Mirror

They also were used as a portal to another world in Lewis Carroll's Through the Looking Glass. For example, a bearing of northwest by north differs by one point from a northwest bearing, and by a point from a north-northwest one. In Tlön, Uqbar, Orbis Tertius, a fictional heresiarch declares that "mirrors and copulation are abominable, since they both multiply the numbers of men.". A "point" is defined as one eighth of a right angle, and therefore equals exactly 11.25 degrees. In Dreamtigers, he writes of fearing that his reflection would move independently or change shape before his eyes. Galileo is a competing system, that will be placed into service by the European Union. Mirrors, along with labyrinths, figure prominently in the work of Argentine writer Jorge Luis Borges, who often used them as symbols of infinity, impersonation, and illusion. It relies on a slightly different geodesic model of the Earth.

Illuminated rotating disco balls covered with small mirrors are used to cast moving spots of light around a dance floor. GLONASS is a positioning system launched by the Soviet Union. Mirrors are often used in magic to create an illusion. The GPS system now permits accurate geographic location with an error of only a few metres, and precision timing to less than a microsecond. The hall of mirrors, commonly found in amusement parks, is an attraction in which a number of distorted mirrors are used to produce unusual reflections of the visitor. In 1974, the first GPS satellite was launched. This technique was used by Native American tribes and numerous militaries to transmit information between distant outposts. Other radionavigation systems include:.

The signal can be used over long distances, possibly up to 60 kilometres on a clear day. It was the first electronic navigation system to provide global coverage. With the sun as light source, a mirror can be used to signal, by variations in the orientation of the mirror. At about the same, TRANSIT, the first satellite-based navigation system was developed. A decorative reflecting sphere of thin metal-coated glass, working as a reducing wide-angle mirror, is sold as a Christmas tree decoration called a bauble. An analogous system for aircraft, VHF omnidirectional range and DME, was developed around the same time. Mirrors, typically large and unframed, are frequently used in interior decoration to create an illusion of space, and amplify the apparent size of a room. It revolutionized navigation by permitting semiautomated equipment to locate geographic positions to less than a half mile (800 m).

Its purpose is to split a beam of light so that half passes straight through, while the other half is reflected — this is useful for interferometry. This used time-of-flight of radio waves from antennas at known locations. The same type of mirror, when used in an optical instrument, is called a half-silvered mirror or beam splitter. Around 1960, LORAN was developed. It may be used to observe criminal suspects or customers (to watch out for theft). Up until 1960 it was commonplace for ships and aircraft to use radio direction-finding on commercial stations in order to locate islands and cities within the last several miles of error. Persons on the dark side see through it - it looks like a transparent window. In the late 19th century Nikola Tesla invented radio and direction-finding was quickly adapted to navigation.

Persons on the brightly lit side see their own reflection - it looks like a normal mirror. Later, mechanical chronometers enabled navigation at sea and in the air using relatively unskilled procedures. It is used between a dark room and a brightly lit room. A number of scientific journals during this period were started especially to chronicle geography. It is a sheet of glass coated with a layer of metal only a few dozen atoms thick, allowing some of the light through the surface (from both sides). These methods were too complex to be used by any but skilled astronomers, but they sufficed to map most of the world. A one-way mirror, also called two-way mirror, reflects about half of the light and lets the other half pass. At first, the best available "clocks" were the moons of Jupiter, and the calculated transits of selected stars by the moon.

Use of a large number of mirrors in a confined space can act to satisfy people's desire for satisfication of their ego, as in the hall of mirrors in the Palace of Versailles. Modern sextants measure to 0.2 minutes of arc, an error that translates to a distance of about 0.2 nautical miles (400 m). Other uses of mirrors in hedonistic acts include the classic 'mirror on the ceiling' for use during sex (see The Eagles' Hotel California), and the use of mirrors for 'cutting' and snorting cocaine. This eliminates the "cosine" error of an astrolabe's short pointer. upskirt. Thus, its "pointer" is as long as the horizon is far away. A mirror is sometimes used for voyeurism, e.g. A sextant uses mirrors to measure the altitude of celestial objects with regard to the horizon.

High quality flat mirrors are essential for making corner reflectors, which are used for emergency location, and even laser ranging to the Moon. In 1730 the sextant was invented and navigators rapidly replaced their astrolabes. Mirrors are also sometimes used as part of security systems, so that a single video camera can show more than one angle at a time. Starting in 1670, the entire world was measured using essentially modern latitude instruments and the best available clocks. Rounded (convex) mirrors are sometimes placed at road junctions, and corners of places such as parking lots or stores, allowing people to see around corners to avoid crashing into other vehicles or shopping carts. After Isaac Newton published the Principia, navigation was transformed. There exist rear view sunglasses, of which the left end of the left glass and the right end of the right glass work as mirrors. Diptychs remained in use during the day, until shadowing astrolabes were constructed.

