Prime Time for Observing Jupiter

March 2016  :  Dave Huestis

It’s been a few months since any of the naked-eye planets have been visible during convenient evening hours for casual stargazers. From mid-November into early 2016, amateur astronomers who wished to view Venus, Saturn, Mars, Jupiter, and a brief and low appearance of Mercury, had to do so during the early morning hours before sunrise. For those of you whose work and family commitments prevented such exploration, the drought of planets in the evening sky is about to end. 

The first planet to be well-placed for scrutiny will be the largest world in the solar system—Jupiter. On March 1 this massive planet (you could fit 1,321 Earths within its volume) will rise above the eastern horizon around 6:00 p.m. I suggest waiting an hour or so for Jupiter to ascend higher before you begin to observe it with a telescope. Since Jupiter will be the brightest object in this area of the sky, you shouldn’t have any difficulty in finding it. 

However, what may first catch your eye before Jupiter increases in altitude will be the blue white star Regulus, the constellation Leo’s brightest star. In sky lore Regulus marks the position of the lion’s heart. It also marks the terminus of a star pattern or asterism known as the sickle, seen as a backwards question mark. The sickle represents the head, mane and shoulders of the lion. Jupiter will be located down and to the left of Regulus, under the lion’s hindquarters. See accompanying star map for reference. 

This favorable prime time appearance of Jupiter is ideal, for on March 8 Jupiter will be at opposition (opposite the Sun in the sky) and at its closest distance to the Earth this year at approximately 412,296,186 miles. Jupiter will be visible all night. And in fact, we’ll be able to observe Jupiter through until mid-August, when it will be low in the west at sunset, requiring a very good western horizon to view. That leaves us five good months to watch the ever changing Jovian system.

Telescopic observations of Jupiter began on January 7, 1610, by Galileo Galilei using a lens 1.5 inches in diameter with a magnification of only 20X. He immediately noticed three little stars in the planet’s vicinity. At first he thought they were background stars, but after careful observation he deduced they were orbiting Jupiter. You can read about Galileo’s discovery in his own words in a brief work titled, Sidereus Nuncius, or The Starry Messenger, published on March 13, 1610. His observations of Jupiter’s satellites begins on page 17 of this document: http://homepages.wmich.edu/~mcgrew/Siderius.pdf 

Galileo’s telescopic work and the calculations that followed proved the Copernican theory’s Sun-centered orbital dynamics of the solar system, still controversial for the time. The planets orbited the Sun, just as Jupiter’s moons orbited about him. Galileo concluded in The Starry Messenger, “…for now we have not one planet only revolving about another, while both traverse a vast orb about the Sun, but our sense of sight presents to us four stars circling about Jupiter, like the Moon about the Earth, while all of them together with Jupiter traverse a great orb moving around the Sun in the space of twelve years.” 

You can relive Galileo’s discovery of Jupiter’s moons, now known as the Galilean satellites, using even a small “department store” refractor. Also easily observed will be Jupiter’s bands and zones, which give the planet a striped appearance. Larger instruments may also reveal the Great Red spot, a centuries old storm in Jupiter’s cloud tops. Use whatever optical aid you have at hand, but if you wish to marvel at the beauty of Jupiter and all it has to offer, then set aside some time to visit one of the local observatories for a splendid visual experience.

Here’s what one could expect to see over a period of a few nights. Just like Galileo, one of the first things that will catch your eye will be the Galilean moons. They are: Io, Europa, Ganymede and Callisto. When several of the Galilean moons are visible at the same time, they often appear in a straight line, parading around Jupiter in the plane of its equator. This astrophysical arrangement presents many interesting phenomena for us earth-bound astronomers to observe. (For those of you who love facts, Jupiter has 63 known moons total, the majority of which cannot be seen visually with even large amateur telescopes in a dark sky.) 

When a moon passes in front of Jupiter and casts a shadow onto the Jovian cloud tops, it is called a shadow transit. Besides seeing the satellite’s shadow, you may also see the bright disk of the satellite traversing Jupiter’s clouds at the same time, though this event is more difficult to observe. A moon may also pass behind the planet, which is called an occultation. Jupiter’s shadow can even eclipse a satellite as well; gradually the moon will either blink out or reappear. Also, it’s fun to watch all four moons line up on one side of the planet. I love to watch Jupiter over an extended period of time during the course of one evening because the view is dynamically changing as you watch.

While you can be pleasantly surprised at the eyepiece of a telescope by any of these satellite events during a random observing session, you can use the following website to plan ahead to ensure you experience a special Galilean satellite event (http://www.skyandtelescope.com/wp-content/observing-tools/jupiter_moons/jupiter.html).

Next it’s time to look at Jupiter’s disk, the ball of the planet. Jupiter is a large planet so even a small telescope will show a keen-eyed observer much detail. The more prominent dark bands/belts and lighter zones in Jupiter’s cloud tops can easily be seen. Though the Great Red Spot (GRS), an almost 400-year-old storm, is not as red (some observers describe it as salmon colored) as it once was many years ago, it should be fairly easy to see if it has rotated into view.

However, since its discovery the GRS has shrunk by 50%, and in more recent years its oval shape has become more circular. Some astronomers speculate that the storm may be dissipating. Only time will tell. A six-inch telescope or larger may be needed to catch a glimpse of it. Keep in mind that Jupiter rotates once in ten hours, making it possible to see the entire planet in one or two nights of observing. You can visit the following website to determine when favorable views of the Great Red spot will occur: http://acquerra.com.au/astro/software/jupiter.html.

Once Jupiter rises high enough to be viewed from the local observatories, it will be the focus of attention for several months. Only the Moon will be able to draw attention away from Jupiter. That is until the beginning of June when Mars, then Saturn, will return to the early evening sky for us to view. By Jove, get out there and enjoy one of the great visual pleasures that a telescope can provide.

Incredible views of Jupiter and his moon’s are available for your viewing pleasure at each of the facilities listed below. Seagrave Memorial Observatory in North Scituate is open every clear Saturday night. Ladd Observatory in Providence is open every Tuesday night. The Margaret M. Jacoby Observatory at the CCRI Knight Campus in Warwick is open every clear Wednesday night. Frosty Drew Observatory in Charlestown is open every clear Friday night. Snow or ice can force closures, so please check the respective websites for any cancellation notices before venturing out for a visit. Also, opening times change during March because on the 13th at 2:00 a.m. we spring ahead one hour to daylight saving time (DST), also known as eastern daylight time (EDT). Verify the schedules on the websites as well. 

And finally, the vernal equinox, (spring) begins on March 20 at 12:30 a.m. EDT.

Keep your eyes to the skies.