Angle of Declination: |
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{{otheruses}} |
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Magnetic Axis and Geographic Axis |
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[[Image:Equatorial coordinates.png|right|250px]] |
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A freely suspended magnet always points in the North-South direction even in the absence of any other magnet. This suggests that the Earth itself behaves as a magnet which causes a freely suspended magnet (or magnetic needle) to point always in a particular direction: North and South. The shape of the Earth's magnetic field resembles that of a bar magnet of length one-fifth of the Earth's diameter buried at its center. |
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In [[astronomy]], '''declination''' (abbrev. '''dec''' or '''δ''') is one of the two coordinates of the [[equatorial coordinate system]], the other being either [[right ascension]] or [[hour angle]]. Dec is comparable to '''[[latitude]]''', projected onto the [[celestial sphere]], and is measured in degrees north and south of the [[celestial equator]]. Therefore, points north of the celestial equator have positive declinations, while those to the south have negative declinations. |
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The South Pole of the Earth's magnet is in the geographical North because it attracts the North Pole of the suspended magnet and vice versa. Thus, there is a magnetic S-pole near the geographical North, and a magnetic N-pole near the geographical South. The positions of the Earth's magnetic poles are not well defined on the globe; they are spread over an area. The axis of Earth's magnet and the geographical axis do no coincide. The axis of the Earth's magnetic field is inclined at an angle of about 15o with the geographical axis. Due to this a freely suspended magnet makes an angle of about 15o with the geographical axis and points only approximately in the North-South directions at a place. In other words, a freely suspended magnet does not show exact geographical South and North because the magnetic axis and geographical axis of the Earth do not coincide. |
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* An object on the [[celestial equator]] has a dec of 0°. |
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Cause of Earth's Magnetism: |
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* An object at the celestial [[north pole]] has a dec of +90°. |
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It is now believed that the Earth's magnetism is due to the magnetic effect of current which is flowing in the liquid core at the center of the Earth. Thus, the Earth is a huge electromagnet. |
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* An object at the celestial [[south pole]] has a dec of −90°. |
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Elements of Earth's Magnetic Field |
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To understand the Earth's magnetic field at any place, we should know the following two quantities or elements |
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The sign is customarily included even if it is positive. Any unit of angle can be used for declination, but it is often expressed in [[minute of arc|degrees, minutes, and seconds of arc]]. |
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1. Declination |
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2. Angle of dip (or Inclination) |
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A celestial object that passes over [[zenith]] has a declination equal to the observer's latitude, with northern latitudes yielding positive declinations. A [[pole star]] therefore has the declination +90° or -90°. Conversely, at northern latitudes φ > 0, celestial objects with a declination greater than 90° - φ, are always visible. Such stars are called [[circumpolar star]]s, while the phenomenon of a sun not setting is called [[midnight sun]]. |
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Declination |
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If instead of measuring from and along the equator the angles are measured from and along the horizon, the angles are called azimuth and altitude (elevation). |
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The vertical plane passing through the axis of a freely suspended magnet is called magnetic meridian. The direction of Earth's magnetic field lies in the magnetic meridian and may not be horizontal. The vertical plane passing through the true geographical North and South (or geographical axis of Earth) is called geographical meridian. The angle between the magnetic meridian and the geographic meridian at a place is called declination at that place. |
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==Stars== |
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The value of the angle of declination is different at different places on Earth. To find the exact geographic directions (North and South) at a place by using a magnetic compass, we should know the angle of declination at that place. The declination is expressed in degrees East (o E) or degrees West (o W). For example a declination of 2 o E means the compass will point 2 degrees east of true geographical North. Thus, the knowledge of declination at a place helps in finding the true geographical directions at that place. In every map used by surveyors, mariners and air pilots, declination for different places is indicated. It should be noted that at the places of zero declination, the compass North will coincide with the true geographical North. |
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Because a [[star]] lies in a nearly constant direction as viewed from earth, its declination is approximately constant from year to year. However, both the [[right ascension]] and declination do change gradually due to the effects of [[precession of the equinoxes]] and [[proper motion]]. |
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Angle of Dip or Inclination |