Some motorcycle helmets have a built-in so-called MROS (Multiple Reflective Optic System): a set of reflective surfaces inside the helmet which together function as a rear-view mirror [1]. Around 400, metallurgy allowed construction of astrolabes graduated in degrees, which replaced the wooden latitude instruments for night use. Rear-view mirror are applied in and on vehicles (such as cars, or bicycles), to allow drivers to see other vehicles coming up behind them. This let masters continue sailing a course when the weather limited visibility of the sky. In astronomy, adaptive optics is a technique to measure variable image distortions and adapt a mirror accordingly on a timescale of milliseconds, to compensate for the distortions. Some time later, around 300, the magnetic compass was invented in China. Such mirrors are often used in lasers. Using these techniques, masters successfully sailed from the eastern Mediterranean to the south coast of the British Isles.

The best mirrors of this type can reflect >99.999% of the light (in a narrow range of wavelengths) which is incident on the mirror. The above instruments were a powerful technology, and appear to have been the technique used by ancient Cretan bronze-age trading empire. By careful choice of the type and thickness of the dielectric layers, the range of wavelengths and amount of light reflected from the mirror can be specified. These were often crucial trade secrets, because they enabled travel to lucrative ports. These are glass (or sometimes other material) substrates on which one or more layers of dielectric material are deposited, to form an optical coating. The most important instrument was a navigators' diary, later called a rutter. For scientific optical work, dielectric mirrors are often used. Time-keeping was by precision hourglasses, filled and tested to 1/4 of an hour, turned by the helmsman, or a young boy brought for that purpose.

The reflectivity as a function of wavelength depends on both the thickness of the coating and on how it is applied. Most sailors could use this instrument to take sun sights, but master navigators knew that sightings of Polaris were far more accurate, because they were not subject to time-keeping errors involved in finding noon. For instance, aluminum mirrors are commonly coated with magnesium fluoride. Latitude was determined with a "cross staff" an instrument vaguely similar to a carpenter's angle with graduated marks on it. Mirror surfaces are sometimes given thin film overcoatings both to retard degradation of the surface and to increase their reflectivity in parts of the spectrum where they will be used. This was placed in front of the helmsman. A hot mirror is the opposite, the coating preferentially reflects infrared. Another early invention was the compass rose, a cross or painted panel of wood oriented with the pole star or diptych.

A cold mirror is made by using a transparent substrate and choosing a coating material that is more reflective to visible light and more transmissive to infrared light. Basically, when the diptych's two sundials indicated the same time, the diptych was aligned to the current latitude and true north. This is exploited in some optical work to make cold mirrors and hot mirrors. When combined with a plumb bob, some diptychs could also determine latitude. The reflectivity of the mirror coating can be measured using a reflectometer and depends on the wavelength of light as well as the metal. Most sailors have always been able find absolute north from the stars, which currently rotate around Polaris, or by using a dual sundial called a diptych. Front silvered mirrors have to be resurfaced occasionally to keep their quality. In the West, navigation was at first performed exclusively by dead-reckoning, the process of estimating one's present position based on the navigators' experience with wind, tide and currents.

A protective overcoat is usually applied before the mirror is removed from the vacuum, because the coating otherwise begins to corrode as soon as it is exposed to oxygen and humidity in the air. This can be accomplished using low-cost quartz clocks because the satellites send time correction signals to the GPS receivers. The coatings are typically applied by vacuum deposition. GPS uses 3D trilateration based on measuring the time-of-flight of radio waves using the well-known speed of light to measure distance from at least three satellites. They reflect 90% to 95% of the incident light when new. A third source along with dead-reckoning will generally resolve to a single position. All of these coatings are easily damaged and require special handling. Signals from these two point establish a hyperbolic curve for possible positions.

Some of them use silver, but most are aluminum, which is more reflective at short wavelengths than silver. The LORAN system is based on measuring the phase shift of radio waves sent simultaneously from a master and slave station. Telescopes and other precision instruments use front silvered mirrors, where the reflecting surface is placed on the front surface of the glass, which gives better image quality. Inexpensive plastic sextants are available, though they have less accuracy than the more expensive metal models. This may be to check physical appearance (including clothing, make-up, hair, etc.) or to control applying make-up, shaving, cutting hair, fixing one's tie, etc. Some sextants create an artificial horizon by reflecting a bubble. A mirror is used for inspecting parts of one's body which are difficult or impossible to see directly, such as the face, neck or the whole body. When the image of the star touches the horizon, the angle can be read from the sextant's scale.

