Arkansas Sky Observatory

SPECTACULAR SATURN OCCULTATION

In the early evening of February 20 (21st U.T.) observers throughout the world will have had 14 opportunities to witness an OCCULTATION of the ringed planet Saturn by the moon. Five such events will occur in 2002.

A lunar occulation occurs when the moon - moving eastward around the earth in its orbit - seemingly "overtakes" or covers up an object more distant. This can be a planet like Saturn, an asteroid, comet or a distant star. Indeed, many occultations of stars occur EVERY single night visible in all amateur telescopes. The timings of such occultations are important to the science and understanding of the moon's orbit about the earth and the dynamics of the Earth-Moon system.

Observers are encouraged to participate in such events whenever possible and predictions for YOUR location can be obtained through the International Occultation Timing Association (IOTA) at:
< http://www.lunar-occultations.com/iota/iotandx.htm > .

Very early....just after dusk on February 20, 2002, observers in most of North America will witness yet another lunar occultation of Saturn. However, unlike the last such event that occured in the chilly moring skies of December 28, 2001, this occulation is much more favorable.

On December 28, the moon was 94% illuminated, just past full, and located VERY low in western skies for most of North America. Hence, the sighting and recording of the disappearance of Saturn behind the dark edge of the moon on that night was hampered by the close proximity of the brighened lunar limb.

Immediately after dusk - and in some locations while still in daylight! - on February 20-21, the dark limb of the First Quarter Moon will take little more than one minute to glide across the distant ringed planet. At this time, the conditions are considerably more favorable because:

1) Saturn will be nearly overhead at the time of occultation;
2) The lunar terminator (divison of night an day) will be midway on the lunar disk (just over 55% illuminated), and thus will NOT overbrighten the much dimmer image of Saturn for visual and photographic imaging of the planet.
3) The early immersion (disappearnce) allows of observers throughout North America and Canada an opportunity to ALSO VIEW the emersion (reappearance) of Saturn as it slowly reappears on the opposite, or western edge!

Observers farther east in North America are much favored for the upcoming occultation. From the far northeastern United States and southeastern Canada, immersion will occur in dark skies after the end of evening twilight; observers in the central U.S. will see the occultation begin at roughly between 5:30 CST and 5:40 CST, or during twilight with not totally dark skies.

The farther west, the more unfavorable the disappearance will be. For Rocky Mountain observers, it is late afternoon and for those even farther west, the event takes place in full daylight. However note that even in daylight hours this event CAN be seen and recorded!

For those with dark skies, and perhaps those east from a line extending diagonally NW to SE from IOWA through Kentucky and even in to Florida, this event will provide an opportunity to repeat a three decade-old observation and perhaps to shed new light on a long-standing mystery:

WAS the "F" ring - the outermost of all Saturn's rings, extending only a short distance outside of the bright outer "A" ring commonly seen in all telescopes (the one "outside" Cassini's division) - actually discovered visually during a lunar occultation or was it found later by sophisticated spacecraft technology?
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AN AMATEUR'S DISCOVERY?
The night of October 17, 1973, presented an occulation of Saturn by the waxing gibbous moon. This occultation it must benoted was the near-reverse of the February 20-21 event since immersion took place at the bright, sunlit limb and the planet re-emerged at the dark limb. When it reappeared, imagine that the faint glint of its rings were expected to "pop out" at any moment against a very dark lunar edge.

Just as in the upcoming occultation, Saturn's rings were tilted at almost their maximum apparent toward the Earth, just like Canadian amateur Glen Reed waited anxiously as he watched the very dark edge of the moon in his 6-inch Newtonian telescope for the outer edge of Ring A to emerge from behind the moon's limb. But rather than a sudden appearance of the bright "A" ring, he noticed a glow which appeared on the the dark limb right where Saturn's bright ring was supposed to appear....but not just yet! What he was seeing was "earlier than predicted," and truly not the bright "A" ring.

It was several seconds before the tip of Ring A reappeared. In his own words, Reed remarked that this glow appeared as "a campfire on the other side of a treeless hill on a dark night." At first averted vision was required, but the brightness of the glow steadily increased until it was washed out by the glare from the "A" ring when it did actually appear about four seconds.