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So far we have only considered one type of magnetic needle which can move only in the horizontal place and points approximately in the North-South direction. Now, if we take a magnetic needle which is free to rotate in the vertical plane, then it will not remain perfectly horizontal. The compass needle makes a certain angle with the horizontal direction. In fact, in the Northern Hemisphere of Earth, the North Pole of the magnetic needle dips below the horizontal line. At any place, the magnetic needle points in the direction of the resultant intensity of Earth's magnetic field at the place. |
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==Varying declination== |
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The declinations of all [[solar system]] objects change much more quickly than those of stars. |
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Angle of Dip at the Poles |
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The magnetic lines of force at the poles of Earth are vertical due to which the magnetic needle becomes vertical. The angle of dip at the magnetic poles of Earth is 90 o. |
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===Sun===<!-- This section is linked from [[Horizontal coordinate system]] --> |
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Angle of Dip at the Equator |
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The declination of the [[Sun]] ('''δ''') is the angle between the rays of the sun and the plane of the earth's equator. Since the angle between the earth axis and the plane of the earth orbit is nearly constant, δ varies with the [[seasons]] and its period is one [[year]], that is the time needed by the earth to complete its revolution around the sun. |
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The lines of force around the magnetic equator of the Earth are perfectly horizontal. So the magnetic needle will become horizontal there. Thus, the angle of dip at the magnetic equator of the Earth will be 0 o. The angle of dip varies from place to place. |
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Historical main field change and declination |
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When the projection of the earth axis on the plane of the earth orbit is on the same line linking the earth and the sun, the angle between the rays of the sun and the plane of the earth equator is maximum and its value is 23°27'. This happens at the [[solstice]]s. Therefore δ = +23°27' at the northern hemisphere summer solstice and δ = -23°27' at the northern hemisphere winter solstice. Due to the changes in the tilt of the Earth's axis, the angle between the rays of the sun and the plane of the earth equator is slightly decreasing. |
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The Earth is like a giant magnet, surrounded by a magnetic field. This magnetic field, which is a vector with both direction and intensity, is generated by a dynamo process in the fluid outer core of the Earth. Due to the chaotic movement of the core fluid, the Earth's magnetic field gradually changes over the years. Figure 1 ( and the corresponding animation [50 MB] ) shows the horizontal direction of the magnetic field lines at the surface of the Earth. The magnetic North and South poles are shown as blue and red stars, respectively (note the change in location of the magnetic poles and the change in the speed of movement). Where the lines are blue, the magnetic field dips into the Earth, where they are red it emerges from the Earth. The transition from red to blue, where the field lines are horizontal, is called the magnetic equator. For more information on the magnetic field elements, see the NGDC/WDC answers to Geomagnetic Frequently Asked Questions |
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When the projection of the earth axis on the plane of the earth orbit is perpendicular to the line linking the earth and the sun, the angle between the rays of the sun and the plane of the earth equator is null. This happens at the [[equinox]]es. Therefore δ is 0° at the equinoxes. |
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Fig.1: (click to download animation for years 1590-2010 [50 MB]) Horizontal direction of the magnetic field, with the magnetic equator displayed in green. |
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When using a magnetic compass for navigation, the compass needle points in the direction of the lines displayed in Figure 1. Obviously, this direction is not equivalent with True North. The compass pointing direction can also differ substantially from the direction to the Magnetic North Pole, since magnetic field lines are not just great circles connecting the magnetic poles. Figure 2 ( and the corresponding animation [44 MB] ) illustrates the orientation of compass needles distributed over the surface of the Earth. True North is indicated by the direction of the black lines. The angle between the pointing direction of a compass needle and True North is called magnetic declination, or sometimes, magnetic variation (see below). |
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Sun's declination is equal to inverse sine of the product of sine of Sun's maximum declination and sine of Sun's tropical longitude at any given moment. Instead of computing sun's tropical longitude, if we need sun's declination in terms of days, following procedure is used. |
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(Fig.2: click to download animation for years 1590-2010 [44 MB]) Orientation of magnetic compass needles. True North is indicated by the direction of the black lines (meridians) |
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Since the eccentricity of the earth orbit is quite low, it can be approximated to a circle, and δ is approximately given by the following expression: |