The "back side" of the mirror is often painted black to completely seal the metal from corrosion. An arm moves a split image of the star relative to the split image of the horizon. This type of mirror reflects about 80% of the incident light. During a sight, the user's view of the star and horizon remains steady as the boat rocks. They are back silvered, where the reflecting surface is viewed through the glass sheet; this makes the mirror durable, but lowers the image quality of the mirror due to extraneous reflections from the front surface of the glass. The angle is measured with a special optical instrument called a "sextant." Sextants use two mirrors to cancel the relative motion of the sextant. Most modern mirrors consist of a thin layer of aluminium deposited on a sheet of glass. Winding the chronometers was a crucial duty of the navigator.

Early mirrors were usually a sheet of polished metal, often silver or copper, for example the Aranmula kannadi. Traditionally, three chronometers are kept in gimbals in a dry room near the center of the ship, and used to set a watch for the actual sight, so that the chronometers themselves do not risk exposure to the elements. In either case, an observer farther from the mirror than you will see your normal orientation directly and in the mirror before you turn, and will then observe that you put your right hand where your left was if you rotate the usual way, or your head where your feet were if you stand on your head. If it is worn constantly, keeping it near body heat, its rate of drift can be measured with the radio, and by compensating for this drift, a navigator can keep time to better than a second per month. If one rotated about a horizontal axis parallel to the mirror surface, one would appear upside down. A quartz wristwatch normally keeps time within a half-second per day. The question is sometimes asked, "Why does the mirror reverse left to right and not top to bottom?" The answer is that one has rotated (as is most comfortable) about a vertical axis to face the mirror. Time is measured with a chronometer, a quartz watch or a shortwave radio broadcast from an atomic clock.

When the rotation is so obvious that it is not worth mentioning, the second component in this decomposition of the effect of the mirror is sometimes emphasized, by saying that a mirror "reverses left and right". Accurately knowing the time of an observation is important. In particular, if one looks at one's image in a vertical mirror in left-right orientation, the image corresponds to a rotation by 180° about the vertical axis in the mirror, combined with a reflection in one's approximate symmetry plane. Most navigation is performed with the sun and moon. In this case the image of the person is in normal standing orientation and vertically in a normal position, at a horizontally different position and with an orientation rotated about a vertical axis, the latter except if the mirror is parallel to the approximate symmetry plane of the person. The numerous celestial objects permit navigators to shoot through holes in clouds. This is the case iff the mirror is vertical. The math required for sight reduction is simple addition and subtraction, if sight-reduction tables are available.

The image is the most realistic if it is still vertical, i.e., if the rotation is about a vertical axis. Usually the navigator knows his position well enough to pick which of the two intersections is the current position. We can apply this to the image in a mirror of, say, a standing person, because people have approximate bilateral symmetry. A second sighting on a different object establishes an intersecting ring. For an object with approximate reflection symmetry, a reflection in some mirror plane corresponds to a combination of:. Conceptually, the angle to the celestial object establishes a ring of possible positions on the surface of the Earth. That is, if the beam of light is shining on a mirror's surface at a 30° angle from vertical, then it reflects from the point of incidence at a 30° angle from vertical in the opposite direction. From a single sight, a time within a second and an estimated position, a position can be determined within a third of a mile (500 m).

A beam of light reflects off a mirror at an angle of reflection that is equal to its angle of incidence. In modern celestial navigation, a nautical almanac and trigonometric sight-reduction tables permit navigators to measure the Sun, Moon, visible planets or any of 57 navigational stars at any time of day or night. Note that spherical concave and convex mirrors do not have a single focal point, as often described in high school physics text books (see spherical aberration in lens (optics) and aberration in optical systems). Once accurate clocks were available, detailed tables for celestial bodies were created so that navigational activities could take place anytime during the day or night, rather than at noon. Finally, there are convex mirrors, where a parallel beam becomes divergent, with the apparent intersection occurring behind the mirror. The need for accurate navigation led to the development of progressively more accurate clocks. There are also parabolic concave mirrors, where a parallel beam of light becomes a convergent beam, whose rays intersect in the focus of the mirror. The difference of longitude is determined knowing that the sun moves to the west at 15 degrees per hour.

In a plane mirror, a parallel beam of light changes its direction as a whole, whilst still remaining parallel; the images formed by a plane mirror are virtual images, of the same size as the original object (see mirror image). Then the local time of local noon is observed by the navigator. . The time of noon at the known location is carried by the navigator on an accurate clock. The most common use is in the home for personal grooming but mirrors are also used in scientific apparatus such as telescopes and lasers, and in industrial machinery. The time of the maximum altitude is easily determined by interpolating between periodic readings. The best known example is the plane mirror. Local noon is determined while shooting the azimuth as described above.

