For the last couple of months, Jupiter has been the most spectacular object in the sky after the sun and moon.
The giant planet lies in the south-east after sunset and dominates the southern sky for most of the night. Look for a very bright, starlike object, shining with yellowish-white colour. Unlike stars, planets don’t twinkle, so Jupiter shines steadily, like a plane’s landing light.
The planets orbit the sun in almost circular concentric orbits. Starting at the Sun they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Pluto used to be included as the ninth and outermost planet.
In addition to having smaller orbits, the inner planets also move faster, so they take a lot less time to orbit the sun: Mercury 0.24 years, Venus 0.61 years, Earth 1 year, Mars 1.9 years, Jupiter 11.9 years, Saturn 29.4 years, Uranus 84.1 years and Neptune 164.5 years.
The result is the inner planets regularly overtake those lying further out. For Earth and Jupiter, this happened on June 10. At that time Earth was exactly between Jupiter and the sun.
That meant that as the Earth turned, Jupiter lay in the south exactly 12 hours after the sun lay in the south. That is why astronomers refer to the planet being in “opposition.” Around opposition is one of the best times to get out the telescope to observe planets in orbits outside the Earth’s. They are at their closest to us, and we have the longest time to see them without the sun being in the sky.
When Galileo first pointed his telescope at Jupiter back in 1610, what he saw revolutionized his picture of the universe. Today we have far better telescopes and around now is the best time to turn them on Jupiter. Even if you have just a pair of binoculars, there will still be something to see, such as what got Galileo so excited.
You will see a tan-coloured disc crossed by darker lines or belts. Near the planet, you will see up to four starlike objects, all in a row, like beads on a wire. These are Jupiter’s four largest moons, moving out from the planet these are Io, Europa, Ganymede and Callisto. Over successive nights you will see them changing position.
They vanish behind the planet and pass in front, and on occasion, you can see the shadow of one of the moons cast on the planet. Galileo observed this, carefully noting how the positions of the moons changed with time and concluded the moons were orbiting the planet. Since this was a time when it was taught that absolutely everything circled the Earth, finding an exception was a wedge opening the doorway to new ideas. This led to a growing realization that a lot of things we observe in the sky become a lot easier to interpret if we assume the Earth and other planets orbit the sun.
If you have a telescope, you can have a closer look at the planet itself. That tan disc is not the body of the planet; it is just the top of a very deep atmosphere. Jupiter has a diameter more than ten times that of the Earth, but rotates on its axis in around ten hours. Whereas a point on Earth’s equator is moving eastwards at around 1670 km per hour, a point on Jupiter’s equator is moving at around 45,000 km/h. This furious rotation pulls the clouds into belts and drives huge and long-lived storms.
On Earth, our weather happens in a thin skin over the planet, a few kilometres thick.
The rough, rocky ball rotating underneath has a major effect on how our weather patterns form and evolve. Jupiter is different. The planet is a gas giant and is mostly atmosphere, so what we are seeing is a weather machine without any ground beneath. This raises a very interesting question.
With no ground anchor underneath, why has the great red spot stayed in the same place for hundreds of years? This is something to think about while looking at that fascinating planet.
Ken Tapping is an astronomer with the Dominion Radio Astrophysical Observatory.