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Because compasses were used before magnetism was fully understood, the magnetic pole located near Earth's geographic North Pole was called a "Magnetic North Pole", and the tip of a compass needle pointing (roughly) towards the Magnetic North Pole was also called the "Magnetic North Pole" of the needle. Now we know that opposite poles attract. Therefore, one of the poles must be a "Magnetic South Pole". Indeed, the magnetic pole near Earth's geographic North Pole, Earth's Magnetic North Pole is, from a physicists point of view, the southern magnetic pole (magnetic field lines are entering Earth). In Figure 2, this is illustrated by the color RED for a physicist's North pole and BLUE for a physicist's South pole. The red (North) tips of the compass needles are therefore attracted to the blue (south) pole of the Earth. However, convention maintains that the magnetic pole located in the northern hemisphere is called the "north" magnetic pole, while that in the southern hemisphere is the "south" magnetic pole, irregardless of the physics. |
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The difference between magnetic North and True North is called magnetic declination (or sometimes magnetic variation) and is measured in degrees east (positive) or west (negative) of True North. Shown in Figure 3 ( and the corresponding animation [43 MB] ) are the lines of equal declination (isogonic lines). On the black, agonic line (declination = 0) True North and Magnetic North are identical. In areas of red lines (positive declination) the compass points East of True North, and in areas of blue lines it points West of True North. The magnetic North and South poles are indicated by black stars. |
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:<math>\delta = -23.45^\circ \cdot \cos \left [ \frac{360^\circ}{365} \cdot \left ( N + 10 \right ) \right ]</math> |
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An erratic looking feature at the North geographic pole is due to the fact that the direction of True North changes drastically when stepping from one side of the geographic North Pole to the other side. In the same way, the direction of True North changes drastically when stepping from one side of the geographic South Pole to the other side. This is merely a mathematical complication arising from the definition of declination. As can be seen in Figure 1, a compass could be used for navigation even at the geographic poles, provided the horizontal magnetic field is strong enough to allow for reliable pointing. Presently this is the case at the geographic South Pole, but not at the geographic North Pole. |
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Fig.3: (click to download animation for years 1590-2010 [43 MB]) Lines of equal declination (isogonic lines) of the Earth's magnetic field. Positive lines in red, negative in blue. Along the black, agonic line (declination = 0), Magnetic North and True North are identical. |
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where ''cos'' operates on [[degree (angle)|degree]]s; if ''cos'' operates on [[radian]]s, 360° in the equation needs to be replaced with 2π and will still output δ in degree; <math>N</math> is Day of the Year, that is the number of days spent since January 1. |
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In these figures and animations, the magnetic field from 1590 to 1980 is given by the GUFM-1 model of Jackson et al. (2000), while the field from 1980 to 2010 is given by the 10th generation of the International Geomagnetic Reference Field. |
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Similar animations are also available as Google Earth KMZ and as NOAA's Science on a Sphere . |
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An alternative form is given as:<ref>[http://holodeck.st.usm.edu/vrcomputing/vrc_t/tutorials/solar/declination.shtml Solar Declination<!-- Bot generated title -->]</ref> |
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:<math>\delta = 23.45^\circ \cdot \sin \left [ \frac{360^\circ}{365} \cdot \left ( N + 284 \right ) \right ]</math> |
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A more precise formula is given by:<ref>Spencer, J.W. 1971: Fourier series representation of the position of the Sun. Search, 2(5), 172.</ref> |
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:<math>\ \delta = \frac{180^\circ}{\pi} \cdot (0.006918 - 0.399912 \cos \gamma + 0.070257 \sin \gamma - 0.006758 \cos 2\gamma + 0.000907 \sin 2\gamma - 0.002697 \cos 3\gamma + 0.00148 \sin 3\gamma)</math> |
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where |
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: <math>\gamma = \frac{2\pi}{365} ( N - 1 ) </math> |
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is the fractional year in radians. |
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More accurate '''''daily''''' values from averaging the four years of a [[leap year|leap-year]] cycle are given in the [http://www.wsanford.com/~wsanford/exo/sundials/DEC_Sun.html '''Table of the Declination of the Sun''']. |
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[[Image:Sun-declination.png|frame|none|A diagram demonstrating how the [[Sun]]'s path over the celestial sphere changes with the varying declination during the year, marking the [[Azimuth]]s in °N where the sun rises and sets at summer and winter [[solstice]] at a place of 56°N latitude.]] |
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===Moon=== |
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Declination of the [[Moon]] is computed by adding Sun's declination (which is called Declination of Place while computing declination of other planets and Moon) to Moon's latitude. Sun's declination (± 23.44°) is much larger in magnitude than Moon's latitude (± 5.14°). Therefore Moon's declination can be said to have an annual cycle synchronous with that of the Sun starting with the vernal equinox. |
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Moon's latitude is a function of the difference between True Moon and its ascending node. Since lunar nodes make one revolution in nearly 19 years, lunar latitude has an approximately 19 year long cycle. Lunar latitude is equal to inverse sine of the product of sine of maximum lunar latitude and sine of difference between Moon and its node. |
