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Glossary term: 赤经(RA)

Description: 赤经是赤道坐标系中的两个坐标之一(另一个是赤纬),天文学家用以定义天体在天空中的位置。从地球看去,天空中的所有不同位置共同构成了一个以地球为中心的遥远球面,称为天球。天空中地球赤道正上方的点构成了天球上的天赤道,地理北极正上方的点就是北天极,而地理南极正上方的点就是南天极。就像地理学家在地球表面定义地理经纬度一样,天文学家也可以在天球上定义经纬度。但如果我们将天体的经度坐标选择为其正下方的点在地球表面上对应的地理经度值,那么恒星的坐标值就会随着地球的自转而改变。为避免这一点,赤道坐标系的赤经是相对于一条不随地球自转改变,而相对于恒星固定不变的“子午线”来测量的。这条子午线的地位与地球上的格林尼治子午线类似。这条子午线与天赤道的交点定义为太阳从南天球运动到北半球的视轨迹与天赤道相交的确切位置。由此定义的经度称为赤经,其数值自西向东增加。如果在地球上向天赤道看去,则随着地球的自转,所有的赤经值都会在约24小时内依次飞掠而过。因此,赤经通常用时间值来表示,用24小时来代替圆周的360度。赤纬是第二个赤道坐标,与地理纬度相对应。地球自转轴的轻微摆动被称为岁差,它使赤道坐标系——恒星和其他天体的赤经和赤纬——随时间发生变化,但这种变化是非常轻微缓慢的。

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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

The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".

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 and 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 署名 4.0 国际 (CC BY 4.0) icons


猎户座呈沙漏状,两串恒星分别向东北和西北方延伸

猎户座星图

Caption: 猎户座及其明亮的恒星和周围的星座。猎户座被金牛座、波江座、天兔座、麒麟座和双子座环绕(从上往下顺时针依次排列)。猎户座最亮的恒星参宿四和参宿七分别位于星座的北端(图中上部)和南端(图中下部),中间是著名的三星 "腰带"。 猎户座横跨天赤道,因此在一年中的某些时候,从地球上的任何地方都能看到它。在世界上最北极或最南极地区,可能看不到猎户座的某些部分。猎户座在北半球冬季和南半球夏季的晚上最为明显。猎户座上方的蓝线标志着黄道,即太阳在一年中划过天空的轨迹。太阳从不经过猎户座,但人们偶尔会在猎户座发现太阳系的其他行星和月球。 在猎户座腰带的正南方,有两个梅西叶天体M42(猎户座星云)和M43,用绿色方框标出。这些星云和M78(腰带左边的绿色方框)是巨大的猎户座分子云团的一部分。它覆盖了猎户座的大部分区域,包括这些分子云正在坍缩形成新星的区域。 该图的 y 轴表示赤纬,单位为度数,以北为上,x 轴表示赤经,单位为小时,以东为左。这里标注的恒星大小与恒星的视星等有关,视星等是衡量恒星视亮度的标准。较大的点代表较亮的恒星。希腊字母表示星座中的亮星。这些恒星按亮度排序,最亮的恒星被标记为α星,第二亮的为β星,等等,不过并不总是完全遵循这种排序。围绕着参宿四的圆圈表示它是一颗变星。虚线是国际天文学联合会划定的星座边界,绿色实线是表示星座的常用形式之一。星座边界和连接恒星的线条都不会出现在实际天空中。
Credit: 由国际天文学联合会天文教育办公室根据国际天文学联合会/《天空与望远镜》的原文改编

License: CC-BY-4.0 Creative Commons 署名 4.0 国际 (CC BY 4.0) icons


Pisces appears as a SW-pointing v-shape with loops at the end of each line. The ecliptic runs WSW to ENE through Pisces.

Pisces Constellation Map

Caption: The constellation Pisces along with its bright stars and surrounding constellations. Pisces is surrounded by (going clockwise from the top) Andromeda, Pegasus, Aquarius, Cetus, Aries and Triangulum. Pisces 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 Pisces from mid March to mid April. Thus the Sun is in Pisces at the March equinox. At this point the ecliptic crosses the celestial equator. The Sun’s location at the spring equinox is used to set the zero point of the Right Ascension positional coordinate. The other planets of the Solar System can often be found in Pisces. Pisces spans the celestial equator and is thus visible at some time in the year from all of planet Earth. In the most arctic or antarctic regions of the world, some parts of the constellation may not be visible. Pisces is most visible in the evenings in the northern hemisphere autumn and southern hemisphere spring The grand design spiral galaxy M74 is marked on this diagram with a small red circle. The y-axis of this diagram is in degrees of declination and 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 署名 4.0 国际 (CC BY 4.0) icons