Jupiter and Saturn - January's Showcase

Sky watchers have been amazed this winter with incredible aurora borealis and fabulous meteor showers. As if these shows were not enough, January adds to an already great winter with the best views of Jupiter and Saturn in years.

Our biggest planet, Jupiter usually appears as a bright star. This January, it is at its closest approach to Earth. With a pair of binoculars, you can easily make out at least four of the planet's 28 moons. If you have a spotting scope handy, you can increase the apparent size of the disk to more clearly see the four visible moons..

Studying planets doesn’t require expensive equipment. I use a $400 spotting scope designed for more terrestrial pursuits, but it provides enough power to clearly see four of the numerous moons of Jupiter and even the rings of Saturn. Even a powerful pair of binoculars can give objects like the moon a whole new excitement. Steady the binoculars on a bench or tripod to avoid any movement. 

How do you tell the difference between a star and a planet? Since stars are a point source of light, they tend to twinkle (the intensity will vary as you watch).  Planets, on the other hand, are actually a small disk and so they don't vary in intensity.  Therefore, if it's very bright in the sky and it doesn't twinkle, there's a good chance you've sighted a planet.

In this age of instant communication, it seems the world gets smaller every day, however when we look at a planet like Jupiter, we realize just how small the Earth really is. A trip around the Earth at the equator would cover around 40,000 km, while the same trip around Jupiter would cover more than 10 times the distance, or 450,000 km. Its mass is estimated to be 317 times that of the Earth and its gravity almost 2½ times that found in the Rockies.

It was Galileo who first glanced at Jupiter through a crude telescope and saw the same four moons you can see from your back yard so many years later. Today, we know there are 28 of these satellites orbiting the planet, even though you will have to be satisfied with the four “Galilean satellites” as these visible moons have become known.  Some of Jupiter’s moons are almost as large as the planet Mercury. One, Europa, is covered with thick sheets of ice which may hide a liquid ocean, while another moon, Io, is spotted with active volcanoes spewing molten sulphur.

It was the study of these moons rotating around Jupiter that helped to confirm Copernicus’s theories of the Earth rotating around the sun.  It was obvious they did not rotate around the Earth as conventional and theological wisdom of the time proclaimed. 

Jupiter is very different from Earth for reasons other than its large size. Despite its enormity, it is much less dense than Earth, in fact, it is only slightly more dense than water. It lacks the large quantities of heavy elements, like iron, that make up the core of Earth-like planets and is made up mostly of light elements like Helium and Hydrogen. In a way, it is chemically more closely related to the stars than to Earth.

Most photographs of Jupiter show it with brightly coloured cloud bands which seem to change very rapidly. Most prominent is a feature known as the Great Red Spot. This spot, many times the size of our minuscule planet may be the centre of a huge global storm.

Jupiter radiates more than twice the heat it receives, giving it a similar formation to many of the more massive stars with which it shares the night sky. Had its mass been 50 to 75 times greater, nuclear reactions in its core would have caused it to glow by its own internal energy source, perhaps leading it to become a star itself. In this situation, we would have two suns and reside in a double or binary star system. Night would become less common as two suns made their separate journeys across the sky.

The Ringed Planet

Seeing the rings of Saturn for the first time is an awe inspiring sight. Over time, the angle of the rings varies, and with this our ability to see them from Earth. This winter, the rings are at their most visible angle. Even with the 45 power magnification of a spotting scope, the rings are clearly visible. 

We often look up at the sky and forget there are eight other planets sharing our orbit around the sun.  Saturn is one of the more exciting celestial bodies that the amateur can scope in with a minimum of equipment.  The first time I saw the rings it almost took my breath away--especially when I realized that I was looking across nearly a billion miles of space. 

The rings are huge and their diametre stretches more than two thirds of the distance of the Earth to the moon.  What looks like a series of solid rings is actually trillions of tiny frozen moonlets spinning around the planet each in their own orbit.  The thickness from top to bottom is only about a mile with the planets gravitational pull keeping them well sorted. 

Controlling at least 30 moons, the largest, Titan can be seen with small telescopes.  It rotates the planet in about 16 days and is larger than the planets Mercury and Pluto. Based upon this comparison, Titan should really be considered a small planet.

Venus is often the most visible planet, and is often referred to as the morning or evening star as it is so bright that it's usually the first so called "star" to appear. 

If you would like more information on these planets, go to http://sse.jpl.nasa.gov/features/planets/planetsfeat.html.