By Jim Haklar
If you look high in the south at sunset this time of year, you’ll see a bright “star” that’s really not a star at all. You’ll be looking at Jupiter – the largest planet in the solar system.
Jupiter is the 5th planet from the sun and is called a “gas giant” planet because it doesn’t have a solid surface (like the Earth). According to NASA, Jupiter’s atmosphere is made up of mostly hydrogen and helium, similar to the Sun, and if Jupiter were more massive it would have become a star. There is a huge storm in Jupiter’s atmosphere called the Great Red Spot. The Great Red Spot has been around for hundreds of years and is so big that the Earth can fit inside it!
Jupiter has over 50 moons and four of them are bright enough to be seen with binoculars. These four moons were discovered by Galileo in 1610 and are called the Galilean moons. The Galilean moons are named Io, Europa, Ganymede, and Callisto. Io, the Galilean moon closest to Jupiter, has volcanos that are active. The surface of Europa is covered in ice and may have an ocean of water underneath. Ganymede is larger than the planet Mercury; it’s the largest moon in the solar system. Callisto’s surface has many craters and is thought to be very old, from the time when the solar system was young.
If you look at Jupiter over several hours you can actually see the moons moving. In a small telescope you can sometimes see the shadow of the moons cross over the planet’s atmosphere. This is called a shadow transit.
Right now Jupiter is easy to see right after sunset. But over the next few months it will start appearing lower at sunset until it passes behind the Sun in August. Then it will once again become visible, but in the morning before sunrise. So try and catch the “King of the Planets” before it’s too late!
About the Author: Jim is an environmental engineer at EPA’s Edison, New Jersey Environmental Center. In his 30 years with the Agency he has worked in a variety of programs including Superfund, Water Management, Public Affairs, and Toxic Substances. He has been an amateur astronomer since he was a teenager, and can often be found after work in the back of the Edison facility with one of his telescopes.