OBSERVING THE PLANETS - Part 2
AN OVERVIEW OF THE JOVIAN WORLD
Jupiter is a dominant object of our skies. Not only can it be seen throughout much of every year as one of, if not THE, brightest objects in the sky, but it also the largest and the one that demonstrates to viewers with even small telescopes "the most bang for the buck" when it comes to visible changes and events.
Jupiter is the largest of the gas giants, and indeed all planets of our solar system. To observers on Earth, its yellowish disk is the largest of the distant planets, except for rare approaches of Venus as it attains a slightly larger disk.
We cannot see any surface detail on Jupiter, just like with any gas giant and there likely is NOT a surface to be seen. What is seen in the visible spectrum through our telescopes are the many bright cloud tops....the dark chasms where clouds plummet downward....violent storms that can attain sizes more than THREE TIMES the entire diameter of Earth.
It is truly a violent world, ever-changing. You can witness first-hand much of this turbulent and active world in even small telescopes. In addition the four large satellites of Jupiter - the Galilean "moons" - can be monitored every night as bright starlike disks that appear to dance around, across and behind the bright mother planet.
There is a lot to be seen on Jupiter, but it takes a trained eye to see most of it. Too many times novice observers have been misled into thinking that subtle detail like the Great Red Spot is "easy pickings" on the planet. Nothing can be further from the truth, as the Jovian contrast is very low and detail is subtle to the unfamiliar.
Fortunately time is on our side. Anything you miss at one moment on the great planet will be back around in less than 10 hours! That's right, even with its huge mass, the 88,000 plus planet rotates on its axis quite fast, at 9h 55 minutes in mid-to-polar latitudes and only 9h 50 minutes around the equator! This "differential rotation" causes the "air" of Jupiter to be "stirred" violently as the slower-moving cloud tops resist the faster moving currents.
The result is a feeding frenzy for amateur astronomers who can witness great storms seen as white ovals (like cyclones) or dark spots (as the Great Red Spot) and as wispy trailing clouds and downwinds.
Being a gaseous giant planet, there are NO surface markings to be visible on Jupiter, only faint "belts" which are downward convection currents. Like canyons in the clouds, sunlight cannot reflect from these areas and hence, they appear darker to us in our telescopes than do the surrounding high clouds, or what we call "zones" on the gas planets. The striking photographs from Rick Krejci which accompany this observing guide demonstrate clearly the remarkable number of belts and zones on Jupiter that can - at one time or another - be seen even in a 3" or larger telescope.
Occasionally, a bright white cloud will be seen on Jupiter, these usually forming just north of the North Equatorial Belt (NEB) in the North Tropical Zone (NTrZ) or immediately south of the South Equatorial Belt (SEB) in the South Tropical Zone (STrZ) - both shown clearly in the attached photos and clearly visible in your telescope. These white clouds are cyclonic storms which force cloudtops high into the Jovian atmosphere and, hence increase their bright reflectivity.
Even as beginners in astronomy, we are used to hearing references to "the Great Red Spot (GRS) on Jupiter.....festoons, white spots, swirls, zones, belts and shadows of its four Galilean satellites as they transit between the giant planet and the sun, casting dark disks onto the clouds tops far below.
Therefore, I have compiled a description of Jupiter and its major features for viewing and a brief overview of what IS visible in common telescopes and what you might expect to see. Note that this discussion is painfully realistic and does not exaggerate what is actually visible in telescopes of various sizes.
Attached are numerous beautiful high resolution DIGITAL composite photographs supplied graciously by Rick Krejci, taken with only an ETX 90 telescope. These clearly show a variety of markings on the giant planet; the reproductions (by the time they reach YOU) are very close to the way that Jupiter can look in a very good 5" to 10" telescope under incredibly steady seeing conditions; smaller telescopes can reveal just about the same detail, but perhaps with not as good contrast and color. Note, that because of the contrast and color enhancement available through Mr. Krejci's imaging system and computer techniques, you WILL NOT be able to see the detail as "vividly" VISUALLY IN ANY MODERATE-SIZED TELESCOPE (size up to about 16"), but the detail is there in very subtle contrast and low color. Careful observation, however, CAN eventually result in much of this detail being seen even in small telescopes.
