Seeing Jupiter's Great Red Spot
by Jeff Polston (October 31, 1997, updated February 1, 2002)
Jupiter, south at top, showing red spot at the upper left.
The planet Jupiter almost always offers a wealth of details for amateur astronomers. Even the smallest of instruments show its two major equatorial belts and the four Galilean moons. Larger instruments (combined with stable skies) show even more belts, in addition to festoons and spots. There is one particular spot that is so well known that it could almost be considered famous. Almost everyone who knows of Jupiter, also has heard of the Great Red Spot. It has been seen by observers for more than 300 years.
The Great Red Spot is an oval feature about 7,500 by 15,600 miles in size. This is big enough to hold a couple of Earth sized planets. It is thought to be a high-pressure region whose cloud tops are significantly higher and colder than the surrounding regions. In other words, it's a big storm.
Even though the Great Red Spot is well known, it can be surprisingly hard to see. Its visibility has changed from year to year and even from month to month. It has gone from a prominent brick red to an almost invisible tan color. The last few times I have observed it, I would describe its color as "salmon" or pinkish tan.
The first hurdle to get across in observing the Great Red Spot is to make sure it is on the side of Jupiter facing Earth. Even the best telescope cannot see it if it is on the other side of the planet. Jupiter's rotation rate is 9 hours and 56 minutes. The Red Spot of course spends half this time hidden on the other side of Jupiter. Even when it is on the side of Jupiter facing the Earth, much of this time it is located near one of Jupiter's limbs. This means is appears dimmer and foreshortened. The best times for observing the Great Red Spot is the time frame from 50 minutes before it transits the Jupiter's central meridian (the imaginary line down the center of the planet's apparent disk from one pole to the other) until 50 minutes after it transits the central meridian. That's 100 minutes of prime observing time. And of course you can observe it longer than this (and I do). Just remember that it might not be under optimal conditions. You can get transit times for the Great Red Spot from popular magazines such as Sky & Telescope. There are also a few software programs out there that will calculate it for you. Visit my Software page and check out such programs as JupSat95
Jupiter, showing how subtle the red spot (lower right corner) can be
The second and most difficult hurdle to get over is actually seeing the Great Red Spot. Like I've said before, it is kind of a "salmon" color so it doesn't really jump out at you. The Great Red Spot is located just below the South Equatorial Belt. In fact, almost half of it is on the South Equatorial Belt. It straddles Jupiter's South Equatorial Belt and the South Tropical Zone. It should go without saying that you need a night of steady atmospheric seeing. You really have to concentrate on the planet, so don't expect to take a quick peek and see it. A lot of the time you will notice the hollow that the Great Red Spot makes into the South Equatorial Belt before you actually recognize the spot itself. Also, sometimes the use of a light green or blue filter will make the Great Red Spot appear a little more distinct. As for the size of telescope needed, I have tracked the Great Red Spot with my 80mm f/11.4 refractor, but larger instruments will give more resolution and make the task easier.
What if you can't see it??? Well, keep trying!!! Planetary observations take a trained eye to see all the details. The first few times I looked for the Great Red Spot I was unable to see it. I didn't even know what to look for at the time. I assumed it would be this gigantic, obvious red spot. It can be very subtle. Sometimes the spot will pop in and out as the steadiness of the atmosphere varies. You have to be diligent so that you'll have your eye at the eyepiece when it does appear. During one observing session, we were glancing at Jupiter with a club members newly constructed 6" reflector. Another member and I could easily see the Great Red Spot but the owner of the scope could not make it out. He was new to observing Jupiter so when he looked, all he could see were the two equatorial belts and the Galilean satellites. To be successful, you need to keep your eye to the eyepiece and take in everything that you see. With patience and determination, you too will see the Great Red Spot of Jupiter.
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