Throughout the evenings of January until the middle of April, the king of the planets in our solar system, Jupiter, will reign over the night skies. Looking overhead and slightly to the south and east in the early evening, the brightest white “star” in sky will be the largest of our planets.
Using a pair of binoculars, looking at Jupiter will easily reveal the
four enormous moons orbiting the planet. Observing these night after night, or even over a few hours, the moons will be seen to change position, sometimes disappearing behind the shadow of the planet, or emerging from behind the limb.
Through a telescope, in addition to the moons, the cloud tops of the planet can be seen in bands completely circling the disk. With larger telescopes within the cloud bands we can see more detail – swirls of storms, including the enormous storm known for the past couple centuries as the “Great Red Spot”. As the four large moons move in front of the planet, we can see their shadows as small black dots move over the cloud tops. After the Earth’s moon, Jupiter is most easily observed and endlessly fascinating object to observe in the night sky.
We have all learned that Jupiter is our largest planet, but how large is large? With an average diameter of 87,000 miles, 11 Earths could be lined up side by side and not quite match the size of this gargantuan body. Its volume is over 1300 times that of Earth’s. Although only about 1/1000th the mass of the Sun, Jupiter alone contains two and a half more mass than all of the other planets in our solar system combined. The mass of Jupiter is so large, that even at a distance of hundreds of millions of miles, the gravity of Jupiter slightly affects the orbits of Earth and the outer planets, as well as more strongly influencing the orbits of many comets and asteroids.
Jupiter is believed to consist of mostly hydrogen and helium, compressed under massive gravitational pressure to a liquid state. At the very center of the planet there may be a relatively small rocky core. The intense gravitational force causes the temperature to rise from the outer regions of the atmosphere toward the center.
Similar in composition to the Sun, Jupiter can be viewed as a “failed star”, which would have reached a temperature high enough to support thermonuclear fusion had its mass been about 10 times greater.
It was Galileo (one of the superheroes of science, in my opinion) who first observed Jupiter with a telescope. Although his telescope was a mere 1” in diameter and of very poor optical quality, immediately Galileo noticed the presence of four “stars” that accompanied the planet night after night, and changed their positions near the planet constantly. Observing Jupiter over several months, he soon reached the realization that these were bodies in orbit about the planet, which he then used as a further justification of the revolutionary viewpoint reached by Copernicus that the Earth and other planets orbited the Sun.
These “Galilean moons” of Jupiter, as we now describe them, played another role in the early advancement of modern physics by allowing a measurement of the speed of light. The orbits of the Galilean moons were well known by the middle of the 17th century, after Galileo’s discovery in 1610. From the known orbits, the exact time of the start and end of each eclipse of each of the Galilean moons could be precisely determined. The innermost of these moons, Io, completes an orbit of Jupiter in about a day and 18 hours, so eclipses are rather commonly observable.
Predictions of the times of eclipses of Io were made throughout the 17th century, but observations of the eclipses showed the predicted times to be off by several minutes, and the amount of error was found to be dependent on the positions of Earth and Jupiter in their orbits about the Sun. The reason for the inaccuracy of the predictions was soon realized to be due to the varying distance from Jupiter to the Earth as the two planets orbit the Sun, and the fact that light does not
travel instantly across space.
Reasoning from the observed delays between predicted and observed times of Io’s eclipses, and estimates of the orbits of Jupiter and Earth as known in the 1670’s, Ole Romer estimated that light would take 22 minutes to cross the orbital diameter of Earth. The actual value is about 16 minutes.
Another very famous feature of Jupiter is the Great Red Spot. This is a massive storm in the upper atmosphere of Jupiter, similar in structure to a hurricane, but with a size sufficient to cover 2-3 Earths placed side by side. The Great Red Spot was not observed by Galileo in the early 1600’s, though his telescopes were powerful enough to see the storm clearly had it been present.
Apart from possible sightings by the famous astronomer Cassini in the late 1600’s, the first definite observation of the storm was in 1831. From this we conclude that the spot is not a permanent feature of the planet.
In more recent years, the Great Red Spot has become significantly smaller, and much less “red”. While I clearly recall observing this storm easily in the 1970s with the small telescope I owned as a child, I now have difficulty finding the feature in my much larger instrument, and have never succeeded in photographing it. Physics simulations, based on measurements of wind speeds obtained from spacecraft that have visited Jupiter, suggest that the storm will persist into the indefinite future. However, the visibility of the spot is known to vary dramatically, at times disappearing completely in visible light and detected only using infrared telescopes.
Jupiter has been visited by 5 spacecraft, starting with Pioneer 10 in 1973, and most recently by the Galileo mission which orbited Jupiter from 1995-2003. The Galileo spacecraft included a probe that entered the atmosphere of Jupiter to directly study the gas composition and temperature profile of the planet’s outer
atmosphere. Between the observations of these spacecraft, and more advanced telescopes, including the Hubble Space Telescope, 67 moons have been discovered orbiting Jupiter.
In addition, in 1979 the Voyager 1 space probe discovered the first of three faint rings of dust surrounding Jupiter. We have since discovered similar rings around Uranus and Neptune, in addition to the famous ring system, formed from particles of ice, around Saturn. The dust rings are much dimmer than Saturn’s ice rings, as the small bits of rock composing them are far less reflective than ice particles. Jupiter’s ring system is not visible from Earth, except in large astronomical research telescopes using methods to block the intense light coming from the planet itself.
This article merely touches upon the vast variety of discoveries that have been made concerning Jupiter and its complex system of rings and moons. I’ll force myself to stop here before this post turns into a monstrosity of text, and also so that I can reserve material for next year’s Jupiter observing season!