Coordonate Ecuatoriale Si Orizontale

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EQUATORIAL COORDINATE SYSTEM

The Columbia Encyclopedia, Sixth Edition | 2008 | Copyright

EQUATORIAL COORDINATE SYSTEM THE MOST COMMONLY USEDASTRONOMICALCOORDINATE SYSTEM FOR INDICATING THE POSITIONS OF STARS OR OTHER CELESTIAL

OBJECTS ON THE CELESTIAL SPHERE

The celestial sphere is an imaginary spherewith the observer at its center. It represents the

entire sky; all celestial objects other than theearth are imagined as being located on itsinside surface. If the earth's axis is extended,the points where it intersects the celestialsphere are called the celestial poles; the northcelestial pole is directly above the earth's NorthPole, and the south celestial pole directly abovethe earth's South Pole. The great circle on thecelestial sphere halfway between the celestialpoles is called the celestial equator; it can bethought of as the earth's equator projected ontothe celestial sphere. It divides the celestialsphere into the northern and southern skies.

Sfera cereasca este o sfer imaginar cu observatorulla centru. Ea reprezint ntregul cer; toate obiectele

cereti, altele dect pmntul sunt imaginate cafiind situate n interior pe suprafaa acesteia.Dac axa Pmntului este prelungit, punctele n

care se intersecteaz sfera cereasca suntnumite poli cereti; polul Nord ceresc estedirect

deasupra Polului Nord al pmntului, i Polul Sudceresc direct deasupra Polul Sud al pmntului.Cercul mare pe sfera cereasca la jumtateadistanei dintre polii cereti este numit ecuatorulceresc, ei poate fi gndit ca ecuatorul Pmntuluiproiectat pe sfera cereasca. Acesta mparte sferacereasca n emisfera nordic i sudic.

AN EQUATORIAL COORDINATE SYSTEM THE MOST COMMONLY USED ASTRONOMICALCOORDINATE SYSTEM FOR INDICATING THE POSITIONS OF STARS OR OTHER

CELESTIAL OBJECTS ON THE CELESTIAL SPHERE

The celestial sphere is an imaginary sphere with theobserver at its center. It represents the entire sky; allcelestial objects other than the earth are imagined asbeing located on its inside surface. If the earth's axisis extended, the points where it intersects thecelestial sphere are called the celestial poles; thenorth celestial pole is directly above the earth's NorthPole, and the south celestial pole directly above theearth's South Pole. The great circle on the celestialsphere halfway between the celestial poles is calledthe celestial equator; it can be thought of as theearth's equator projected onto the celestial sphere. Itdivides the celestial sphere into the northern andsouthern skies. An important reference point on thecelestial equator is the vernal equinox , the point atwhich the sun crosses the celestial equator in March.

Sfera cereasc este o sfer imaginar cuobservator la centru. Ea reprezint ntregul cer;toate obiectele cereti, altele dect pmntulsuntimaginate ca fiind situate pe suprafaa acesteia, n interiorul. Dac axa Pmntului esteprelungit, punctele n care se intersecteazsfera cereascasunt numite poli cereti; polul

nord ceresc estedirect deasupra pmntului PolulNord, i polul sud ceresc direct deasuprapmntului Polul Sud. Cercul mare pe sferacereasca la jumtatea distanei ntre poliicereti este numit ecuatorul ceresc, ea poate

fi gndit ca ecuatorul Pmntului proiectatepesfera cereasca. Acesta mparte sferacereasc ncer de nord i de sud. Un important

punct de referin pe ecuatorul ceresc esteechinociul de primvar, la punctul n care

soarele traverseaz ecuatorul ceresc n luna
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martie.

To designate the position of a star, the astronomerconsiders an imaginary great circle passing throughthe celestial poles and through the star in question.This is the star'shour circle , analogous to ameridian of longitude on earth. The astronomerthen measures the angle between the vernalequinox and the point where the hour circleintersects the celestial equator. This angle is calledthe star's right ascension and is measured in hours,minutes, and seconds rather than in the morefamiliar degrees, minutes, and seconds. (There are360 degrees or 24 hours in a full circle.) The rightascension is always measured eastward from the

vernal equinox. Next the observer measures alongthe star's hour circle the angle between thecelestial equator and the position of the star. Thisangle is called thedeclinationof the star and ismeasured in degrees, minutes, and seconds northor south of the celestial equator, analogous tolatitude on the earth. Right ascension anddeclination together determine the location of a staron the celestial sphere. The right ascensions anddeclinations of many stars are listed in variousreference tables published for astronomers andnavigators. Because a star's position may changeslightly (seeproper motion and precession of theequinoxes), such tables must be revised at regularintervals. By definition, the vernal equinox islocated at right ascension 0 h and declination 0.

