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Sundial
A sundial is a horology, horological device that tells the time of day (referred to as civil time in modern usage) when direct sunlight shines by the position of the Sun, apparent position of the Sun in the sky. In the narrowest sense of the word, it consists of a flat plate (the ''dial'') and a gnomon, which casts a shadow onto the dial. As the Sun diurnal motion, appears to move through the sky, the shadow aligns with different hour-lines, which are marked on the dial to indicate the time of day. The ''style'' is the time-telling edge of the gnomon, though a single point or ''nodus'' may be used. The gnomon casts a broad shadow; the shadow of the style shows the time. The gnomon may be a rod, wire, or elaborately decorated metal casting. The style must be polar alignment, parallel to the axis of the Earth's rotation for the sundial to be accurate throughout the year. The style's angle from horizontal is equal to the sundial's geographical latitude. The term ''sundial'' can r ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Equation Of Time
The equation of time describes the discrepancy between two kinds of solar time. The two times that differ are the apparent solar time, which directly tracks the diurnal motion of the Sun, and mean solar time, which tracks a theoretical mean Sun with uniform motion along the celestial equator. Apparent solar time can be obtained by measurement of the current position (Hour angle#Solar hour angle, hour angle) of the Sun, as indicated (with limited accuracy) by a sundial. ''Mean'' solar time, for the same place, would be the time indicated by a steady clock set so that over the year its differences from apparent solar time would have a mean of zero. The equation of time is the east or west component of the analemma, a curve representing the angular offset of the Sun from its mean position on the celestial sphere as viewed from Earth. The equation of time values for each day of the year, compiled by astronomical observatory, observatories, were widely listed in almanacs and ephemeri ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Analemmatic Sundial
Analemmatic sundials are a type of horizontal sundial that has a vertical gnomon and hour markers positioned in an elliptical pattern. The gnomon is not fixed and must change position daily to accurately indicate time of day. The time of day is read by the position of the shadow on the ellipse."Analemmatic sundials: How to build one and why they work" C.J. Budd and C.J. Sangwin, maths.org, 1 June 2000 Like most sundials, analemmatic sundials mark rather than clock time. Description An analemmatic sundial is completely defined by: * The size of its ellipse (chosen by the designer). * The |
Position Of The Sun
The position of the Sun in the sky is a function of both the time and the geographic coordinate system, geographic location of observation on Earth's surface. As Earth's orbit, Earth orbits the Sun over the course of a year, the Sun appears to move with respect to the fixed stars on the celestial sphere, along a circular path called the ecliptic. Earth's rotation about its axis causes diurnal motion, so that the Sun appears to move across the sky in a Sun path that depends on the observer's geographic latitude. The time when the Sun culmination, transits the observer's meridian (astronomy), meridian depends on the geographic longitude. To find the Sun's position for a given location at a given time, one may therefore proceed in three steps as follows: # calculate the Sun's position in the ecliptic coordinate system, # convert to the equatorial coordinate system, and # convert to the horizontal coordinate system, for the observer's local time and location. This is the coordinate s ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Gnomon
A gnomon (; ) is the part of a sundial that casts a shadow. The term is used for a variety of purposes in mathematics and other fields, typically to measure directions, position, or time. History A painted stick dating from 2300 BC that was excavated at the archeological site of Taosi is the oldest gnomon known in China. The gnomon was widely used in ancient China from the second millennium BC onward in order to determine the changes in seasons, orientation, and geographical latitude. The ancient Chinese used shadow measurements for creating calendars that are mentioned in several ancient texts. According to the collection of Zhou Chinese poetic anthologies ''Classic of Poetry'', one of the distant ancestors of King Wen of the Zhou dynasty used to measure gnomon shadow lengths to determine the orientation around the 14th century BC. The ancient Greek philosopher Anaximander (610–546 BC) is credited with introducing this Babylonian instrument to the Ancient Greeks. The ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Solar Time
Solar time is a calculation of the passage of time based on the position of the Sun in the sky. The fundamental unit of solar time is the day, based on the synodic rotation period. Traditionally, there are three types of time reckoning based on astronomical observations: #Apparent solar time, apparent solar time and #Mean solar time, mean solar time (discussed in this article), and ''sidereal time'', which is based on the apparent motions of stars other than the Sun. Introduction A tall pole vertically fixed in the ground casts a shadow on any sunny day. At one moment during the day, the shadow will point exactly north or south (or disappear when and if the Sun moves directly overhead). That instant is called solar noon, ''local apparent noon'', or 12:00 local apparent time. About 24 hours later the shadow will again point north–south, the Sun seeming to have covered a 360-degree arc around Earth's axis. When the Sun has covered exactly 15 degrees (1/24 of a circle, both a ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Horology
Chronometry or horology () is the science studying the measurement of time and timekeeping. Chronometry enables the establishment of standard measurements of time, which have applications in a broad range of social and scientific areas. ''Horology'' usually refers specifically to the study of mechanical timekeeping devices, while ''chronometry'' is broader in scope, also including biological behaviours with respect to time (biochronometry), as well as the dating of geological material (geochronometry). Horology is commonly used specifically with reference to the mechanical instruments created to keep time: clocks, watches, clockwork, sundials, hourglasses, Water clock, clepsydras, timers, time recorders, marine chronometers, and atomic clocks are all examples of Measuring instrument, instruments used to measure time. People interested in horology are called ''horologists''. That term is used both by people who deal professionally with timekeeping apparatuses, as well as enthus ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Daylight Saving Time