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For greater accuracy, Reduced Latitude is used instead of Moon's true latitude, which is obtained by multiplying lunar latitude with a multiplier having a maximum value of 1 for tropical Moon at 180° and 0.91745 for tropical Moon at 0°. This is caused by a third cycle in lunar declination which has a period of one lunar month and a maximum range of ± 0.425°. Summing all three components gives a range of maximum declination from +28°35' to +18°18' and the minimum from -18°18' to -28°35' for lunar declination. |
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The third component of lunar declination is computed from following formula : |
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Multiplier = Cos D / [Cos {Sin¯(Sin M * Sin D)}] |
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where D is Sun's maximum declination (± 23.44°) and M is Moon's tropical longitude. This multiplier is multiplied into Moon's latitude to get Reduced Latitude. The minimum value of Multiplier is for tropical Moon at zero longitude, which is equal to cosine of Sun's maximum declination, being equal to 0.91745. |
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This multiplier is used to determine the reduced latitude of other planets as well. |
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==See also== |
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* [[right ascension]], [[celestial coordinate system]] |
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* [[geographic coordinates]], [[ecliptic]] |
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* [[Setting circles]] |
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---- |
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'''Declination''' is used in some contexts that rule out astronomical declination, to mean the same as ''[[magnetic declination]]''. |
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Declination is occasionally and erroneously used to refer to the linguistic term [[declension]]. |
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==References== |
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<references/> |
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==External links== |
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*[http://www.wsanford.com/~wsanford/exo/sundials/DEC_Sun.html '''Table of the Declination of the Sun:''' Mean Value for the Four Years of a Leap-Year Cycle ] |
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*[http://www.sunlit-design.com/products/thesunapi/documentation/sdxDecl.php Declination function for Excel, CAD or your other programs.] The Sun API is free and extremely accurate. For Windows computers. |
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[[Category:Celestial coordinate system]] |
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[[Category:Angle]] |
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[[Category:Technical factors of astrology]] |
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[[ar:میل]] |
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[[ast:Declinación (astronomía)]] |
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[[bg:Деклинация]] |
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[[ca:Declinació (astronomia)]] |
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[[cs:Deklinace]] |
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[[da:Deklination (astronomi)]] |
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[[de:Deklination (Astronomie)]] |
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[[et:Kääne (astronoomia)]] |
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[[el:Απόκλιση αστέρος]] |
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[[es:Declinación (astronomía)]] |
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[[eo:Deklinacio (astronomio)]] |
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[[fa:میل]] |
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[[fr:Déclinaison (astronomie)]] |
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[[ga:Diallas]] |
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[[ko:적위]] |
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[[hr:Deklinacija (astronomija)]] |
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[[id:Deklinasi]] |
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[[it:Declinazione (astronomia)]] |
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[[la:Declinatio (astronomia)]] |
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[[lv:Deklinācija (astronomijā)]] |
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[[lb:Deklinatioun (Astronomie)]] |
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[[lt:Deklinacija]] |
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[[ml:ഡെക്ലിനേഷന്]] |
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[[nl:Declinatie (astronomie)]] |
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[[ja:赤緯]] |
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[[no:Deklinasjon]] |
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[[nn:Deklinasjon]] |
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[[pl:Deklinacja (astronomia)]] |
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[[pt:Declinação]] |
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[[ru:Склонение (астрономия)]] |
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[[sk:Deklinácia]] |
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[[sl:Deklinacija]] |
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[[th:เดคลิเนชัน]] |
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[[vi:Xích vĩ]] |
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[[zh:赤纬]] |
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Old page wikitext, before the edit (old_wikitext) |
'{{otheruses}}
[[Image:Equatorial coordinates.png|right|250px]]
In [[astronomy]], '''declination''' (abbrev. '''dec''' or '''δ''') is one of the two coordinates of the [[equatorial coordinate system]], the other being either [[right ascension]] or [[hour angle]]. Dec is comparable to '''[[latitude]]''', projected onto the [[celestial sphere]], and is measured in degrees north and south of the [[celestial equator]]. Therefore, points north of the celestial equator have positive declinations, while those to the south have negative declinations.
