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Saturn's vanishing act.

West Coast observers will see the ringed planet disappear behind the Moon on the morning of September 10th.

THIS YEAR NORTH AMERICAN observers have already seen lunar occultations of three planets; the Moon passed in front of Venus and Mercury in July, while was Jupiter's turn in August. The trend continues this month when Saturn slips behind the last-quarter Moon on the morning of September 10th, presenting some interesting viewing and astrophotographic opportunities.

For observers on the East Coast both the disappearance and reappearance of Saturn will occur in a daylit sky. But the farther west you live, the better your viewing circumstances become. As seen from Honolulu, Hawaii, Saturn disappears at the bright limb of the Moon close to 1:05 a.m. (11:05 Universal Time), only to reappear at the dark limb some 44 minutes later. The entire event will be observable under dark skies against the backdrop of the constellation Taurus.

Meanwhile, a large swath of the West Coast south of Oregon will view the planet's disappearance in a dark or twilight sky, with reappearance taking place in twilight or after sunrise. For example, observers in Southern California see the start of the occultation in darkness close to 4:51 a.m. (11:51 UT) with emersion in strong twilight more than an hour later. (See the table on page 94 for exact times at various U.S. and Canadian cities.)

Owing to the angular size of Saturn and its rings, the occultation occurs at a "leisurely" pace. The Moon's slow eastward drift requires typically 45 seconds to cover the planet's disk and 100 seconds to traverse the entire ring system. Observers with access to a stopwatch and an accurate time source can make timings of the various stages: the preceding ansae (tips) of rings A and B, preceding limb of disk, center of disk, following limb of disk, and following ring ansae. These events will be much easier to view at the disappearance of the planet, but observers with well-aligned and accurately driven equatorial mounts will be able to keep Saturn within the field of view until its eagerly awaited reappearance at the Moon's dark limb.

You may also be able to time the immersion and emersion of Saturn's brighter moons, though the brightness of the lunar surface may render them invisible close to the time of disappearance. On that morning, Rhea (magnitude 9.7) will lie 54 arcseconds to the upper right of Saturn's center, at the 2 o'clock position as seen in the conventional inverted image of an equatorially mounted telescope. Brighter Titan (8.3) will be 103 arcseconds to the upper left of Saturn's center, at the 11 o'clock position in an inverted eyepiece view. The southern hemisphere of Saturn is currently tilted toward Earth by an angle of 26[degrees], so the ring system is very well displayed.

Observers near the Canadian border on the West Coast are close to the northern limit of the occultation track. Anyone south of this line will see Saturn disappear, while those farther north will see the Moon pass below the planet. For an observer on the line, the upper limb of the Moon literally grazes Saturn as it passes by. The mountains and crater rims close to the north lunar pole will be seen superimposed on the ringed planet's disk as the Moon glides by--a dramatic sight worth photographing or videotaping if you can. A commercial camcorder or webcam used afocally (behind the eyepiece) with a modest-aperture, driven telescope should have enough light sensitivity to record the event.

The northern graze line makes landfall close to the Oregon-Washington border at 5:38 a.m. local time (12:38 UT). Approximately 70 miles southeast of this point lies the city of Portland, where Saturn's occultation starts 12 minutes earlier and lasts 12 minutes longer. The line continues across Washington, passing just south of Seattle at 5:43 a.m. (12:43 UT). Observers in the city will see Saturn miss the Moon's limb by just 30 arcseconds, or 112 times the planet's width. From Vancouver, just over the border into Canada, the Moon-Saturn separation appears four times greater.

The graze line then clips the northern tip of Idaho and the upper-northwest corner of Montana, crossing the 49th parallel into Canada at 6:54 a.m. Mountain Time (12:54 UT). The northern limit of the occultation path sweeps onward through southern Alberta, Saskatchewan, across Lake Winnipeg in Manitoba, through Ontario and Quebec, to emerge at the Gulf of St. Lawrence. Viewed from Boston, Massachusetts, immersion occurs at 9:27 a.m. (13:27 UT), and emersion follows one hour later. Observers in the east with particularly transparent skies may care to try and view both the disappearance and reappearance of Saturn in daylight, but it will be a challenge. A well-aligned equatorial mount or a computerized Go To mount will increase your chances of securing an observation; a polarizing or light red filter may also improve the view.

