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Glossarbegriffe: Rektaszension (RA)

Description: Die Rektaszension ist eine der beiden Koordinaten im äquatorialen Koordinatensystem (die andere ist die Deklination), die Astronomen verwenden, um die Positionen von Himmelsobjekten am Himmel zu bestimmen. Von der Erde aus gesehen bilden die verschiedenen Positionen am Himmel zusammen eine weit entfernte Kugel mit der Erde im Zentrum: die Himmelskugel. Die Punkte am Himmel, die sich direkt über dem Äquator der Erde befinden, bilden auf dieser Himmelskugel den Himmelsäquator. Der Punkt direkt über dem geografischen Nordpol der Erde ist der himmlische Nordpol. Der Punkt über dem Südpol der Erde ist der himmlische Südpol. Genauso wie Geographen die geographische Länge und Breite auf der Erdoberfläche definieren, kann man die Länge und Breite auf der Himmelskugel definieren. Würden wir aber den Längengrad eines Himmelsobjekts als den des Ortes auf der Erde direkt darunter wählen, würde sich dieser Längengrad eines Sterns im Laufe der Zeit ändern, wenn sich die Erde dreht. Stattdessen messen die Äquatorialkoordinaten die Rektaszension als eine Form der himmlischen Länge relativ zu einem "Nullmeridian" am Himmel, der sich nicht mit der Erde dreht, sondern in Bezug auf die Fixsterne feststeht. Dieser himmlische Nullmeridian ist mit dem auf der Erde gebräuchlichen Nullmeridian von Greenwich vergleichbar. Er wird durch seinen Schnittpunkt mit dem Himmelsäquator definiert: genau an dem Punkt, an dem die scheinbare Bahn der Sonne den Himmelsäquator von der südlichen zur nördlichen Himmelshalbkugel kreuzt. Dieser himmlische Längengrad wird als Rektaszension bezeichnet. Ihr Wert nimmt nach Osten hin zu. Blickt man in Richtung des Himmelsäquators, dann ziehen die Längengrade im Laufe von (etwa) 24 Stunden an einem Beobachter vorbei. Aus diesem Grund wird die Rektaszension in der Regel im Zeitmaß angegeben, wobei 24 Stunden den vollen 360 Grad entsprechen. Die Deklination, die zweite Polarkoordinate, entspricht der geografischen Breite. Durch eine leichte Schwankung der Erdrotationsachse, die als Präzession bezeichnet wird, ändert sich das äquatoriale Koordinatensystem und damit auch die Rektaszension und Deklination von Sternen und anderen Himmelsobjekten im Laufe der Zeit, allerdings nur sehr geringfügig und sehr langsam.

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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

Bildunterschriften: 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.
Bildnachweis: Adapted by the IAU Office of Astronomy for Education from the original by IAU/Sky & Telescope

License: CC-BY-4.0 Creative Commons Namensnennung 4.0 International (CC BY 4.0) icons


Orion appears as an hourglass-shaped pattern with two strings of stars extending northeast and northwest

Orion Constellation Map

Bildunterschriften: The constellation Orion along with its bright stars and surrounding constellations. Orion is surrounded by (going clockwise from the top) Taurus, Eridanus, Lepus, Monoceros and Gemini. Orion’s brightest stars Betelgeuse and Rigel appear at the northern (upper on this diagram) and southern (lower) end of the constellation respectively with the famous three star “belt” in the middle. Orion 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. Orion is most visible in the evenings in the northern hemisphere winter and southern hemisphere summer. The blue line above Orion marks the ecliptic, the path the Sun appears to travel across the sky over the course of a year. The Sun never passes through Orion, but one can occasionally find the other planets of the Solar System and the Moon in Orion. Just south of Orion’s belt lie two Messier objects M42 (the Orion nebula) and M43, marked by green squares. These nebulae along with M78 (here the green square to the left of the belt) are part of the huge Orion Molecular Cloud Complex. This covers most of the constellation and includes regions where these molecular clouds are collapsing to form young starts. 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 circle around Betelgeuse 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. Neither the constellation boundaries, nor the lines joining the stars appear on the sky.
Bildnachweis: Adapted by the IAU Office of Astronomy for Education from the original by IAU/Sky & Telescope

License: CC-BY-4.0 Creative Commons Namensnennung 4.0 International (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

Bildunterschriften: 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.
Bildnachweis: Adapted by the IAU Office of Astronomy for Education from the original by IAU/Sky & Telescope

License: CC-BY-4.0 Creative Commons Namensnennung 4.0 International (CC BY 4.0) icons

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