* An object on the [[celestial equator]] has a dec of 0°.
* An object at the celestial [[north pole]] has a dec of +90°.
* An object at the celestial [[south pole]] has a dec of −90°.
The sign is customarily included even if it is positive. Any unit of angle can be used for declination, but it is often expressed in [[minute of arc|degrees, minutes, and seconds of arc]].
A celestial object that passes over [[zenith]] has a declination equal to the observer's latitude, with northern latitudes yielding positive declinations. A [[pole star]] therefore has the declination +90° or -90°. Conversely, at northern latitudes φ > 0, celestial objects with a declination greater than 90° - φ, are always visible. Such stars are called [[circumpolar star]]s, while the phenomenon of a sun not setting is called [[midnight sun]].
If instead of measuring from and along the equator the angles are measured from and along the horizon, the angles are called azimuth and altitude (elevation).
==Stars==
Because a [[star]] lies in a nearly constant direction as viewed from earth, its declination is approximately constant from year to year. However, both the [[right ascension]] and declination do change gradually due to the effects of [[precession of the equinoxes]] and [[proper motion]].
==Varying declination==
The declinations of all [[solar system]] objects change much more quickly than those of stars.
===Sun===<!-- This section is linked from [[Horizontal coordinate system]] -->
The declination of the [[Sun]] ('''δ''') is the angle between the rays of the sun and the plane of the earth's equator. Since the angle between the earth axis and the plane of the earth orbit is nearly constant, δ varies with the [[seasons]] and its period is one [[year]], that is the time needed by the earth to complete its revolution around the sun.
When the projection of the earth axis on the plane of the earth orbit is on the same line linking the earth and the sun, the angle between the rays of the sun and the plane of the earth equator is maximum and its value is 23°27'. This happens at the [[solstice]]s. Therefore δ = +23°27' at the northern hemisphere summer solstice and δ = -23°27' at the northern hemisphere winter solstice. Due to the changes in the tilt of the Earth's axis, the angle between the rays of the sun and the plane of the earth equator is slightly decreasing.
When the projection of the earth axis on the plane of the earth orbit is perpendicular to the line linking the earth and the sun, the angle between the rays of the sun and the plane of the earth equator is null. This happens at the [[equinox]]es. Therefore δ is 0° at the equinoxes.
Sun's declination is equal to inverse sine of the product of sine of Sun's maximum declination and sine of Sun's tropical longitude at any given moment. Instead of computing sun's tropical longitude, if we need sun's declination in terms of days, following procedure is used.
Since the eccentricity of the earth orbit is quite low, it can be approximated to a circle, and δ is approximately given by the following expression:
:<math>\delta = -23.45^\circ \cdot \cos \left [ \frac{360^\circ}{365} \cdot \left ( N + 10 \right ) \right ]</math>
where ''cos'' operates on [[degree (angle)|degree]]s; if ''cos'' operates on [[radian]]s, 360° in the equation needs to be replaced with 2π and will still output δ in degree; <math>N</math> is Day of the Year, that is the number of days spent since January 1.