Pentru a desemna poziia unei stele, astronomulconsider un cerc imaginar mare care trece prinpolii cereti i prin stele n cauz. Acesta estecercul orar al stelei, analog unei meridianul delongitudine de pe pmnt. astronom Msurile apoiunghiul dintre echinociul vernal i punctul n careintersecteaz cercul or ecuatorul ceresc. Acestunghi este numit ascensiunea stelei dreapta i semsoar n ore, minute i secunde, mai degrabdect n grade mai familiar, minute i secunde. (Nusunt de 360 de grade sau 24 de ore ntr-un cerccomplet.) Ascensiunea dreapta este ntotdeaunamsurat est de la echinociul de primvar.

Lng msurile de observator de-a lungul cerculuistelei ora unghiul dintre ecuatorul ceresc ipoziia de stele. Acest unghi este numitdeclinaia de stele i se msoar n grade,minute i secunde la nordsau la sud de ecuatorulceresc, analog la latitudinea pe pmnt.Ascensiunea dreapt i declinaia stabilimpreun locaia de o stea pesfera cereasca.ascensiuni dreptul i declinationsde multestele sunt enumerate n diverse tabele de referinpublicat de astronomi i navigatori.Deoarecepoziia unei stele se poate schimba uor (a sevedea micare corect i echinoctiilor),astfel detabele trebuie s fie revizuite la intervaleregulate. Prin definiie, echinociul de primvar seafl la dreapta ascensiunea 0 h i declinaia 0 .

Another useful reference point is the sigma point,the point where the observer's celestial meridianintersects the celestial equator. The right ascensionof the sigma point is equal to the observer's localsidereal time. The angular distance from the sigmapoint to a star's hour circle is called itshour angle ;it is equal to the star's right ascension minus thelocal sidereal time. Because the vernal equinox isnot always visible in the night sky (especially in thespring), whereas the sigma point is always visible,the hour angle is used in actually locating a body inthe sky.

Un alt punct de referin util este punctul sigma,punctul n care meridianul observatoruluicerescintersecteaz ecuatorul ceresc. ascensiuneadreapta punctului de sigma este egal cu timpulobservatorului sideral local. distana unghiular dinpunctul de sigma la cercul unei stele or senumete unghiul su or; este egal cuascensiunea stelei dreptul de minus timp siderallocal. Deoarece echinociul de primvar nu este

ntotdeauna vizibil n cerul de noapte (maiales nprimvar), n timp ce punctul de sigmaestentotdeauna vizibil, unghiul de or este folosit

nlocalizarea de fapt, un organism n cer.
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CELESTIAL SPHERE


CELESTIAL SPHERE IMAGINARY SPHERE OF INFINITE RADIUS WITH THE EARTH AT ITS CENTER

It is used for describing the positions andmotions of stars and other objects. For thesepurposes, any astronomical object can bethought of as being located at the point wherethe line of sight from the earth through theobject intersects the surface of the celestialsphere. In astronomical coordinate systems ,the coordinate axes are great circles on thecelestial sphere. In most systems of this type,the reference points are fixed on the sphere,so the two coordinates needed to locate abody are relatively constant.

Este folosit pentru descrierea poziiilor i propuneri destele i alte obiecte. Pentru aceste scopuri, oriceobiect astronomic pot fi considerateca fiind situat lapunctul n care linia de vedere de la pmnt prinintermediul obiectului intersecteazsuprafaa sferei ceresti. n sistemele de coordonate astronomice, axe decoordonate sunt cercuri mari pe sfera cereasca. n celemai multe sisteme de acest tip, punctele dereferin sunt fixate pe sfera, astfel nct celedou coordonatenecesare pentru a localiza unorganism sunt relative constante.

