Arkansas Sky Observatory

"...What they observed was entirely unexpected. During the nine minutes before Uranus completely blocked out the light from the star, the light disappeared -- for seconds at a time!"
From an article in Time Magazine, April 11, 1977 about the discovery of Uranus' Rings

And perhaps an equally revealing and rewarding opportunity with Saturn's rings await in September, 2002

On March 10th of 1977, the occultation of the star SAO 158687 by Uranus resulted in the discovery of its ring system. At first, the multiple events of the star dimming were thought to be caused by a system of satellites (Moons) in orbit around Uranus some 40,000 km from the surface. See the full chronology of this discovery at The Rings of Uranus website from Harvard.

Even though we already know that Saturn has rings, one of the best ways to understand how they constantly change is by using data from a stellar occultation by the ring system. There is still a lot to learn, and amateur astronomers can play a big role in this study.

Events such as this are rather rare and give us the opportunity to learn more about our planetary system. An occultation is the movement of a closer body over that of another further in distance, covering the latter so it is blocked from view. In the case of celestial and planetary objects, they may only last only a few seconds in many cases. Since a star is seen only as a pinpoint from our vantage point, its "smallness" can be used to a great advantage when passing through tiny gaps in Saturn's rings, or being covered by thin outer and inner ring systems normally not visible with even the largest of Earth based telescopes. When one of these nearly invisible rings covers the tiny star image, the star will dim, sometimes significantly, or even "blank" out altogether if the ring is opaque or dense enough. When the star enters a gap, such as the Cassini Division, it will suddenly re-appear to almost full brightness for a moment, then fade once again as Saturn moves its rings over the star. Since Saturn has many divisions and "waves" in its ring system, a star might be seen blinking on and off or "flashing" during the passage. These blinks, and the exact times they occur, are extremely important in the study of planetary ring systems.

In July of 1989, Saturn occulted 28 Sagittarii, a fifth magnitude star. This was particularly rare for such a bright star to conjunct with a planet. 28 Sagittarii was so bright that even when it went behind the outside ring of Saturn (the "A" ring), the star was still visible! It did dim slightly then brighten up many times. With our upcoming event in September, the star is only tenth magnitude. Since it is much dimmer than 28 Sagittarii, the star should allow for more noticeable changes in apparent brightness as it passes behind this ring.

Saturn's Ring System

Saturn's rings are actually made up of several sets of "sub-rings". In most smaller telescopes, the Cassini Division is clearly visible as a large gap between the rings. There are many smaller gaps in these rings, and consequently, more thinner rings between the gaps. These finer details can only be seen with larger instruments, and some have only been visible with Voyager images. The major divisions of the rings have been classified with letters of the alphabet starting with 'A' and proceeding in order of discovery. Galileo was the first to see the rings of Saturn in 1610.

As shown in the diagram above, many of the "sub-rings" are very faint due primarily to the fact they are so thin.

Much of how and why the rings exist as they do, is still a puzzle to scientists. When Voyager I and II captured the finest images ever of the ring system, it only added more to the confusion. The many moons of Saturn and the gravitational effect of Saturn itself can only explain some of it. So, as mentioned before, there is still much to learn.

Stellar Occultation's by the Ring System

When the pinpoint light of a star passes (actually Saturn passes in front of the star) behind the ring system(s), it affords an opportunity to see glimpses of that star dim and brighten as it passes through the gaps and rings. These events sometimes last only seconds, if not milliseconds. Being the case, very accurate timings and estimates of the change in brightness are very important. Even in smaller telescopes this dimming can indicate the existence of a ring you otherwise can not see, such as the 'F' ring. The duration of the dimming will indicate the width of a given ring, while the duration of brightening will indicate the width of a gap. At certain points, the ring system becomes so complex with tiny gaps and rings, the star will appear to flash in and out. Following is an example of how complex the rings are:

This is a portion of ring 'A'. Note the gap on the left is Encke's Division, and the large gap on the far right is Cassini's Division. Obviously there are many smaller, and nearly transparent gaps between these two major divisions. These are where possible brightening or flashes of the occulted star might occur. As the star moves behind the more opaque rings, it will quickly dim.

The latter part of 2002 offers us three opportunities to observe Saturn passing its rings over a star in the western hemisphere.

The event in November will occur at nearly sunrise in some parts of North America and may not be visible. The other two will begin just before or after midnight in the central portion of North America.

We will cover in detail the event of 19 Sep 2002 in this article, and have more information of the other two as they grow near.

Details of the 19 Sep 2002 Event

Saturn will occult the star GSC 1324-2107, magnitude 10.06. Scopes of 6" and greater should be usable to record this occultation and the occulting ring event with some level of accuracy, but also with some difficulty due to the difference in brightness of Saturn itself, and the star. IR imaging would be ideal to level the brightness ratio and allow a good possibility of detecting the events. Be certain to have everything set up and ready for the event, and DO NOT start watching for it at any specific time quoted here. The timings below refer to those at a specific location! Begin watching, or check from your location, at least one hour ahead to get an idea of when the moment will become critical to watch carefully.

