Apr 2017

Jupiter Viewing Starts!


If you have noticed a bright light in the eastern sky (perhaps while taking your pet for a walk) you are not alone. Venus in the west is gone (returning soon to the morning sky) and Mercury will not last long as a substitute evening star. Now we turn to the eastern sky to watch our next featured planet, Jupiter, a planet almost as bright as Venus, and in many ways much more interesting.

Jupiter is the largest planet of the 8 planets in our solar system (9 if you still pine for Pluto). Jupiter and our Sun constitute 99.99% of the total system mass and each has a gravity pull so strong that according to current theories, a major planet was prevented from forming in the space between -only jagged chunks of space debris, the asteroids, are found orbiting there. One Jove-centric pundit defined our solar system as “Sun, Jupiter and assorted debris”, and it is true that the “leftovers” don’t amount to much, mass-wise. Earth, of course, has water, an atmosphere, and conditions that allow living organisms to thrive, so it is pretty special. Jupiter is unique, too, but only because it is such an extreme contrast to our own beautiful world.

The gravity of Jupiter is over 2.5 times Earth’s so, if yours truly, at a trim 180 pounds, were on Jupiter, I would need to support 455 pounds just to stand up, -if there was any place to stand. A solid surface like that on Earth cannot be found on Jupiter, and what we see in telescopes are the tops of swirling white and brownish clouds of ammonia crystals and darker-coloured sulphur compounds. Below the clouds (about 50 km thick) is a mixture of very cold hydrogen and helium gas which gets thicker and thicker all the way to the “surface” of a strange ocean of metallic liquid hydrogen. The atmospheric pressure here would crush the strongest pressurized vessel from Earth. So if the gravity doesn’t kill you, the pressure will.

Exploration of Jupiter can only happen from orbit or floating/flying in the upper atmosphere, but even here spacecraft are severely challenged. Wind speeds reach 1600 km/h (10 times those in Earth’s jet streams), and lightning occurs in “superbolts” the equivalent of 100 of the terrestrial variety. Huge storms in the atmosphere of this huge planet are common, -one Jovian hurricane, the Great Red Spot (first seen in 1665 and raging still) is bigger than the entire Earth with wind speeds over 400 km/h (twice those of a category 4 hurricane on Earth). Even Jupiter’s magnetic field is huge, extending several million km into space, so we are lucky that we are 600 million km away, -it is a safe enough distance.

Jupiter reaches a point in our sky called opposition on Apr 7 when it is exactly opposite the Sun and highest in our sky at midnight. This is prime-time for Jupiter, when it is well above the turbulent air near the horizon. But if it is cloudy Apr 7, don’t panic. Planets at opposition are in good viewing position for a month before and several months after the opposition date. For the best views on any clear night, observe the planet when it is away from the disturbing air near the horizon. Summer star parties will obviously feature both Jupiter and Saturn, which itself reaches opposition June 15.

Jupiter Apr1 - Sep 1 2017

Jupiter is near the star Spica, the brightest star of Virgo, but Spica does not move, so use it as a reference point to watch Jupiter doing what distinguishes planets from background stars, i.e. wander. Jupiter slowly slips westward until June 8 (retrograding), then, in the summer, it starts back towards Spica. By the end of Jupiter-viewing in autumn, the pair will be setting in the west just after sunset, with Jupiter closest to Spica and right above it.

Observing nights at the Fox Observatory will certainly include views of Jupiter with its always-changing arrangement of 4 bright moons. These are even visible in binoculars, but true to its status as King of Planets, Jupiter has 63 more moons circling it, the most of any planet. By Jove, Jupiter really is an attention-getter!

John H.