For a complete historical review of suspected sightings, both modern and old, of the "F" ring, there is a wonderful article in the February, 2002 issue of Sky and Telescope by Thomas Dobbins and William Sheehan.
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AN OPPORTUNITY?

Of course, the situation is not quite the same for the upcoming occultation. For one, it will be IMPOSSIBLE to see such a faint ring as the "F" ring reappear against the bright edge of the moon, which is the February 20-21 situation.

So, even though the reappearance is something that must be attempted to record both visually and photographically, there is little chance to re-create Reed's sighting in this manner.

However, another method is possible for the keen-eyed visual observer with at least a 6-inch telescope (the larger the better) or those with CCD and digital cameras that can take rapid-sequence images. During the early evening disapperance, entire event will require no more than about a minute for the whole of Saturn and its rings to be covered by the moon. Most "green observers" to occultations are shocked by how rapidly they go by...instantly with a star and very rapidly with a planet.

The trick here is to record Saturn all the way until it is gone....past that actually. Since it only requires one minute to disappear, rapid imaging and very sensitive equipment MIGHT catch a glimpse of the "afterburn" of ring "F." Watch Saturn slowly disappear and BE READY as it is nearly completely gone.....in the moment just AFTER it disappears you want to look carefully - and very quickly! - for any glow left in the wake of Saturn's last position...the last glimmer of "ring A" that you saw. This will at best last only 4 SECONDS! So image/look fast!

A video camera set to its greatest sensitivity should be able to record any spurious glow after "ring A" has vanished; likewise a good and very sensitive digital camera (operate it manually and use either a "sequence" mode or the "movie mode" if available) and shoot as rapidly as possible; begin imaging the second that you see the "first bite" out of the approaching Saturn ring....then continue until a full 15 seconds after you think Saturn has disappeared!

Visual observers should use a minimum magnification of 150X; use as much power as the seeing conditions will allow. This will aid you in two ways:

1) that the narrow field of view concentrates just on the area around Saturn and excludes the sunlit portion of the Moon, permitting the eye to become dark adapted....which you very much will need;

2) the increased magnification will allow you to concentrate precisely on the point of disappearance in the seconds that immediately follow the final sighting of "ring A".

If using a refractor, chromatic aberration will almost assuredly give a false impression of a glow from the spurious flare of false colors created. The diffraction from the spider vanes of a Newtonian or Cassegrain reflector may also create the false impression of a glow surrounding the tip of "ring A" just before and after immersion. Hence, only apochromatic refractors (3-element color corrected) and catadioptric telescopes like Schmidt-Cassegrains and Maksutovs are preferred if possible.
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AND....WHILE YOU'RE AT IT....

ENJOY this gorgeous phenomenon....although we are blessed with 14 of them in the span of only a year, this is an ideal event for Feb. 20-21. More details concerning exact times, lunar altitudes, disappearance and reappearance points on the lunar limb, etc., WILL be posted on the IOTA site as time grows closer. Look for further updates and bulletins here at the Arkansas Sky Observatory....as always your observations and images are encouraged and welcomed here at ASO!

Dr. Clay

NEW - Local Predictions for Saturn Occultation

Here are the latest specific times for both DISAPPEARANCE and REAPPEARANCE of Saturn for the upcoming spectacular Saturn occultation of the moon on the evening of November 20 (local time) and 21 (Universal Time and in Europe).

The last event of December 28 was well observed and photographed, yet it was a very difficult event to see due to the very low eastern position of the event.

Note that for the United States, this event takes place VERY early (see my previous discussion about this event) in the evening; for eastern U.S. and SE Canadian observers, the event will take place in darkness with the moon and Saturn nearly directly overhead at the end of twilight.

Central US observers will be observing just before 00:00 U.T. (Nov. 21) or right before 6 p.m. local time, so evening twilight will still be present in the sky.

BE prepared for observing and photographing this event NO LATER than 30 minutes ahead of YOUR predicted time from the tables at the URL given below:

< http://www.lunar-occultations.com/iota/0221drut.htm >

which are provided courtesy of the International Occultation Timing Association (IOTA). These tables provide complete listings of BOTH the disappearance (early evening) and reappearance (later evening, about one hour or less AFTER the disappearance!) for nearly every major city in the viewable path of this beautiful event!

More information regarding the importance of this and some tips on photographing the spectacle will be posted about one week prior to the occultation!

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