If you live in the western hemisphere, particularly the Americas, then you will most likely have the full effect of this occultation. Those along the east coast of North America and the west coast of South America will see the star pass dead center towards Saturn's disk. Those on either side of this line will still see the event, but at a slightly different vector. The map below indicates the coverage and visibility of this September event. The convergence of the green and gray is where the most direct, and longest duration of the Saturn disk occultation will be seen. The colored area ONLY indicates disk occultation, so this means that occultation by the rings may extend into Africa and parts of Asia because the rings extend well beyond the diameter of the disk. Of course areas still in daylight will not be a candidate for the event.

The following are projected images (simulations made with Project Pluto's Guide) of the path the star will take behind the rings of Saturn. Times are given for each event. These projections are only for some of the major ring features, so do not look only for these! Watch for the entire duration, if at all possible. The projected images AND times are for a geocentric position in the western hemisphere and will not apply to all locations. If you do not have software or sources for timings in your area, use those, otherwise simply start watching early.

Just prior to the conjunction of the outer part of ring A

08:07:15 UTC

Projected Timings of Various Events

The following are projected times for the major events. They are based on a geocentric position ONLY, and are only intended to provide approximate times. Each location across the western hemisphere will observe slightly different times for these events.

EVENT	TIME - UT
1) Disappearance - West Anse	08:08:45
2) Reappearance - Encke Div. West Anse	08:13:20
3) Reappearance - Cassini Div. West Anse	08:31:43
4) Disappearance - Cassini Div. West Anse	08:35:58
5) Reappearance - from B ring West Anse	09:18:28
6) Disappearance - Disk W115.35 S29.79	09:30:16
7) Reappearance - Disk W29.22 S17.66	12:20:16
8) Disappearance B Ring East Anse	13:01:47
9) Reappearance - Cassini Div. East Anse	13:41:17
10) Disappearance - Fr. Cassini Div East Anse	13:48:17
11) Reappearance - Encke Div East Anse	14:05:02
12) Disappearance - Fr Encke Div East Anse	14:05:32
13) Event Over - Star emerges from A Ring	14:10:02

Simulated images of certain events during the occultation

How to Record Data for the Event

Visual Method

First and foremost is accurate timings of the dimming and brightening of the star as it passes through the rings. An old method that has stood the test of time is a good quality tape recorder and a radio tuned to WWV. During this time of night, propagation on either 5MHz or 10MHz should be good for a clear signal of WWV. Just prior to the first event (when the star just goes behind the rings), set your tape recorder to record and your radio tuned with a good signal. Make sure that the recorder is close enough to both your observing position and the radio so that you get a clean recording of both your voice and WWV. Practice sessions should be done long before the event takes place to get this right. As you watch closely through the eyepiece, as soon as you see the star dim or go away completely, say a loud "D" (for dim), then if and when the star pops out again or gets back to full brightness, say a loud "B" (for brighten). Any sound can be used as long as you use something different for dimming and brightening. Do not use a handheld device to make sounds, as there is more delay from the brain to the fingers than there is to the mouth! Too bad we can't use the blink of our eyes to indicate this!!

Be prepared! Once the star gets behind the rings, you may see one flash after another in just a short period of time. Again, practice your reflexes and using your sounds very clear and distinctly. Practice will also help you avoid getting the two sounds backwards.

Also, it is important to get into the most comfortable position possible to avoid strain on the back, etc. Fatigue will only make your timings less accurate as time goes on. Once the star goes behind the disk of Saturn, you can rest for a good while (over two hours) since there will be nothing to see during that portion of the occultation.

Web Cams, Video, Etc.

Another method that can be most accurate is the use of video or web cams. A web cam should be fast enough to record at least 3 frames per second. If your software allows you to show the time in each frame, down to the second, turn the time display on, but be sure you set your computer clock to the correct time from WWV, or any reliable source. If you use a web cam, make certain you have plenty of hard drive space for the AVI or MPEG! For the first part of the ring occultation until the star hides behind the disk, you can estimate a total time of almost an hour and a half! A LARGE AVI to say the least. A video (tape or disk) camera that has a low lux capability would be ideal to capture the event. If you have sound recording ability, turn it on and have a radio with WWV going in the background. Later, you can watch the event with your VCR or DVD, rewind, fast forward, etc. to pin exact timings down to the second!

Once you are done, go over your tape or DVD and write down each and every "event" (brightening or dimming) with the exact time. Record your latitude, longitude, scope used, name, and method for recording and submit these to ASO (location of the online submission form will be announced after the event). We will then forward ALL observations to I.O.T.A. (International Occultation Timing Association) for reduction to useable data. All observers of this event will be given due credit no matter how much or how little you provide. Any one of us, or group of us, might even discover a new ring or division! The main thing is that you have an opportunity to contribute to the science and research of astronomy with this exciting event.

Good luck with your observation of this unique event! Please check the website often for the location of the online submission form for your observations.