If the prospect of observing a Saturn occultation in daylight from eastern North America is not appealing, you need only wait until November 30th for a more favorable event. On this date, anyone in the contiguous United States south and east of a line drawn approximately between Los Angeles and Hudson Bay will see the ringed planet slip behind the Moon in the evening sky. These two worlds perform one last celestial pirouette before the end of the year, in the early hours of December 28th.
Occultation of Saturn on September 10, 2001

                    Time     Saturn       Saturn
City                zone   disappears   reappears        P.A.

Albuquerque, NM     MDT    6:15 a.m.     7:43 a.m.   260[degrees]
Boston, MA          EDT    9:27 a.m.    10:27 a.m.   296[degrees]
Chicago, IL         CDT    7:60 a.m.     9:12 a.m.   281[degrees]
Dallas, TX          CDT    7:36 a.m.     9:04 a.m.   250[degrees]
Denver, CO          MDT    6:25 a.m.     7:45 a.m.   276[degrees]
El Paso, TX         MDT    6:12 a.m.     7:42 a.m.   248[degrees]
Fresno, CA          PDT    4:56 a.m.     6:12 a.m.   276[degrees]
Honolulu, HI        HST    1:05 a.m.     1:49 a.m.   294[degrees]
Kansas City, MO     CDT    7:44 a.m.     9:05 a.m.   273[degrees]
Los Angeles, CA     PDT    4:52 a.m.     6:15 a.m.   265[degrees]
New York, NY        EDT    9:22 a.m.    10:28 a.m.   288[degrees]
Phoenix, AZ         MDT    5:02 a.m.     5:30 a.m.   258[degrees]
Portland, OR        PDT    5:26 a.m.     5:51 a.m.   323[degrees]
Regina, SK          CDT    7:00 a.m.     7:18 a.m.   333[degrees]
Sacramento, CA      PDT    4:59 a.m.     6:07 a.m.   285[degrees]
San Diego, CA       PDT    4:51 a.m.     6:18 a.m.   260[degrees]
San Francisco, CA   PDT    4:56 a.m.     6:05 a.m.   283[degrees]
San Jose, CA        PDT    4:55 a.m.     6:07 a.m.   281[degrees]
Tucson, AZ          MDT    5:02 a.m.     6:32 a.m.   253[degrees]
Washington, DC      EDT    9:17 a.m.    10:29 a.m.   279[degrees]


City                  Saturn          Sun

Albuquerque, NM     70[degrees]    11[degrees]
Boston, MA          34[degrees]    42[degrees]
Chicago, IL         49[degrees]    30[degrees]
Dallas, TX          60[degrees]    24[degrees]
Denver, CO          66[degrees]    12[degrees]
El Paso, TX         72[degrees]    11[degrees]
Fresno, CA          73[degrees]    -6[degrees]
Honolulu, HI        30[degrees]   -57[degrees]
Kansas City, MO     56[degrees]    24[degrees]
Los Angeles, CA     76[degrees]    -4[degrees]
New York, NY        36[degrees]    42[degrees]
Phoenix, AZ         76[degrees]     4[degrees]
Portland, OR        63[degrees]   -10[degrees]
Regina, SK          59[degrees]     8[degrees]
Sacramento, CA      71[degrees]    -8[degrees]
San Diego, CA       78[degrees]    -3[degrees]
San Francisco, CA   71[degrees]    -9[degrees]
San Jose, CA        72[degrees]    -8[degrees]
Tucson, AZ          76[degrees]     5[degrees]
Washington, DC      39[degrees]    41[degrees]

The table shows the local times (and time zones) of Saturn's
disappearance and reappearance for selected cities in the United
States and Canada. For reappearance, the position angle (P.A.)
of the Saturn is measured around the lunar limb from celectial
north (0[degrees]) through east (90[degrees]), south
(180[degrees]), and west (27[degrees]). Finally, the table gives
the altitudes of Saturn and the Sun above the horizon at the
time of reappearance. More information can be obtained from our
Web site at

Associate editor ADRIAN ASHFORD'S most memorable grazing lunar occultation observation was that of Venus in October 1980.
COPYRIGHT 2001 All rights reserved. This copyrighted material is duplicated by arrangement with Gale and may not be redistributed in any form without written permission from Sky & Telescope Media, LLC.
No portion of this article can be reproduced without the express written permission from the copyright holder.
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Article Details
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Author:Ashford, Adrian R.
Publication:Sky & Telescope
Date:Sep 1, 2001
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