Glossary term: Orbit

Description: An orbit is the path of a moving object in a system around the center of mass of that system, caused by the mutual gravitational force between the objects in the system. For systems such as the Solar System, where the central body is much more massive than the other bodies, this center of mass lies inside or close to the most massive object (in the case of the Solar System, the Sun). In a binary star system the center of mass the stars orbit often lies between the two stars.

Orbits are typically elliptical in shape with the center of mass of the system lying at one focus of the ellipse. The size and shape of the orbit is defined by the semi-major axis and the eccentricity of the ellipse. More eccentric orbits have higher ellipticities. Most planets in the Solar System have orbital eccentricities very close to zero, for example, Venus (0.007) and Earth (0.017). The exceptions are Mercury (0.206) and the dwarf planet Pluto (0.244).

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Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

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 Media

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

Jupiter's Rotation, by Vishal Sharma, India

Caption: 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.
Credit: Vishal Sharma/IAU OAE

License: CC-BY-4.0 Creative Commons Attribution 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

Caption: 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).
Credit: Nicolas Hurez, Paul-Antoine Matrangolo and Carl Pennypacker/IAU OAE

License: CC-BY-4.0 Creative Commons Attribution 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

Caption: 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.
Credit: Ralf Burkart/IAU OAE

License: CC-BY-4.0 Creative Commons Attribution 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

Caption: 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.
Credit: ESO/Lagrange/SPHERE consortium credit link

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

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