Keep in mind that the following description details features which can be seen under OPTIMAL viewing conditions: 1) when the air is very steady; 2) when Saturn is high in the sky; 3) when your telescope has cooled down (1-2 hours minimum) and 4) under planetary viewing magnification, or about 50x per inch aperture minimum.
BELTS AND ZONES OF THE JOVIAN CLOUDS
We have already discussed that the ZONES of Jupiter are seen as bright areas and represent cloud tops of gases forced high into the Jovian atmosphere by upward convection currents.
By contrast, the dark BELTS are cooler areas between the zones which represent plummeting clouds forming deep and narrow canyons between zones. Many times, as can be seen in the accompanying photographs, the belts will appear "double" or separated into two components with a narrow but definitely discernable zone between the two components. This is CLEARLY seen in the South Equatorial Belt (SEB) and the North Equatorial Belt (NEB) in the photographs; when such a zone appears it is termed either a north or south "equatorial belt zone."
The nomenclature for these belts and zones is standard for all gas giant planets: Jupiter, Saturn, Uranus and Neptune, but only Jupiter shows the number and the intensity demonstrated in the accompanying chart. Here is a drawing (from "A Complete Manual of Amateur Astronomy", P. Clay Sherrod, Prentice-Hall 1982, used by permission) that demonstrates the astronomical nomenclature for all gaseous planets: Jupiter, Saturn, Uranus and Neptune. It is the same for all planets. Note that, ASTRONOMICALLY, south is always shown at the TOP of photos and diagrams. Now photos and the drawing can be compared; although the electronic files of the photos are certainly not as clear as the originals, try to make out which "belts" and "zones" you can make out on Jupiter
You should become very familiar with these markings, and learn to differentiate between NORTH and SOUTH when viewing in your telescope; remember, a telescope with a right angle prism gives images with NORTH UP, and the image reversed right-to-left. However, the photographs shown here always have SOUTH UP, as this is the correct and accepted orientation in the astronomical community.
A couple of quick notes regarding belts and zones:
1) the equatorial belt (EB) is neither north nor south;
2) the polar regions are NOT "belts", they are "regions;"
3) there are NO "tropical belts," only "tropical zones;"
4) the belts are not always seen the same SIZE nor INTENSITY, sometimes much more vivid than others, and many times wider or even double from the night or week before.
5) be sure to make observing notes concerning the COLOR of the belts and zones, as they change over time; you will see yellows, white, blue, orange and tan in the zones and dark tan, brown, black, red and orange in the belts.
6) REMEMBER: "belts" are always dark; "zones" are always bright.
THE GREAT RED SPOT
To amateur astronomers perhaps the best known feature of Jupiter is the "Great Red Spot" (GRS), a very prominent and darkened oval in the southern hemisphere of the planet. Now known to be an immense storm of cyclonic nature, detailed photographs from the Hubble Space Telescope (HST) show a spinning nature to the GRS, with considerable central detail within the spot itself.
The spot, as well as some conspicuous darkening within it, can be clearly seen in the pair of photographs by Mr. Krejci; the GRS is in the upper left in the top photo and in the upper right in the lower photo (marked with arrow). Note that the spot is "embedded" (actually it "plows through" with its own motion) in the SEB, south component, and is encroaching on the STrZ; a very dark center can be clearly seen in the lower photograph.
The influence of the GRS on other Jovian features is great as demonstrated in this pair of images. Note how the SEBs has "curved" to conform to the shape of the GRS, and the trailing (left) edge of the SEBs is considerably darkened and displaced to the south behind the Red Spot.
In the upper photograph, note the duplicity of the SEB and the very distinct and bright South Equatorial Belt Zone (SEBz). This appears to "begin" immediately following the GRS (lower photo) but is absent preceding the GRS. Such tiny phenomena are the real captivation of the planet to amateur astronomers.
