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Glossary term: 赤經(RA)

Description: 赤經是赤道坐標系中的兩個坐標之一(另一個是赤緯),天文學家用以定義天體在天空中的位置。從地球看去,天空中的所有不同位置共同構成了一個以地球為中心的遙遠球面,稱為天球。天空中地球赤道正上方的點構成了天球上的天赤道,地理北極正上方的點就是北天極,而地理南極正上方的點就是南天極。就像地理學家在地球表面定義地理經緯度一樣,天文學家也可以在天球上定義經緯度。但如果我們將天體的經度坐標選擇為其正下方的點在地球表面上對應的地理經度值,那麼恆星的坐標值就會隨著地球的自轉而改變。為避免這一點,赤道坐標系的赤經是相對於一條不隨地球自轉改變,而相對於恆星固定不變的“子午線”來測量的。這條子午線的地位與地球上的格林尼治子午線類似。這條子午線與天赤道的交點定義為太陽從南天球運動到北半球的視軌跡與天赤道相交的確切位置。由此定義的經度稱為赤經,其數值自西向東增加。如果在地球上向天赤道看去,則隨著地球的自轉,所有的赤經值都會在約24小時內依次飛掠而過。因此,赤經通常用時間值來表示,用24小時來代替圓周的360度。赤緯是第二個赤道坐標,與地理緯度相對應。地球自轉軸的輕微擺動被稱為歲差,它使赤道坐標系——恆星和其他天體的赤經和赤緯——隨時間發生變化,但這種變化是非常輕微緩慢的。

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Term and definition status: The original definition of this term in English have been approved by a research astronomer and a teacher
The translation of this term and its definition is still awaiting approval

This is an automated transliteration of the simplified Chinese translation of this term

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Related Diagrams


The bright stars in Andromeda form a Y-shape. Pegasus to the lower right. In the center is M31, marked with a red ellipse.

Andromeda Constellation Map

Caption: The constellation Andromeda showing the bright stars and surrounding constellations. Andromeda is surrounded by (going clockwise from the top) Cassiopeia, Lacerta, Pegasus, Pisces, Aries, Triangulum and Perseus. The brightest star in Andromeda (Alpheratz) is in the lower part of the constellation. Together with three stars in Pegasus it forms the asterism known as the "Great Square of Pegasus". The next two bright stars in the constellation (Mirach and Almach) form a line extending north-east from Alpheratz. Andromeda is a northern constellation and is most visible in the evenings in the Northern Hemisphere autumn. It is visible from all of the Northern Hemisphere and most temperate regions of the Southern Hemisphere but is not visible from Antarctic and Subantarctic regions. The most famous object in Andromeda, the Andromeda Galaxy is marked here with a red ellipse and its Messier catalog number M31. The yellow circle on the left marks the position of the open cluster NGC 752 and the green circle on the right marks NGC 7662 (the blue snowball nebula), a planetary nebula. The y-axis of this diagram is in degrees of declination with north as up and the x-axis is in hours of right ascension with east to the left. The sizes of the stars marked here relate to the star's apparent magnitude, a measure of its apparent brightness. The larger dots represent brighter stars. The Greek letters mark the brightest stars in the constellation. These are ranked by brightness with the brightest star being labeled alpha, the second brightest beta, etc., although this ordering is not always followed exactly. The dotted boundary lines mark the IAU's boundaries of the constellations and the solid green lines mark one of the common forms used to represent the figures of the constellations. Neither the constellation boundaries, nor the lines joining the stars appear on the sky.
Credit: Adapted by the IAU Office of Astronomy for Education from the original by IAU/Sky & Telescope

License: CC-BY-4.0 Creative Commons 姓名標示 4.0 國際 (CC BY 4.0) icons


The four bright stars of Crux form a kite shape with the long axis pointing vertically

Crux Constellation Map

Caption: The constellation Crux (commonly known as the Southern Cross or Crux Australis) showing its bright stars and surrounding constellations. The Southern Cross is surrounding by (going clockwise from the top) Centaurus, Carina and Musca. The brightest star is alpha Crucis which appears at the bottom of the constellation's famous kite shape. The Southern Cross is visible from southern and equatorial regions of the world. In more southerly parts of the world it is circumpolar so is always above the horizon. In other parts of the southern hemisphere and in equatorial regions it is most visible in the evenings in the southern hemisphere autumn. The yellow circles show the locations of two open clusters, NGC 4755 (known as the Jewel Box) and NGC 4609. The line joining gamma and alpha Crucis (the third and first brightest stars in the Southern Cross) points in the approximate direction of the South Celestial Pole. This has led to the Southern Cross playing an important role in celestial navigation, allowing navigators from different astronomical traditions to find their bearings. The y-axis of this diagram is in degrees of declination with north as up and the x-axis is in hours of right ascension with east to the left. The sizes of the stars marked here relate to the star's apparent magnitude, a measure of its apparent brightness. The larger dots represent brighter stars. The Greek letters mark the brightest stars in the constellation. These are ranked by brightness with the brightest star being labeled alpha, the second brightest beta, etc., although this ordering is not always followed exactly. The dotted boundary lines mark the IAU's boundaries of the constellations and the solid green lines mark one of the common forms used to represent the figures of the constellations. Neither the constellation boundaries, nor the lines joining the stars appear on the sky.
Credit: Adapted by the IAU Office of Astronomy for Education from the original by IAU/Sky & Telescope.