CELESTIAL SPHERE IMAGINARY SPHERE OF INFINITE RADIUS WITH THE EARTH AT ITS CENTER
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EQUATORIAL COORDINATE SYSTEM


EQUATORIAL COORDINATE SYSTEM THE MOST COMMONLY USEDASTRONOMICALCOORDINATE SYSTEM FOR INDICATING THE POSITIONS OF STARS OR OTHER CELESTIAL

OBJECTS ON THE CELESTIAL SPHERE

The celestial sphere is an imaginary sphere withthe observer at its center. It represents the entiresky; all celestial objects other than the earth are

imagined as being located on its inside surface. Ifthe earth's axis is extended, the points where itintersects the celestial sphere are called thecelestial poles; the north celestial pole is directlyabove the earth's North Pole, and the southcelestial pole directly above the earth's South Pole.The great circle on the celestial sphere halfwaybetween the celestial poles is called the celestialequator; it can be thought of as the earth's equatorprojected onto the celestial sphere. It divides the

sfera cereasca este o sfer imaginar cuobservatorla centru. Ea reprezint ntregul cer; toateobiectele cereti, altele dect pmntul sunt

imaginate ca fiind situate pe suprafaa acesteia, ninteriorul. Dac axa Pmntului este prelungit,punctele n care se intersecteaz sfera cereascasunt numite poli cereti; polul nord ceresc estedirect deasupra pmntului Polul Nord, i polulsudceresc direct deasupra pmntului PolulSud. cercmare pe sfera cereasca la jumtateadistanei ntrepolii cereti este numit ecuatorulceresc, ea poatefi gndit ca ecuatorul Pmntului proiectate pesfera cereasca. Acesta mparte sfera ce
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celestial sphere into the northern and southernskies.

reasca ncer de nord i de sud.

AN EQUATORIAL COORDINATE SYSTEM THE MOST COMMONLY USEDASTRONOMICALCOORDINATE SYSTEM FOR INDICATING THE POSITIONS OF STARS OR OTHER CELESTIALOBJECTS ON THECELESTIAL SPHERE

The celestial sphere is an imaginary sphere withthe observer at its center. It represents the entiresky; all celestial objects other than the earth areimagined as being located on its inside surface. Ifthe earth's axis is extended, the points where itintersects the celestial sphere are called thecelestial poles; the north celestial pole is directlyabove the earth's North Pole, and the southcelestial pole directly above the earth's South Pole.The great circle on the celestial sphere halfwaybetween the celestial poles is called the celestialequator; it can be thought of as the earth's equatorprojected onto the celestial sphere. It divides thecelestial sphere into the northern and southernskies. An important reference point on the celestialequator is the vernal equinox , the point at whichthe sun crosses the celestial equator in March.

sfera cereasca este o sfer imaginar cuobservatorla centru. Ea reprezint ntregul cer; toateobiectele cereti, altele dect pmntul suntimaginate ca fiind situate pe suprafaa acesteia, ninteriorul. Dac axa Pmntului este prelungit,punctele n care se intersecteaz sfera cereascasunt numite poli cereti; polul nord ceresc estedirect deasupra pmntului Polul Nord, i polulsudceresc direct deasupra pmntului PolulSud. cercmare pe sfera cereasca la jumtateadistanei ntrepolii cereti este numit ecuatorulceresc, ea poatefi gndit ca ecuatorul Pmntului proiectate pesfera cereasca. Acesta mparte sfera cereasca ncer de nord i de sud. Un important punctde referin pe ecuatorul ceresc este echinociuldeprimvar, la punctul n care soareletraverseazecuatorul ceresc n luna martie.

To designate the position of a star, the astronomerconsiders an imaginary great circle passing throughthe celestial poles and through the star in question.This is the star'shour circle , analogous to ameridian of longitude on earth. The astronomerthen measures the angle between the vernalequinox and the point where the hour circleintersects the celestial equator. This angle is calledthe star's right ascension and is measured in hours,minutes, and seconds rather than in the more

familiar degrees, minutes, and seconds. (There are360 degrees or 24 hours in a full circle.) The rightascension is always measured eastward from thevernal equinox. Next the observer measures alongthe star's hour circle the angle between thecelestial equator and the position of the star. Thisangle is called thedeclinationof the star and ismeasured in degrees, minutes, and seconds northor south of the celestial equator, analogous to

Pentru a desemna poziia unei stele, astronomulconsider un cerc imaginar mare care treceprinpolii cereti i prin stele n cauz. Acestaestecercul stelei or, analog unei meridianuldelongitudine de pe pmnt. AstronomMsurile apoiunghiul dintre echinociul vernal i punctul n careintersecteaz cercul or ecuatorulceresc. Acestunghi este numit ascensiunea stelei dreapta i se msoar n ore, minute i secunde,mai degrabdect n grade mai familiar, minute isecunde. (Nusunt de 360 de grade sau 24 de ore

