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.
Uses As a relative bearing 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).Recent USAF practice allows the designation "right" and "left," as in 150 degrees right, thus the degrees can only go up to 180, meaning 6:00. 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.
As a true bearing
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 “12 o clock” which is defined as the longitudinal axis of the vessel or imaginary vessel to the bearing. The clock direction is given as one might read the “time” from a giant clock laid horizontally below the object of interest.
If such a clock were aligned so that its 12 o clock always aligned with either true north or south, it could be used as reference for determining absolute bearing or true bearing.
For example, a physical clock laid flat on the ground can be oriented so that its 12 o clock index points in the direction of the noon sun. In this case, the other positions on the clock face will correlate to absolute cardinal points of the compass.
Clock compass myth
A myth exists that a running clock or wristwatch showing local mean time can easily be oriented in this way using a procedure based only on knowing the position of the sun. The Boy Scout Handbook included such a procedure as recently as 2016.Official Boy Scout Handbook, Thirteenth Edition, 2016, p. 356 archive-url=https://archive.org/details/officialboyscout0000boys/page/356/mode/2up|
This myth arises from the intuitive notion that the sun “rises in the East” and “sets in the West” and (in the Northern Hemisphere) “lies to the south at noon”, coupled with a faulty assumption that the sun progresses around the compass dial at a constant rate throughout the day.
Unfortunately the real behavior of the sun is more complex than commonly understood.
The bearing from the observer to the sun is called the “solar azimuth”. There are different conventions for when this value is “zero”, for instance if one chooses the idea of “azimuth” to behave similarly to the idea of “bearing” on a map, true north will be defined as 0 degrees, increasing clockwise through all the points of the compass, passing through 180 degrees when facing South, continuing all the way to 360 degrees after a full rotation. Other conventions are possible: some astronomical formulas are based on the opposite convention that the solar azimuth is zero at local noon, at which time observers in the Northern Hemisphere would say the sun lies to the South.
A correct formula for azimuth according to the US Naval ObservatoryComputing Altitude and Azimuth from Greenwich Apparent Sidereal Time, United States Naval Observatory url=https://aa.usno.navy.mil/faq/alt_az can be written as:
tan(A) = sin(ω) / (cos(ω)·sin(φ) - tan(δ)·cos(φ))
Where
A is Azimuth angle measured clockwise from south: due west is +90° , due East is -90°
φ is an angle corresponding to your latitude (if you are at “48 degrees North latitude” then the angle is +48 °)
δ is an angle corresponding to the solar declination for today’s date (this is an angle between -23.45° on the winter solstice and +23.45° on the summer solstice, zero on the equinoxes, related to the axial tilt of the earth relative to the plane of its orbit around the sun, easy to look up from a table)
ω is an angle corresponding to time of day. This is known as “hour angle” and is calculated like this: if H is the number of hours AFTER NOON (where H is negative before noon) then ω = H * 15°
Clearly this is more complex than naive calculations that assume azimuth is merely equal to hour angle: at a minimum these methods do not factor in latitude or date.
Additionally, most clocks measure “local mean time” which generally differs from “local solar time”. Converting normal clock time to solar time requires three adjustments, accounting for 1) daylight savings time, 2) longitudinal offset from the central meridian of the local time zone, and 3) the “Equation of Time”, an offset from -15 to +15 minutes which varies by date. With accurate knowledge of these three factors, the clock can be adjusted to show “solar time” instead of “mean time” and, in that case, when solar noon arrives, the time on the clock will indeed show 12:00 just as the sun reaches its “culmination” or highest point in the sky.
If all the necessary corrections above are considered, a properly oriented clock face can be used to make inferences about cardinal directions. Otherwise, wristwatch-sun orientation methods could produce significant error.
Examples From aviation 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.
For airplanes in rapid maneuvers, air traffic controllers will issue the eight cardinal compass points instead.
From community planning 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.
From medicine 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.
From golf 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.
From microscopy 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.
Errors
Air traffic controls can only infer the aircraft heading from the ground track of an aircraft, which may not reflect the aircraft's actual heading due to drift angle caused by the wind. As a result, pilots should give due consideration and apply drift correction when an air traffic controller provides traffic advisories. Additionally, the error could also occur when the radar traffic information is issued while the aircraft is changing course.
Instrumentation Although the raw clock position is invaluable or indispensable 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.
Origin of the clock positions