Glossarbegriffe: Umlaufbahn

Also known as Orbit

Description: Eine Umlaufbahn beschreibt die Bewegung eines Objekts in einem System um den Massenschwerpunkt dieses Systems, der durch die gegenseitige Anziehungskraft der Objekte im System entsteht. In Systemen wie dem Sonnensystem, in dem der Zentralkörper viel massereicher als die anderen Objekte ist, liegt dieser Massenschwerpunkt innerhalb oder in der Nähe des massereichsten Objekts. Im Fall des Sonnensystems ist das die Sonne. In einem Doppelsternsystem liegt der Massenschwerpunkt, um den sich die Sterne bewegen, oft zwischen den beiden Sternen.

Umlaufbahnen haben in der Regel eine elliptische Form. Dabei liegt der Massenschwerpunkt des Systems in einem der zwei Brennpunkte der Ellipse. Die Größe und Form der Umlaufbahn wird durch die Hauptachse und die Exzentrizität der Ellipse bestimmt. Die Exzentrizität einer Ellipse beschreibt vereinfacht gesagt, wie sehr die Ellipse von der Kreisform abweicht: Je größer die Exzentrizität, desto weniger kreisförmig ist eine Umlaufbahn. Die meisten Planeten im Sonnensystem bewegen sich auf fast kreisförmigen Umlaufbahnen mit einer Bahnexzentrizität sehr nahe bei Null, z. B. Venus (0,007), Erde (0,017), mit Ausnahme von Merkur (0,206) und dem Zwergplaneten Pluto (0,244).

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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

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The planet Jupiter with the two of the four Galilean moons (visible as bright dots) orbiting it.

Jupiter's Rotation, by Vishal Sharma, India

Bildunterschriften: Third place in the 2021 IAU OAE Astrophotography Contest, category Galilean moons: Jupiter’s Rotation, by Vishal Sharma, India. This time-lapse beautifully shows the rotation of Jupiter and the passage of two Galilean moons on the right side of the frame. Jupiter completes one rotation in just under 10 hours and we see as the Great Red Spot makes its way from left to right. The two moons travel a noticeable fraction of their orbit in this short time. This image was taken in 2020 in the North of India.
Bildnachweis: Vishal Sharma/IAU OAE

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

The planet Jupiter, seen here as a bright disk, is orbited by the four Galilean moons, seen here as bright dots

Jupiter Moons Movie2, by Nicolas Hurez, Paul-Antoine Matrangolo, and Carl Pennypacker, United States of America

Bildunterschriften: Second place in the 2021 IAU OAE Astrophotography Contest, category Galilean moons. This sequence shows the orbit of the four Galilean moons around the planet Jupiter. Almost two entire orbits of the innermost moon, Io, can be seen, with the other moons (Europa and Ganymede, but in particular Callisto) being further away, orbiting noticeably slower. The images were obtained in 2018 with the Las Cumbres Global Observatory at different locations on Earth, allowing a continuous sequence of images over approximately half a week without gaps during the day. With clear skies and over the course of several nights, the motion of the Galilean moons can also be observed with binoculars (ideally steady your elbows on a surface).
Bildnachweis: Nicolas Hurez, Paul-Antoine Matrangolo and Carl Pennypacker/IAU OAE

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

Jupiter with coloured horizontal bands of clouds. The shadow of the moon Io is seen as a dark circle in the top left

Jupiter, Io and its shadow, by Ralf Burkart, Germany

Bildunterschriften: First place in the 2021 IAU OAE Astrophotography Contest, category Galilean moons. This time-lapse of Jupiter taken in 2017 from Germany beautifully illustrates the transit of one of the Galilean moons, Io, in front of Jupiter. As this is simply a moon casting a shadow on a planet it is equivalent to a lunar eclipse on Earth observed from further away. While the shadow of the moon is clearly visible from the beginning, it might be difficult to spot the moon itself against the background of the beautiful atmospheric bands of Jupiter the first time the video is seen. Watching it repeatedly allows appreciating the rapid motion and rotation in this fantastic observation.
Bildnachweis: Ralf Burkart/IAU OAE

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

beta Pictoris b moves from bottom right towards the center of the image, reappearing 22 months later on the top left

The orbit of beta Pictoris b

Bildunterschriften: This series of images shows the orbital motion of the extrasolar planet (exoplanet) beta Pictoris b. The planet is the bright dot in each image. The planet's host star is hidden behind the black circle in the middle of each image. This is done to remove the much brighter host star which would otherwise drown out the light from the planet. The planet's orbit is viewed edge-on. Seeing the orbit from this perspective makes it look like the planet moves along a straight line. Between February 2015 and November 2016 beta Pictoris b appears to move closer and closer to its host star. The planet then moved so close to the star that it was not seen for almost two years, after which it reappeared on the other side of the star.
Bildnachweis: ESO/Lagrange/SPHERE consortium credit link

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

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