ntr-un cerccomplet.) inaltarea dreaptaeste ntotdeaunamsurat est de la echinociul deprimvar.Lng msurile de observator de-alungul cerculuistelei ora unghiul dintre ecuatorul ceresc i poziiade stele. Acest unghi este numitdeclinaia de stelei se msoar n grade, minutei secunde la nordsau la sud de ecuatorul ceresc,analog lalatitudinea pe pmnt. Ascensie dreapti declinaia stabili mpreun locaia de o stea pe
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latitude on the earth. Right ascension anddeclination together determine the location of a staron the celestial sphere. The right ascensions anddeclinations of many stars are listed in variousreference tables published for astronomers andnavigators. Because a star's position may change

slightly (seeproper motion and precession of theequinoxes), such tables must be revised at regularintervals. By definition, the vernal equinox islocated at right ascension 0 h and declination 0.

sfera cereasca. ascensiuni dreptul i declinationsde multe stele sunt enumerate n diverse tabele dereferin publicat de astronomi i navigatori.Deoarece poziia unei stele se poate schimbauor (a sevedea micare corect i echinoctiilor),astfel detabele trebuie s fie revizuite la intervale regulate.

Prin definiie, echinociul de primvar se afl ladreapta ascensiunea 0 h i declinaia 0 .

Another useful reference point is the sigma point,the point where the observer's celestial meridianintersects the celestial equator. The right ascensionof the sigma point is equal to the observer's localsidereal time. The angular distance from the sigmapoint to a star's hour circle is called itshour angle ;

it is equal to the star's right ascension minus thelocal sidereal time. Because the vernal equinox isnot always visible in the night sky (especially in thespring), whereas the sigma point is always visible,the hour angle is used in actually locating a body inthe sky.

Un alt punct de referin util este punctul sigma,punctul n care meridianul observatorului cerescintersecteaz ecuatorul ceresc. Ascensiuneadreapta punctului de sigma este egal cu timpulobservatorului sideral local. distana unghiular dinpunctul de sigma la cercul unei stele or

senumete unghiul su or; este egal cuascensiunea stelei dreptul de minus timp siderallocal. Deoarece echinociul de primvar nu este

ntotdeauna vizibil n cerul de noapte (maiales nprimvar), n timp ce punctul de sigma este

ntotdeauna vizibil, unghiul de or este folositnlocalizarea de fapt, un organism n cer.
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HOUR CIRCLE


HOUR CIRCLE IN ASTRONOMY, A SECONDARY AXIS IN THE EQUATORIAL COORDINATESYSTEM

The hour circle of a celestial body is the greatcircle on thecelestial sphere that passesthrough both the body and the north celestialpole. A star's hour circle is used in determiningitsright ascension and declination .

Cercul or a unui corp ceresc este cerc marepesfera cereasca care trece att prin corp si polulnord ceresc. O stea cercul lui or este folosit ndeterminarea ascensiune dreapt i declinaia.
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RIGHT ASCENSION


RIGHT ASCENSION IN ASTRONOMY, ONE OF THE COORDINATES IN THE EQUATORIALCOORDINATE SYSTEM

The right ascension of a celestial body is the angulardistance measured eastward from the vernal equinoxalong the celestial equator to its intersection with the

body's hour circle.

ascensia dreapt a unui corpceresc este distanaunghiular msurat spreest de la echinociul de primvar de-a lungul

ecuatorului ceresc pn la intersecia sa cucercul organismului or.

RIGHT ASCENSION IN ASTRONOMY, ONE OF THE COORDINATES IN THE EQUATORIALCOORDINATE SYSTEM

The right ascension of a celestial body is the angulardistance measured eastward from the vernal equinoxalong the celestial equator to its intersection with the

body's hour circle.

ascensia dreapt a unui corp ceresc estedistanaunghiular msurat spre est dela echinociul de primvar de-a lungul

ecuatorului ceresc pn la intersecia sa cucercul organismului or.
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CELESTIAL SPHERE, RIGHT ASCENSION (in hours), AND DECLINAT ION (in deg. + or -)

A Celestial meridian is half a great circle (cutscelestial sphere in half) through the north and southcelestial ples, analogous to a meridian on Earth.The celestial equator and ecliptic are great circlesthat intersect at two points. The intersection near thesun's position on March 21 is called the First Point of

Aries, or the Vernal Equinox. Its meridian is theprime meridian, analogous to the Greenwichmeridian. The analog of longitude in the celestial

Un meridian Celestial este o jumtatede cercmare (reduceri sfera cereasca n jumtate),prinnord i sud poli cereti, analog la un meridianpePamant. ecuatorul ceresc si ecliptica sunt cercurimari care se intersecteaz

n dou puncte.Intersecia lng poziia soarelui pe 21 martie se numete primulpunct de Berbec, sau echinociul deprimvar. Meridian sa este primul


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sphere is RIGHT ASCENSION. Each 15 degreesalong the celestial equator is marked as ONE HOUROF RIGHT ASCENSION, increasing toward the east.