Daylight saving time (DST), also referred to as daylight savings time, daylight time (Daylight saving time in the United States, United States and Daylight saving time in Canada, Canada), or summer time (British Summer Time, United Kingdom, Summer time in Europe, European Union, and others), is the practice of advancing clocks to make better use of the longer daylight available during summer so that darkness falls at a later clock time. The standard implementation of DST is to set clocks forward by one hour in spring (season), spring or late winter, and to set clocks back by one hour to standard time in the autumn (or ''fall'' in North American English, hence the mnemonic: "spring forward and fall back"). Overview As of 2023, around 34 percent of the world's countries use DST. Some countries observe it only in some regions. In Canada, all of Yukon Time Zone, Yukon, most of Time in Saskatchewan, Saskatchewan, and parts of Nunavut, Ontario, British Columbia and Quebec do not ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Sphere
A sphere (from Ancient Greek, Greek , ) is a surface (mathematics), surface analogous to the circle, a curve. In solid geometry, a sphere is the Locus (mathematics), set of points that are all at the same distance from a given point in three-dimensional space.. That given point is the center (geometry), ''center'' of the sphere, and the distance is the sphere's ''radius''. The earliest known mentions of spheres appear in the work of the Greek mathematics, ancient Greek mathematicians. The sphere is a fundamental surface in many fields of mathematics. Spheres and nearly-spherical shapes also appear in nature and industry. Bubble (physics), Bubbles such as soap bubbles take a spherical shape in equilibrium. The Earth is spherical Earth, often approximated as a sphere in geography, and the celestial sphere is an important concept in astronomy. Manufactured items including pressure vessels and most curved mirrors and lenses are based on spheres. Spheres rolling, roll smoothly in ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Polar Alignment
Polar alignment is the act of aligning the rotation around a fixed axis, rotational axis of a telescope's equatorial mount or a sundial's gnomon with a celestial pole to parallel Earth's axis. Alignment methods The method to use differs depending on whether the alignment is taking place in daylight or at night. Furthermore, the method differs if the alignment is done in the Northern Hemisphere or Southern Hemisphere. The purpose of the alignment also must be considered; for example, the value of accuracy is much more significant in astrophotography than in casual stargazing. Aiming at the pole stars In the Northern Hemisphere, sighting Polaris the North Star is the usual procedure for aligning a telescope mount's polar axis parallel to the Earth's axial tilt, axis. Polaris is approximately three-quarters of a degree from the North Celestial Pole, and is easily seen by the naked eye. Sigma Octantis, σ Octantis, sometimes known as the South Star, can be sighted in the Souther ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Cylinder (geometry)
A cylinder () has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base. A cylinder may also be defined as an infinite curvilinear surface in various modern branches of geometry and topology. The shift in the basic meaning—solid versus surface (as in a solid ball versus sphere surface)—has created some ambiguity with terminology. The two concepts may be distinguished by referring to solid cylinders and cylindrical surfaces. In the literature the unadorned term "cylinder" could refer to either of these or to an even more specialized object, the '' right circular cylinder''. Types The definitions and results in this section are taken from the 1913 text ''Plane and Solid Geometry'' by George A. Wentworth and David Eugene Smith . A ' is a surface consisting of all the points on all the lines which are parallel to a given line and which pass through ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Cone (geometry)
In geometry, a cone is a three-dimensional figure that tapers smoothly from a flat base (typically a circle) to a point not contained in the base, called the '' apex'' or '' vertex''. A cone is formed by a set of line segments, half-lines, or lines connecting a common point, the apex, to all of the points on a base. In the case of line segments, the cone does not extend beyond the base, while in the case of half-lines, it extends infinitely far. In the case of lines, the cone extends infinitely far in both directions from the apex, in which case it is sometimes called a ''double cone''. Each of the two halves of a double cone split at the apex is called a ''nappe''. Depending on the author, the base may be restricted to a circle, any one-dimensional quadratic form in the plane, any closed one-dimensional figure, or any of the above plus all the enclosed points. If the enclosed points are included in the base, the cone is a solid object; otherwise it is an open surface ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
True North
True north is the direction along Earth's surface towards the place where the imaginary rotational axis of the Earth intersects the surface of the Earth on its Northern Hemisphere, northern half, the True North Pole. True south is the direction opposite to the true north. It is important to make the distinction from ''magnetic'' north, which points towards North magnetic pole, an ever changing location close to the True North Pole determined Earth's magnetic field. Due to fundamental limitations in map projection, true north also differs from the grid north which is marked by the direction of the grid lines on a typical printed map. However, the geographic coordinate system, longitude lines on a globe lead to the true poles, because the three-dimensional representation avoids those limitations. The celestial pole is the location on the imaginary celestial sphere where an imaginary extension of the rotational axis of the Earth intersects the celestial sphere. Within a margin of ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