An alternative form is given as:<ref>[http://holodeck.st.usm.edu/vrcomputing/vrc_t/tutorials/solar/declination.shtml Solar Declination<!-- Bot generated title -->]</ref>
:<math>\delta = 23.45^\circ \cdot \sin \left [ \frac{360^\circ}{365} \cdot \left ( N + 284 \right ) \right ]</math>
A more precise formula is given by:<ref>Spencer, J.W. 1971: Fourier series representation of the position of the Sun. Search, 2(5), 172.</ref>
:<math>\ \delta = \frac{180^\circ}{\pi} \cdot (0.006918 - 0.399912 \cos \gamma + 0.070257 \sin \gamma - 0.006758 \cos 2\gamma + 0.000907 \sin 2\gamma - 0.002697 \cos 3\gamma + 0.00148 \sin 3\gamma)</math>
where
: <math>\gamma = \frac{2\pi}{365} ( N - 1 ) </math>
is the fractional year in radians.
More accurate '''''daily''''' values from averaging the four years of a [[leap year|leap-year]] cycle are given in the [http://www.wsanford.com/~wsanford/exo/sundials/DEC_Sun.html '''Table of the Declination of the Sun'''].
[[Image:Sun-declination.png|frame|none|A diagram demonstrating how the [[Sun]]'s path over the celestial sphere changes with the varying declination during the year, marking the [[Azimuth]]s in °N where the sun rises and sets at summer and winter [[solstice]] at a place of 56°N latitude.]]
===Moon===
Declination of the [[Moon]] is computed by adding Sun's declination (which is called Declination of Place while computing declination of other planets and Moon) to Moon's latitude. Sun's declination (± 23.44°) is much larger in magnitude than Moon's latitude (± 5.14°). Therefore Moon's declination can be said to have an annual cycle synchronous with that of the Sun starting with the vernal equinox.
Moon's latitude is a function of the difference between True Moon and its ascending node. Since lunar nodes make one revolution in nearly 19 years, lunar latitude has an approximately 19 year long cycle. Lunar latitude is equal to inverse sine of the product of sine of maximum lunar latitude and sine of difference between Moon and its node.
For greater accuracy, Reduced Latitude is used instead of Moon's true latitude, which is obtained by multiplying lunar latitude with a multiplier having a maximum value of 1 for tropical Moon at 180° and 0.91745 for tropical Moon at 0°. This is caused by a third cycle in lunar declination which has a period of one lunar month and a maximum range of ± 0.425°. Summing all three components gives a range of maximum declination from +28°35' to +18°18' and the minimum from -18°18' to -28°35' for lunar declination.
The third component of lunar declination is computed from following formula :
Multiplier = Cos D / [Cos {Sin¯(Sin M * Sin D)}]
where D is Sun's maximum declination (± 23.44°) and M is Moon's tropical longitude. This multiplier is multiplied into Moon's latitude to get Reduced Latitude. The minimum value of Multiplier is for tropical Moon at zero longitude, which is equal to cosine of Sun's maximum declination, being equal to 0.91745.
This multiplier is used to determine the reduced latitude of other planets as well.
==See also==
* [[right ascension]], [[celestial coordinate system]]
* [[geographic coordinates]], [[ecliptic]]
* [[Setting circles]]
----
'''Declination''' is used in some contexts that rule out astronomical declination, to mean the same as ''[[magnetic declination]]''.
Declination is occasionally and erroneously used to refer to the linguistic term [[declension]].
==References==
<references/>
==External links==
*[http://www.wsanford.com/~wsanford/exo/sundials/DEC_Sun.html '''Table of the Declination of the Sun:''' Mean Value for the Four Years of a Leap-Year Cycle ]
*[http://www.sunlit-design.com/products/thesunapi/documentation/sdxDecl.php Declination function for Excel, CAD or your other programs.] The Sun API is free and extremely accurate. For Windows computers.