As the Earth rotates toward the east, the celestialsphere seems to move from east to west. Thismotion brings successive hours of right ascension onto your local meridian. Your local sidereal time is theright ascension of your local meridian.

meridian,analog la meridianul Greenwich. analogdelongitudine n sfera cereasca este ascensiadreapt. Fiecare 15 grade de-alungul ecuatoruluiceresc este marcat ca o orde ascensia dreapt,crescndspre est. Ca Pamantul se

roteste sprersrit, sfera cereasca pare s semute dela estla vest. Aceast micare aduce or succesive aleascensiunii chiar pe la meridianulde local. Timpulsideral localeste ascensiunea dreptul demeridianul local.


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DECLINATION



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DECLINATION IN ASTRONOMY, ONE OF THE COORDINATES IN THEEQUATORIAL COORDINATESYSTEM

The declination of a celestial body is its angulardistance north or south of the celestial equatormeasured along its hour circle .

DECLINATION IN ASTRONOMY, ONE OF THE COORDINATES IN THE EQUATORIAL COORDINATESYSTEM

The declination of a celestial body is its angulardistance north or south of the celestial equatormeasured along its hour circle .
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CELESTIAL SPHERE


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A Celest ial meridian is a great circle (cuts

celestial sphere in half) through the north andsouth celestial poles, analogous to a rneridianon Earth.

The DECLINATION of a star is the angle "d"from the celestial equator to the starrneasured along the celestial meridian of thestar.


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Use diagram only to obtain numencal valuesAPPROXIMATE MEAN DECLINATION 1975FOR CENTRE OF MAP Annual changedecreasing 1.4'

N'utiliser le diagramme que pour obtenir lesvaleurs numriques DCLINAISONMOYENNE APPROXIMATIVE

AU CENTRE DE LA CARTE EN 1975Variation annuelle dcroissante 1.4'

ONE THOUSAND METREUNIVERSAl TRANSVERSE MERCATOR GRID

ZONE 20 QUADRILLAGE DE MILLE MTRESTRANSVERSE UNIVERSEL DE MERCATOR


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http://oceanservice.noaa.gov/education/kits/tides/media/tide04_400.gif
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CIRCLE

http://www.encyclopedia.com/topic/hour_angle.aspx

HOUR ANGLE
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HOUR ANGLE IN ASTRONOMY, A COORDINATE IN THE EQUATORIAL COORDINATE SYSTEM

The hour angle of a celestial body is the angular

distance, expressed in hours, minutes, andseconds (one hour equals 15 degrees), measuredwestward along the celestial equator from theobserver's celestial meridian to the hour circleofthe object being located. The hour angle is used inmeasuring astronomical time; localsidereal timeisequal to the hour angle of the vernal equinox.

hour angle in astronomy, a coordinate in theequatorial coordinate system .

The hour angle of a celestial body is the angular distance, expressed in hours, minutes, and seconds(one hour equals 15 degrees), measured westward along the celestial equator from the observer'scelestial meridian to the hour circle of the object being located. The hour angle is used in measuringastronomical time; local sidereal time is equal to the hour angle of the vernal equinox.

The four stars at the corners of the Great Squareof Pegasus are:

And 00 h 08 m1 + 29O 05

Peg 23 h 04 m1+ 28O 05

Peg 23 h 05 m1+ 15O 12

Peg 00 h 13 m1+ 15O

11
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It is necessary to plot the four stars, at leastapproximately, to find out which pairs form thediagonals !

Then to find the length of each diagonals,use thecosine rule:

cos S1S2=cos S1P cos S2P + sin S1P sin S2P cos P

This gives And to Peg = 20.1O

and Peg to Peg = 20.5O


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http://www.encyclopedia.com/topic/sidereal_time.aspx#1E1-sidrltim

SIDERAL TIME

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sidereal time (ST), time measured relative to thefixed stars; thus, the sidereal day is the periodduring which the earth completes one rotation onits axis so that some chosen star appears twice onthe observer's celestial meridian. Because the

earth moves in its orbit about the sun, the siderealday is about 4 min shorter than the solar day (seesolar time ). Thus, a given star will appear to rise 4min earlier each night, so that different stars arevisible at different times of the year. The localsidereal time of an observer is equal to the hourangleof the vernal equinox.

