Jupiter is the largest planet in our solar system. It’s so large that more than 1,300 Earths could fit inside Jupiter. It’s easy to see on a clear night during the times of year when it’s visible in the night sky. Jupiter is known as a Gas Giant because its enormous atmosphere is made of 75% hydrogen and 25% helium. Even the core of Jupiter, deep under its enormous gas atmosphere, is made up of mostly condensed hydrogen and helium. The dense liquid hydrogen core behaves like a liquid metal, and it’s fast rotation is responsible for generating the strong magnetic field that surrounds Jupiter.
Jupiter has a chaotic, turbulent atmosphere that is heavily banded with clouds. It’s easy to see these clouds in a moderately sized telescope. The atmosphere is home to a gigantic cyclonic storm called the Great Red Spot. This storm was observed over 350 years ago, and has continued to circulate in Jupiter’s atmosphere since that time. When the first measurements were made in the late 1800s, the storm was found to be 25,000 miles across. The storm is now slowly shrinking over time, and scientists believe that one day it will disappear.
Jupiter has four large moons, Ganymede, Io, Europa, and Callisto. They're called the Galilean moons because the astronomer Galileo described observing and studying them with one of the first telescopes in the year 1610. Galileo observed these four moons changing positions over time, and determined that these moons were orbiting Jupiter. This observation led to the understanding that the Earth is not the center of the Universe, and not all objects in the night sky orbit around the Earth.
Among the Galilean moons, Ganymede is the largest. Ganymede is the largest moon in our solar system. It’s larger than the planet Mercury. If Ganymede was orbiting the Sun instead of orbiting Jupiter it would be classified as a planet.
With the addition of larger telescopes and the use of photographic equipment, more and more moons orbiting Jupiter have been discovered. To date there are 95 moons orbiting Jupiter that are officially recognized by the International Astronomical Union, but that doesn’t count hundreds more smaller moons that have only briefly been observed. Jupiter has so many moons that not all of them have been named. The region of space occupied by Jupiter and all of its moons is known as the Jovian system.
Even though Jupiter is about 480 million miles from Earth, it’s easy to see it in the night sky with just a pair of binoculars. On any given night, some or all of the Galilean moons will also be visible. Observing Jupiter with a pair of binoculars is a great exercise for young astronomers just beginning to learn the night sky.
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