License: CC-BY-4.0 Creative Commons 姓名標示 4.0 國際 (CC BY 4.0) icons


獵戶座呈沙漏狀,兩串恆星分別向東北和西北方延伸

獵戶座星圖

Caption: 獵戶座及其明亮的恆星和周圍的星座。獵戶座被金牛座、波江座、天兔座、麒麟座和雙子座環繞(從上往下順時針依次排列)。獵戶座最亮的恆星參宿四和參宿七分別位於星座的北端(圖中上部)和南端(圖中下部),中間是著名的三星 "腰帶"。 獵戶座橫跨天赤道,因此在一年中的某些時候,從地球上的任何地方都能看到它。在世界上最北極或最南極地區,可能看不到獵戶座的某些部分。獵戶座在北半球冬季和南半球夏季的晚上最為明顯。獵戶座上方的藍線標誌著黃道,即太陽在一年中劃過天空的軌跡。太陽從不經過獵戶座,但人們偶爾會在獵戶座發現太陽系的其他行星和月球。 在獵戶座腰帶的正南方,有兩個梅西葉天體M42(獵戶座星雲)和M43,用綠色方框標出。這些星雲和M78(腰帶左邊的綠色方框)是巨大的獵戶座分子雲團的一部分。它覆蓋了獵戶座的大部分區域,包括這些分子雲正在坍縮形成新星的區域。 該圖的 y 軸表示赤緯,單位為度數,以北為上,x 軸表示赤經,單位為小時,以東為左。這裡標注的恆星大小與恆星的視星等有關,視星等是衡量恆星視亮度的標準。較大的點代表較亮的恆星。希臘字母表示星座中的亮星。這些恆星按亮度排序,最亮的恆星被標記為α星,第二亮的為β星,等等,不過並不總是完全遵循這種排序。圍繞著參宿四的圓圈表示它是一顆變星。虛線是國際天文學聯合會劃定的星座邊界,綠色實線是表示星座的常用形式之一。星座邊界和連接恆星的線條都不會出現在實際天空中。
Credit: 由國際天文學聯合會天文教育辦公室根據國際天文學聯合會/《天空與望遠鏡》的原文改編

License: CC-BY-4.0 Creative Commons 姓名標示 4.0 國際 (CC BY 4.0) icons


Libra appears as a triangle pointing north (up) with two lines hanging down. It is bisected by the ecliptic running ESE-WNW

Libra Constellation Map

Caption: The constellation Libra along with its bright stars and surrounding constellations. Libra is surrounded by (going clockwise from the top) Serpens Caput, Virgo, Hydra, Centaurus, Lupus, Scorpius and Ophiuchus. Libra lies on the ecliptic (shown here as a blue line), this is the path the Sun appears to take across the sky over the course of a year. The Sun is in Libra from late October to late November. The other planets of the Solar System can often be found in Libra. Libra lies just south of the celestial equator and is thus visible at some time in all but the most arctic regions. Libra is most visible in the evenings in the northern hemisphere late spring/early summer and southern hemisphere late autumn/early winter. The y-axis of this diagram is in degrees of declination with north as up and the x-axis is in hours of right ascension with east to the left. The sizes of the stars marked here relate to the star's apparent magnitude, a measure of its apparent brightness. The larger dots represent brighter stars. The Greek letters mark the brightest stars in the constellation. These are ranked by brightness with the brightest star being labeled alpha, the second brightest beta, etc., although this ordering is not always followed exactly. The dotted boundary lines mark the IAU's boundaries of the constellations and the solid green lines mark one of the common forms used to represent the figures of the constellations. Neither the constellation boundaries, nor the line marking the ecliptic, nor the lines joining the stars appear on the sky.
Credit: Adapted by the IAU Office of Astronomy for Education from the original by IAU/Sky & Telescope

License: CC-BY-4.0 Creative Commons 姓名標示 4.0 國際 (CC BY 4.0) icons


Scorpius appears as a letter T joined to a letter J. The ecliptic runs ESE to WNW and clips one arm of the T

Scorpius Constellation Map

Caption: The constellation Scorpius (often commonly called Scorpio) along with its bright stars and surrounding constellations. Scorpius is surrounded by (going clockwise from the top) Ophiuchus, Serpens Caput, Libra, Lupus, Norma, Ara, Corona Australis and Sagittarius. Scorpius’s brightest star Antares appears in the heart of the constellation with the famous tail of Scoprius in the south-east (lower left). Scorpius lies on the ecliptic (shown here as a blue line), this is the path the Sun appears to take across the sky over the course of a year. The Sun only spends a short amount of time in late November in Scorpius. The other planets of the Solar System can often be found in Scorpius. Scorpius lies south of the celestial equator. The whole constellation is not visible from the most arctic regions of the world with parts of Scorpius obscured for observers in northern parts of Asia, Europe and North America. Scorpius is most visible in the evenings in the northern hemisphere summer and southern hemisphere winter. The yellow circles mark the positions of the open clusters M6, M7 & NGC 6231 while the yellow circles with plus signs superimposed on them mark the globular clusters M4 and M80. The y-axis of this diagram is in degrees of declination with north as up and the x-axis is in hours of right ascension with east to the left. The sizes of the stars marked here relate to the star's apparent magnitude, a measure of its apparent brightness. The larger dots represent brighter stars. The Greek letters mark the brightest stars in the constellation. These are ranked by brightness with the brightest star being labeled alpha, the second brightest beta, etc., although this ordering is not always followed exactly. The circle around Antares indicates that it is a variable star. The dotted boundary lines mark the IAU's boundaries of the constellations and the solid green lines mark one of the common forms used to represent the figures of the constellations. The blue line marks the ecliptic, the path the Sun appears to travel across the sky over the course of one year. Neither the constellation boundaries, nor the line marking the ecliptic, nor the lines joining the stars appear on the sky.
Credit: Adapted by the IAU Office of Astronomy for Education from the original by IAU/Sky & Telescope

License: CC-BY-4.0 Creative Commons 姓名標示 4.0 國際 (CC BY 4.0) icons