sidereal time (ST), time measured relative to thefixed stars; thus, the sidereal day is the periodduring which the earth completes one rotation onits axis so that some chosen star appears twice onthe observer's celestial meridian. Because theearth moves in its orbit about the sun, the siderealday is about 4 min shorter than the solar day (seesolar time ). Thus, a given star will appear to rise 4min earlier each night, so that different stars arevisible at different times of the year. The localsidereal time of an observer is equal to the hourangleof the vernal equinox.
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SOLAR TIME


SOLAR TIME TIME DEFINED BY THE POSITIONOF THE SUNThe solarday is the time it takes for the sun to

return to the same meridian in the sky. Local solartime is measured by a sundial . When the center of

the sun is on an observer's meridian, the observer'slocal solar time is zero hours (noon). Because theearth moves with varying speed in its orbit atdifferent times of the year and because the plane ofthe earth's equator is inclined to its orbital plane,the length of the solar day is different depending onthe time of year. It is more convenient to definetime in terms of the average of local solar time.Such time, called mean solar time, may be thoughtof as being measured relative to an imaginary sun
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(the mean sun) that lies in the earth's equatorialplane and about which the earth orbits withconstant speed. Every mean solar day is of thesame length.

The difference

solar time time defined by the position of the sun.The solardayis the time it takes for the sun toreturn to the same meridian in the sky. Local solartime is measured by a sundial . When the center ofthe sun is on an observer's meridian, the observer'slocal solar time is zero hours (noon). Because the

earth moves with varying speed in its orbit atdifferent times of the year and because the plane ofthe earth's equator is inclined to its orbital plane,the length of the solar day is different depending onthe time of year. It is more convenient to definetime in terms of the average of local solar time.Such time, called mean solar time, may be thoughtof as being measured relative to an imaginary sun(the mean sun) that lies in the earth's equatorialplane and about which the earth orbits withconstant speed. Every mean solar day is of thesame length. The difference between the localsolar time and the mean solar time at a givenlocation is known as the equation of time. Tablesused by navigators list the equation of time fordifferent times of year so that an observer cancalculate his mean solar time from his local solartime (found by determining the sun's hour angle ).Mean solar time is the basis forcivil timeandstandard time.
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day


day period oftimefor the earth to rotate once on itsaxis. The ordinary day, or solar day, is measuredrelative to the sun, being the time betweensuccessive passages of the sun over a stationaryobserver'scelestial meridian. The length of a solarday varies during the course of a year, so forpurposes of time measurement an average, ormean, solar day is used (see solar time ), equal toexactly 24 hr. The sidereal day, used byastronomers, is measured relative to the fixed starsrather than the sun (see sidereal time); it is about 4min shorter than the mean solar day. The term day

is also used to refer to that part of each 24-hrperiod during which the sun's direct rays are notblocked by the earth, this period of daylight hoursextending from sunrise to sunset; the remainingportion of the 24 hr is called night. If the plane ofthe earth's orbit about the sun coincided day periodoftime for the earth to rotate once on its axis.
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The ordinary day, or solar day, is measured relativeto the sun, being the time between successivepassages of the sun over a stationary observer'scelestial meridian . The length of a solar day variesduring the course of a year, so for purposes of timemeasurement an average, or mean, solar day isused (seesolar time ), equal to exactly 24 hr. Thesidereal day, used by astronomers, is measuredrelative to the fixed stars rather than the sun (seesidereal time); it is about 4 min shorter than themean solar day. The term day is also used to referto that part of each 24-hr period during which thesun's direct rays are not blocked by the earth, thisperiod of daylight hours extending from sunrise to

sunset; the remaining portion of the 24 hr is callednight. If the plane of the earth's orbit about the suncoincided with the plane of the equator, day andnight would each be 12 hr long everywhere on theearth all year long. However, because of the tilt ofthe earth's axis of rotation, the times of sunrise andsunset vary from day to day, with the result that inthe Northern Hemisphere there are long days andshort nights in the summer and short days and longnights in the winter. See equinox;solstice .

celestial meridian


celestial meridian vertical circle passing through thenorth celestial pole and an observer's zenith. It isan axis in thealtazimuth coordinate system .
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CELESTIAL MERIDIAN VERTICAL CIRCLEPASSING THROUGH THE NORTH CELESTIALPOLE AND AN OBSERVER'S ZENITH

It is an axis in thealtazimuth coordinate system.
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