[[Category:Celestial coordinate system]]
[[Category:Angle]]
[[Category:Technical factors of astrology]]
[[ar:میل]]
[[ast:Declinación (astronomía)]]
[[bg:Деклинация]]
[[ca:Declinació (astronomia)]]
[[cs:Deklinace]]
[[da:Deklination (astronomi)]]
[[de:Deklination (Astronomie)]]
[[et:Kääne (astronoomia)]]
[[el:Απόκλιση αστέρος]]
[[es:Declinación (astronomía)]]
[[eo:Deklinacio (astronomio)]]
[[fa:میل]]
[[fr:Déclinaison (astronomie)]]
[[ga:Diallas]]
[[ko:적위]]
[[hr:Deklinacija (astronomija)]]
[[id:Deklinasi]]
[[it:Declinazione (astronomia)]]
[[la:Declinatio (astronomia)]]
[[lv:Deklinācija (astronomijā)]]
[[lb:Deklinatioun (Astronomie)]]
[[lt:Deklinacija]]
[[ml:ഡെക്ലിനേഷന്]]
[[nl:Declinatie (astronomie)]]
[[ja:赤緯]]
[[no:Deklinasjon]]
[[nn:Deklinasjon]]
[[pl:Deklinacja (astronomia)]]
[[pt:Declinação]]
[[ru:Склонение (астрономия)]]
[[sk:Deklinácia]]
[[sl:Deklinacija]]
[[sh:Deklinacija (astronomija)]]
[[sr:Deklinacija (astronomija)]]
[[fi:Deklinaatio]]
[[sv:Deklination (astronomi)]]
[[th:เดคลิเนชัน]]
[[vi:Xích vĩ]]
[[zh:赤纬]]'
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New page wikitext, after the edit (new_wikitext) |
'Angle of Declination:
Magnetic Axis and Geographic Axis
A freely suspended magnet always points in the North-South direction even in the absence of any other magnet. This suggests that the Earth itself behaves as a magnet which causes a freely suspended magnet (or magnetic needle) to point always in a particular direction: North and South. The shape of the Earth's magnetic field resembles that of a bar magnet of length one-fifth of the Earth's diameter buried at its center.
The South Pole of the Earth's magnet is in the geographical North because it attracts the North Pole of the suspended magnet and vice versa. Thus, there is a magnetic S-pole near the geographical North, and a magnetic N-pole near the geographical South. The positions of the Earth's magnetic poles are not well defined on the globe; they are spread over an area. The axis of Earth's magnet and the geographical axis do no coincide. The axis of the Earth's magnetic field is inclined at an angle of about 15o with the geographical axis. Due to this a freely suspended magnet makes an angle of about 15o with the geographical axis and points only approximately in the North-South directions at a place. In other words, a freely suspended magnet does not show exact geographical South and North because the magnetic axis and geographical axis of the Earth do not coincide.
Cause of Earth's Magnetism:
It is now believed that the Earth's magnetism is due to the magnetic effect of current which is flowing in the liquid core at the center of the Earth. Thus, the Earth is a huge electromagnet.
Elements of Earth's Magnetic Field
To understand the Earth's magnetic field at any place, we should know the following two quantities or elements
1. Declination
2. Angle of dip (or Inclination)
Declination
The vertical plane passing through the axis of a freely suspended magnet is called magnetic meridian. The direction of Earth's magnetic field lies in the magnetic meridian and may not be horizontal. The vertical plane passing through the true geographical North and South (or geographical axis of Earth) is called geographical meridian. The angle between the magnetic meridian and the geographic meridian at a place is called declination at that place.
The value of the angle of declination is different at different places on Earth. To find the exact geographic directions (North and South) at a place by using a magnetic compass, we should know the angle of declination at that place. The declination is expressed in degrees East (o E) or degrees West (o W). For example a declination of 2 o E means the compass will point 2 degrees east of true geographical North. Thus, the knowledge of declination at a place helps in finding the true geographical directions at that place. In every map used by surveyors, mariners and air pilots, declination for different places is indicated. It should be noted that at the places of zero declination, the compass North will coincide with the true geographical North.
Angle of Dip or Inclination
So far we have only considered one type of magnetic needle which can move only in the horizontal place and points approximately in the North-South direction. Now, if we take a magnetic needle which is free to rotate in the vertical plane, then it will not remain perfectly horizontal. The compass needle makes a certain angle with the horizontal direction. In fact, in the Northern Hemisphere of Earth, the North Pole of the magnetic needle dips below the horizontal line. At any place, the magnetic needle points in the direction of the resultant intensity of Earth's magnetic field at the place.
Angle of Dip at the Poles
The magnetic lines of force at the poles of Earth are vertical due to which the magnetic needle becomes vertical. The angle of dip at the magnetic poles of Earth is 90 o.
Angle of Dip at the Equator
The lines of force around the magnetic equator of the Earth are perfectly horizontal. So the magnetic needle will become horizontal there. Thus, the angle of dip at the magnetic equator of the Earth will be 0 o. The angle of dip varies from place to place.
