Glossary term: Galilean Satellites
Description: The Galilean satellites are the four biggest and brightest satellites, or moons, orbiting the planet Jupiter: Io, Europa, Ganymede, and Callisto. Closest to Jupiter is Io, which has hundreds of active volcanoes. The main reason for Io's vulcanism is that Io is getting "kneaded" by the tidal effects of Jupiter's gravity. Europa's entire surface is covered with ice. Under the ice is believed to be an ocean of liquid water which is one of the best candidates for harboring life outside of Earth in the Solar System. Ganymede is the largest moon in our Solar System, and also the moon with the largest mass, at twice the mass of Earth's Moon. Callisto is almost exactly the same size as the planet Mercury, but has only about one third of Mercury's mass.
The four Galilean moons were discovered by Galileo Galilei in 1610, as one of several discoveries in Galilei's pioneering campaign of using a telescope for astronomical observations. Galilei was able to document that, over time, the four moons orbit Jupiter like a miniature solar system. This was a clear demonstration that astronomical objects can orbit a center other than the Earth; this observation played an important role in the shifting of the scientific consensus from the Earth-centered (geocentric) to the Sun-centered (heliocentric) view of the Solar System.
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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".
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In Other Languages
- Arabic: أقمار غاليليو
- German: Galileische Satelliten
- French: Satellites galiléens
- Italian: Satelliti galileiani
- Japanese: ガリレオ衛星 (external link)
- Simplified Chinese: 伽利略卫星
- Traditional Chinese: 伽利略衛星
Related Media
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
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, 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
Io
Caption: NASA's Galileo spacecraft acquired its highest resolution images of Jupiter's moon Io on 3 July 1999. Io is one of the four Jovian moons discovered by Galileo Galilei in 1610. Io's colours are witness to its extensive volcanic activity as they stem from sulphuric compounds. Tidal forces from Jupiter and the neighbouring moons are the cause for Io's volcanism.
Credit: NASA/JPL/University of Arizona
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License: PD Public Domain icons
Europa
Caption: This image taken by NASA's Galileo space probe in September 1996 shows Jupiter's ice-covered satellite, Europa, in approximate natural color. Long, dark lines are fractures in the crust, some of which are more than 3,000 kilometers (1,850 miles) long. The bright feature containing a central dark spot in the lower third of the image is a young impact crater some 50 kilometers (31 miles) in diameter. Europa is about 3,160 kilometers (1,950 miles) in diameter, or about the size of Earth's moon.
Credit: NASA/JPL/DLR
License: PD Public Domain icons
