In recognition of Jupiter reaching opposition this month, the night sky features the giant planet. Jupiter may be the amateur astronomers favorite planet to observe, due to the fact that so many interesting features can be seen in the outer atmosphere of this massive planet. It contains the lion's share of the mass of the solar system outside of the Sun, 2.5 times all other planets combined. It is over 5 times further from the Sun than planet Earth. It spends 10 months in the sky every 13 months. It takes the giant planet 11.86 years to return to the same spot in the sky, so that it spends about a year in each constellation of the zodiac. When it reaches opposition this year, the disk will appear over 44" in diameter.

The four largest moons of Jupiter are called Galilean moons because they were discovered by Galileo with the small telescope he first pointed at the heavens. It became obvious to Galileo that the small stars he observed by the planet were actually satellites like our own moon. Over the course of an evening, he could tell that they orbited the giant planet. Eager to share his discovery, he published Starry Messenger. But in Galileo's day, the pope decreed that no object in the heaven orbited any other object but God's perfect creation, mother Earth. The four Gallilean moons may be observed at opposition by a star gazer with a steady hand through a pair of binoculars. Slight surface color variations have been reported by observers. The four giant moons are not the only satellites of Jupiter, to-date we have discovered that the giant planet has 67 companion satellites

The planet is made of hydrogen gas and does not have a surface like Earth or Venus. Instead, observers on Earth see the only the outer layers of clouds in the planet's atmosphere. The planet rotates in an incredible 9 hours and 51 minutes at the equator, with speeds at the poles being somewhat less. This means that features rotate very quickly across the surface. Serious observers who make sketches plan on drawing the surface features in 10 minutes. Astronomers track features by imagining a central line stretching from pole to pole, called the central meridian. As features cross this line, they are said to be transiting the meridian. And what a variety of features can be seen! Prominent bands mark the North and South Equatorial Belts (NEB and SEB respectively) and these can be seen in even small telescopes. Unless of course Jupiter pulls one of its usual, unusual tricks and one of these belts disappears temporarily. Other features appear as white, red or bluish ovals. Every night the disk of our giant neighbor holds its own surprises! One feature that has been considered permanent is a great storm that has been called the Great Red Spot (GRS) It was first identified in 1664 and was estimated to be 36,000 km in length in 1880, but today it covers 15,000 km. There is speculation that the GRS is not as permanent as once thought and may dissipate, especially as the dark red color turned to an orangish pink. Observations this year, however, finds the GRS once again becoming a bright cherry color. Always mysterious, Jupiter can still surprise us.

--See You Under the Stars!
Astra for Astra's Almanac

The star chart above was generated by Stellarium, a free open source planetarium program. This image was created by Dawn Jenkins, using Stellarium and a graphic editing program to format the image for this web page. The second image above was created by Thuvan Dihn and licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license. It has been modified as allowed under the license by Dawn Jenkins and may be distributed as stated on the Wikimedia Commons website.