is not confined to transportation. It has general application to circumstances in which the location of one object with respect to another must be systematized.
This is a system of denoting impromptu relative bearing widely used in practical navigation to give the position of an observed object readily and comprehensibly. "Relative" means that it does not state or imply any compass directions whatsoever. The vessel can be pointed in any direction. The clock numbers are relative to the direction in which the vessel points. The angular distance between adjacent clock numbers is 30 degrees, a round unit that simplifies mathematical juggling. A quick clock number can be shouted by a lookout, whereas after a calculation and comparison of compass points, which might be unknown anyway, it might be too late for the vessel to avoid danger.
As an example of a standard use, the clock position of every approaching vessel is monitored. If the clock number for the observed vessel does not change, it is on a collision course for the observer vessel, as vessels that pass by must change relative bearing. In warfare the clock system is especially useful in drawing attention to enemy locations.
The clock system is easily converted into a 360 degree system for more precise denotation. One bearing, or point, is termed an azimuth. The convention is that of analytic geometry: the y-axis at zero degrees is the longitudinal axis of the vehicle. Angles grow larger in the clockwise direction. Thus, directly to port is at 270 degrees. Negative angles are not used. In navigational contexts, the bearing must be stated as 3 digits: 010 (not so in other contexts). These circles are not to be confused with latitude and longitude, or with any sort of compass reading, which are not relative to the vehicle, but to the magnetic and spin axes of the Earth.
For maritime and aviation applications, the clock bearing is almost always a relative bearing; i.e., the angle stated or implied is angular distance from the longitudinal axis of the vessel or imaginary vessel to the bearing. However, if the 12:00 position is associated with a true bearing, then the observed position is also.
For example, clock position on a watch can be used to find the approximate bearing of true north or south on a day clear enough for the sun to cast a shadow. The technique takes a line of sight (LOS) on the visible sun, or on the direction pointed to by a shadow stick, through the hour hand of the watch. It exploits the one true bearing of the sun in its course across the sky: the LOS from the observer to the zenith of its course. There the sun is seen mid-way between sunrise and sunset. A vertical plane including sun and observer is perpendicular to the plane of the sun’s course. Its intersection with the surface of the earth is a meridian, a line passing through a geographical pole. If the sun is in the southern half of the sky, the zenith bearing points true south; if northern, north. The time at that moment is 12:00 P.M., solar time. The clock position to the observer is 12.
If the watch is set to uncorrected solar time, both hands point to the sun. In a 12-hour watch, the sun and the hour hand both advance, but not at the same rate; the sun covers 15 degrees per hour, and watch 30. To keep the hour hand on the sun, 12:00 must recede from the zenith at the same rate the hour hand advances. Thus when the observer takes an arbitrary LOS, the zenith LOS – true north or south – is to be found at half the angle between 12 and the LOS.
The zenith LOS is only an approximation due to changes in the time kept by the watch. That time is based on mean solar time rather than observed solar time. Also, time changes with longitude, and the institution of daylight savings time. The time generally available for watch settings in the observer’s region is called civil time. It can be corrected to solar time, but LOS on a watch is generally too imprecise to make the trouble worth the effort.
In World War II aircraft pilots needed a quick method of communicating the relative position of threats, for which the clock system was ideal. The gunners of a bomber, or the other aircraft in the squadron, had to be kept informed for purposes of immediate response. However, in aviation, a clock position refers to a horizontal direction. The pilots needed a vertical dimension, so they supplemented the clock position with the word high or low to describe the vertical direction; e.g., 6 o'clock high means behind and above the horizon, while 12 o'clock low means ahead and below the horizon.
The horizon line was only visible in clear weather in daylight, and was only useful as a reference line in straight and level flight, when it appeared on the nose of the aircraft. The vocabulary therefore was only of use during daylight patrols or missions. The reference line and reference clock positions did not exist during combat aerobatics, at night, or during cloudy weather, when other means had to be found for locating the combatants, such as radar.
In 1916, J.B. Plato devised a clock system to identify farms around reference points in rural areas. A clock face was imagined centered on a rural community with 12:00 pointing true north. The circle was divided into concentric numbered bands at each mile of radius. The bands were divided into 12 segments at each position of the clock numbered after the clock hour. Within a segment, every building was assigned a letter. For instance, Alton 3-0 L meant house L in segment 3 of the central circle of 1 mile radius at Alton, where 3 was at 3:00.
Medical pathology uses the clock system to describe the location of breast tumors. A clock face is considered imposed over each breast, left and right, centered on the alveolar region, with the positions shown around it. Tumors are located at one or more subsites, or clock positions, identified by one or more clock numbers. In addition the numbers are arranged in quadrants: Upper Outer Quadrant (UOQ), Lower Inner Quadrant (LIQ), and so on. Codes are assigned to the quadrants, the alveolar region, and the whole breast.
Golf players use the clock system to study the course of the ball in putting situations. For holes that are on a slope, the hole is imagined to be the center of a clock face with 12:00 at the high point and 6:00 at the low point. The ball will only run true when hit from the high or low points; otherwise, its course will break, or bend on the slope. Some golfers practice clock drill - hitting the ball from all the positions of the clock - to learn how it breaks.
An article in the Journal of Applied Microscopy for 1898 recommends the use of a polar coordinate system in the form of a clockface for recording the positions of microscopic objects on a slide. The face is conceived centered on the circle visible under the lens. The pole is the center. Angle is given as a clock number, and distance as a decimal percentage of the radius through the object. For example, “3,9” means 3:00 o’clock at 9 tenths of the radius.
Although the raw clock position is invaluable or indispensible in many circumstances requiring rapid response, for ordinary careful navigation it is not sufficiently precise. It can be made precise by various methods requiring the use of instruments.
The clock face with its clock positions is a heritage of Roman civilization, as is suggested by the survival of Roman numerals on old clocks and their cultural predecessors, sundials. The mechanical clock supplanted the sundial as the major timekeeper, while the Hindu–Arabic numeral system replaced the Roman as the number system in Europe in the High Middle Ages. The Romans, however, had adapted their timekeeping system from the Ancient Greek. The historical trail leads from there to ancient Mesopotamia through the ancient Greek colonies placed on the coast of Anatolia in the 1st millenium BC. The first known historian, Herodotus of Halicarnassus, who was a native of that border region, made the identification:
The polos (“pole”) was a sundial of a concave face resembling the concavity of the universe (named a “pole” in this case). The gnomon was the pointer.
The Babylonian time system is documented by thousands of Mesopotamian cuneiform tablets. The Babylonians inherited the better part of their system from the Sumerians, whose culture they absorbed. Tablets of different periods reveal the development of a sexagesimal numbering system from decimal and duodecimal systems, which reveals itself in the construction of unique symbols for numerals 1-59 from natural finger decimals (ten fingers, ten symbols). Why they developed this system is a matter for academic debate, but there are multiple advantages, including division by several factors, offering several possible subdivisions, one of which is by 12’s. Classical civilization took over the time system entire, and modern from classical. The modern system is basically the Sumerian.
The 1949 movie Twelve O'Clock High takes its title from the system. In this case, the position would be ahead and above the horizon, an advantageous position for the attacker. The phrase "on your six" refers to the six o'clock or the adjacent positions; that is, the expression cautions that someone is behind.
Media related to Relative bearing at Wikimedia Commons