The GRS is clearly visible in small and large telescopes. During recent years, the GRS has experienced a significant "fading" to our line of sight and is a bit more difficult to spot and certainly less vividly red than in some years. For those who persevere, though, the spot will eventually regain its majesty.
Although it can FAINTLY be detected (particularly in the lower photograph), the GRS displays a very thin ring around it at the present time; I have seen this for several years now, the ring outlining the pinkish spot and appearing very thin and only somewhat darker than the spot itself. My ETX 125 shows it during very steady conditions, but the ring - and the dark central spot within the GRS - are not visible in smaller telescopes right now.
Check the listings in "Sky & Telescope" magazine for transit times (look in the "calendar" section) of the GRS. Most uniformed viewers seem to think that the spot should be visible any time they set out to observer, but this is not the case, the spot rotating on the planet once around every 10 hours or so.
STORMS AND FESTOONS
Much of the fun in observing Jupiter is watching the nightly changes that take place in the visible clouds. Even the smallest telescope can reveal changes from one night to the next....even during one night's observing, the fast spinning of Jupiter allows you to see many "faces" as the planet turns yet more of its "surface" toward us.
Among the many interesting features that can be monitored in small and medium telescopes are the "white ovals" and "dark ovals", storms similar in nature (cyclonic) to the GRS, but with vortex motion that forces the cloud tops HIGHER (as opposed to lower with the Red Spot) in the atmosphere than the surrounding ZONES. Hence, the higher clouds reflect more sunlight and appear brighter.
In Rick Krejci's two photos here, you can clearly see some rapidly-changing features that are visible to you. Look at the lower photograph first, demonstrating two very bright white ovals in the North Tropical Zone (NTrZ) and encroaching into the north component of the North Equatorial Belt (NEBn). These are bright cloud tops.
Not seen right now, as far as my observations go, are any "dark ovals" which are also cyclonic storms of about the same size as the white ovals, but are representative of "downdraft cyclones" in which clouds are pulled inward rather than thrust high into the atmosphere.
NOTE: The dark spot at the top (south) in the lower photograph is the SHADOW of the Jovian satellite "Io", discussed in the following section.
The storm spots are not long-lived, but some last for many years. Indeed, the Great Red Spot has been seen for well over 300 years, even though in fades in and out of intensity from year to year. The white ovals appear to last longer than the dark ovals, and in some cases both only last a few weeks and quickly dissipate. Their motion is independent, but they do move very similar in rotation to the belts around them.
Observers with small telescopes can easily detect the bright and dark ovals provided that the observer is viewing a the time the planet has turned them to our direction here on Earth. Since the ovals can suddenly appear overnight (as can much transient and dramatic outbursts), observers are advised to watch nightly and attempt to spot these outbreaks early in their infancy. Who knows....you might discover a new disturbance on the giant planet!
Now look closely at the top image, examining the region between the two Equatorial Belts. The white arrows point to three clearly-visible "FESTOONS" that gently curve in the Equatorial Zone (EZ). If you look carefully at the top photo, you will see that the festoons appear to originate at the south edge of the North Equatorial Belt (NEB) and curve southward (toward the top) and to the right before reaching the equator of Jupiter.
Likely, within only a few days after Mr. Krejci took this magnificent image, these festoons were either gone, or greatly altered from the form in which you see them in his photograph. The festoons are "streaks" of swirling gases intermixing rapidly in the turbulent equatorial zone of the planet. It is a very active region and change can be seen on a night-to-night basis.
OBSERVING THE GALILEAN SATELLITES
In amateur telescopes (indeed, in ANY telescope less than 36" diameter), only FOUR of Jupiter's satellites can be detected visually. These are the same four that were discovered by Galileo in 1610, and hence entitled the "Galilean Satellites. Charts are published in "Sky & Telescope" magazine each month detailing the daily motion of these satellites as they orbit Jupiter.