A mirror is a surface with good specular reflection that is smooth enough to form an image. Noon was an easy event to observe. a reflection in the approximate symmetry plane of the object (due to the assumption this is a minor change). Longitude is calculated as a time difference between the same celestial event at different locations. a translation if the mirror is parallel to the symmetry plane of the object, and otherwise a rotation about the line of intersection of the two planes by an angle which is twice the angle between the two planes. Since periodic readings of the altitude will plot a sine wave, the maximum reading is the one used for local noon. Local noon is easily determined by recording periodic readings of the altitude of the sun.

The sun's angle over the horizon at noon was measured, and compared to the known angle at the same date as the known port. However, prior to the development and formulation of its key principles in the latter part of the 17th century by Isaac Newton and Gottfried Leibniz, tables of the sun's altitude during the year for a known port were used. Calculating the anticipated altitude of the sun for a given day and known position is done easily using Calculus. Determining latitude by the sun was a little more difficult since the sun's altitude at noon during the year changes for a given location.

Navigators could determine their latitude by measuring the angular altitude of Polaris any time that it was visible (excepting, of course, in those southern latitudes from where it cannot be observed). Anciently the home port was used as the known location, currently the Greenwich Meridian or Prime Meridian is used as the known location for celestial charts. Celestial navigation systems are based on observation of the positions of the Sun, Moon and stars relative to the observer and a known location. This is known as a fix.

Addition lines of position can be measured in order to validate the results taken against other reference points. These lines of position can be plotted on a nautical chart, with the intersection being the ship's current location. This is done by correctly identifying reference points and measuring their bearings from the ship. Periodically, the navigator needs to confirm the accuracy of the dead reckoning or estimated position calculations using position fixing techniques.

If the set and drift, due to tide and wind, can be determined, an estimated position can also be calculated. A navigator uses the ship's last known position and dead reckoning, based on the ship's logged compass course and speed, to calculate the current position. Traditional maritime navigation with a compass uses multiple redundant sources of position information to locate the ship's position. These were made obsolete by satellite navigation systems.

The invention of the radio lead to radio beacons and radio direction finders providing accurate land-based fixes even hundreds of miles from shore. Later developments included the placing of lighthouses and buoys close to shore to act as marine signposts identifying ambiguous features, highlighting hazards and pointing to safe channels for ships approaching some part of a coast after a long sea voyage. The development of accurate systems for taking lines of position based on the measurement of stars and planets with the sextant allowed ships to navigate the open ocean without needing to see land marks. Nautical charts were developed to record new navigational and pilotage information for use by other navigators.

The magnetic compass allowing a course to be maintained and estimates of the ship's location to be calculated. Early navigators used pilotage, relying on local knowledge of land marks and coastal features, forcing all ships to stay close to shore. Knowing the ship's current position is the main problem for all navigators. There are several different branches of navigation, including but not limited to:.

They built a replica of an ancient double-hulled canoe called the Hokule'a, whose crew, in 1976, successfully navigated the Pacific Ocean from Hawaii to Tahiti using no instruments. In 1973, the Polynesian Voyaging Society was established in Hawaii to research Polynesian navigation methods. The first settlers of the Hawaiian Islands were said to have used these navigation methods to sail to the Hawaiian Islands from the Marquesas Islands. The guild secrets might have been lost, had not one of the last living navigators trained a professional small boat captain so that he could write a book.

Generally each island maintained a guild of navigators who had very high status, since in times of famine or difficulty, only they could trade for aid or evacuate people. These, and outrigger canoe construction methods, were kept as guild secrets. In Eastern Polynesia, navigators, in order to locate directions at various times of day and year, memorized extensive facts concerning:. The Polynesian navigators routinely crossed thousands of miles of open ocean, to tiny inhabited islands, using only their own senses and knowledge, passed by oral tradition, from navigator to apprentice.

. Prominent examples are the Phoenicians, the Ancient Greeks, the Malays, the Persians, Arabians, the Norse and, perhaps more than any others, the peoples of the Pacific Ocean, particularly Polynesians and Micronesians. In the pre-modern history of human migration and discovery of new lands by navigating the oceans, a few peoples have excelled as sea-faring explorers. There are several traditions of navigation.

Alpha, a longwave system developed by the Soviet Union. Omega, a longwave system developed by the United States Navy. Decca. collision avoidance using radar.

position fixing - determining current position by visual and electronic means. waypoint navigation - using electronic equipment such as radio navigation and satellite navigation system to follow a course to a waypoint. dead reckoning - using compass and log to monitor expected progress on a journey. pilotage - using visible natural and man made features such as sea marks and beacons.

celestial navigation - navigation by observation of the sun, moon and stars. Wayfinding Main Page. Wayfinding Summary. angles for approaching harbors.

colors of the sea and sky, especially how clouds would cluster at the locations of some islands. directions of swells on the ocean, and how the crew would feel their motion. wildlife species (which congregate at particular positions). times of travel.

weather. the motion of specific stars, and where they would rise and set on the horizon of the ocean.