Historical main field change and declination
The Earth is like a giant magnet, surrounded by a magnetic field. This magnetic field, which is a vector with both direction and intensity, is generated by a dynamo process in the fluid outer core of the Earth. Due to the chaotic movement of the core fluid, the Earth's magnetic field gradually changes over the years. Figure 1 ( and the corresponding animation [50 MB] ) shows the horizontal direction of the magnetic field lines at the surface of the Earth. The magnetic North and South poles are shown as blue and red stars, respectively (note the change in location of the magnetic poles and the change in the speed of movement). Where the lines are blue, the magnetic field dips into the Earth, where they are red it emerges from the Earth. The transition from red to blue, where the field lines are horizontal, is called the magnetic equator. For more information on the magnetic field elements, see the NGDC/WDC answers to Geomagnetic Frequently Asked Questions
Fig.1: (click to download animation for years 1590-2010 [50 MB]) Horizontal direction of the magnetic field, with the magnetic equator displayed in green.
When using a magnetic compass for navigation, the compass needle points in the direction of the lines displayed in Figure 1. Obviously, this direction is not equivalent with True North. The compass pointing direction can also differ substantially from the direction to the Magnetic North Pole, since magnetic field lines are not just great circles connecting the magnetic poles. Figure 2 ( and the corresponding animation [44 MB] ) illustrates the orientation of compass needles distributed over the surface of the Earth. True North is indicated by the direction of the black lines. The angle between the pointing direction of a compass needle and True North is called magnetic declination, or sometimes, magnetic variation (see below).
(Fig.2: click to download animation for years 1590-2010 [44 MB]) Orientation of magnetic compass needles. True North is indicated by the direction of the black lines (meridians)
Because compasses were used before magnetism was fully understood, the magnetic pole located near Earth's geographic North Pole was called a "Magnetic North Pole", and the tip of a compass needle pointing (roughly) towards the Magnetic North Pole was also called the "Magnetic North Pole" of the needle. Now we know that opposite poles attract. Therefore, one of the poles must be a "Magnetic South Pole". Indeed, the magnetic pole near Earth's geographic North Pole, Earth's Magnetic North Pole is, from a physicists point of view, the southern magnetic pole (magnetic field lines are entering Earth). In Figure 2, this is illustrated by the color RED for a physicist's North pole and BLUE for a physicist's South pole. The red (North) tips of the compass needles are therefore attracted to the blue (south) pole of the Earth. However, convention maintains that the magnetic pole located in the northern hemisphere is called the "north" magnetic pole, while that in the southern hemisphere is the "south" magnetic pole, irregardless of the physics.
The difference between magnetic North and True North is called magnetic declination (or sometimes magnetic variation) and is measured in degrees east (positive) or west (negative) of True North. Shown in Figure 3 ( and the corresponding animation [43 MB] ) are the lines of equal declination (isogonic lines). On the black, agonic line (declination = 0) True North and Magnetic North are identical. In areas of red lines (positive declination) the compass points East of True North, and in areas of blue lines it points West of True North. The magnetic North and South poles are indicated by black stars.
An erratic looking feature at the North geographic pole is due to the fact that the direction of True North changes drastically when stepping from one side of the geographic North Pole to the other side. In the same way, the direction of True North changes drastically when stepping from one side of the geographic South Pole to the other side. This is merely a mathematical complication arising from the definition of declination. As can be seen in Figure 1, a compass could be used for navigation even at the geographic poles, provided the horizontal magnetic field is strong enough to allow for reliable pointing. Presently this is the case at the geographic South Pole, but not at the geographic North Pole.
Fig.3: (click to download animation for years 1590-2010 [43 MB]) Lines of equal declination (isogonic lines) of the Earth's magnetic field. Positive lines in red, negative in blue. Along the black, agonic line (declination = 0), Magnetic North and True North are identical.
In these figures and animations, the magnetic field from 1590 to 1980 is given by the GUFM-1 model of Jackson et al. (2000), while the field from 1980 to 2010 is given by the 10th generation of the International Geomagnetic Reference Field.
Similar animations are also available as Google Earth KMZ and as NOAA's Science on a Sphere .'
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