This photo demonstrates what you can expect to see during one of the many weekly "transits" (when the satellite passes IN FRONT of Jupiter) of a Jovian moon. Here, we see the SHADOW (yes...it is a solar eclipse on Jupiter!) of the satellite IO, the closest of the Galilean satellites to the planet. Although not visible in this photo, Mr. Krejci has been able to capture BOTH the satellite AND the shadow simultaneously in animated time-lapse motion in the ETX 90.
This is a phenomenon that YOU can see with your telescope! Clearly visible are both the disk of the small satellite AND the shadow that it MAY cast as it transits the planet (not all transits lead to eclipses). Another satellite fancy would be for you to observe one of the four satellites as it DISAPPEARS BEHIND the giant gas globe of Jupiter. The telescope (and/or camera) will clearly show the small satellite as it slips behind the planet, and in many cases, when one might re-emerge on the opposite side!
OBSERVING TIPS FOR JUPITER
As mentioned, the detail is there to be seen on Jupiter, but the contrast on the planet is VERY low, making the detail difficult to discern. To see all that we have discussed requires two things: 1) your familiarity of WHERE the features are located and what to look for; and, 2) understanding what techniques and tools you can use to bring out the subtle detail.
MAGNIFICATION - As with all planets, keep in mind that "magnification" is necessary, but not too much. (see the observing guide on "Seeing and Transparency" on this web site) Magnification is limited by the steadiness of the night air, and if the stars twinkle and images of planets are not sharp....you WILL NOT see the detail no matter what scope you are using! Whatever telescope you use to observe Jupiter, use this rule for magnification: when the steadiness is "come and go" and rather average, limit your magnification to 50x per inch of aperture; this means about 175x in a 90mm scope and about 250x in a 125mm scope. A 200mm (8") scope MIGHT stretch out the 400x, but that power will be pushing the limit even on a good night. On a night exceptional seeing, up the power-per-inch rule to 75x or 80x per inch to see even finer detail. Moonlight will not interfere with planetary viewing. When choosing the perfect magnification for any night, you might do what I follow: get an eyepiece (do not use zooms for planetary detail....they do not show fine detail like eyepieces alone) and magnification that shows a wealth of detail in your opinion; then go a little "higher" and see if that detail holds; if it doesn't the first eyepiece is your limit. If high magnification continues to show the same fine detail (always judge by the most difficult and tiny feature you can make out) you see at lower power, keep raising the magnification until you begin to see breakdown of detail. You have reached above your limit at that point!
FILTERS FOR JUPITER - I have mentioned time and again that detail is very subtle on Jupiter. There are TWO filters that you can screw onto the base of your eyepieces to really bring out considerable more detail than you can see without them: 1) the Wratten #58 green filter is incredible for increasing contrast between the belts and zones, revealing dark and bright spots, intensifying the GRS, and showing fainter belts than you might see without it; 2) a good "light pollution" or "nebula" filter is also recommended for Jupiter work; it does not provide much increase for other planets, but is excellent for Jupiter providing an overall increase in contrast planet-wide.
AVERTED VISION - this is an astronomical trick, not used too much anymore since most people never "look" through telescopes. When you look DIRECTLY at any bright object, particularly a small one like a planet, you are viewing with the worst part of your eye. In this position you view only with the RODS of your retina which - the older you get - deteriorate with age and use. Try this: with Jupiter centered, attempt to move your eye so that you never actually stare RIGHT AT the planet but rather skim past it, or look just to one side. This activates the "CONES" of your retina which are more color and light sensitive. It is amazing what you can see with this "averted vision" than without it! Try it also on double stars and deep sky objects.
The mighty planet Jupiter puts on a new show for those ready to find its features every night. It may not be as spectacular at first sight as the ringed majesty of Saturn, but its secrets are far more obvious and merely awaiting the trained eye and inquisitive mind to unravel.
Don't know what to "look at" tonight? Take a trip to Jupiter and you're guaranteed an award of bright ovals....red spots....wispy festoons....whirling cloud tops and dancing satellites.
All from the pleasure of your backyard.
P. Clay Sherrod
Arkansas Sky Observatory
Conway / Petit Jean Mountain
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