© Norman Sperling, April 24, 2011
Last New Year's Day I got an eMail with an old, familiar ring to it: "Spectacular graze in 98 days". A grazing occultation, a special kind of eclipse where the north or south pole of the Moon grazes by a star, can be really nifty to watch. The star is at full brightness, and then abruptly disappears as a mountain covers it up. It may reappear through a valley, disappear behind another mountain, and can do so several pairs of times.
In the 1960s, computers advanced enough to make worthwhile predictions, so astronomers learned how to make scientifically valuable observations of grazing occultations ... using portable telescopes and cassette recorders. An observer watches the star through a telescope, and tells the recorder the instant that the image turns off and on. Timing is maintained either by recording the shortwave signals of WWV or WWVH directly, or by recording a nearby clear-channel AM radio station that somebody else is recording simultaneously with WWV.
With such simple equipment, teams of observers, strung out perpendicular to the graze path, can determine the profiles of mountains and valleys on the Moon to an accuracy of a few tens of meters, from a distance of 400,000 km!
To organize the whole operation, Dr. David Dunham and others set up the International Occultation Timing Association (IOTA). The last half century has seen marked improvements in computers, telescopes, eyepieces, voice recorders, video recorders, mapping, and communications, and IOTA has used each to refine its procedures. The leaders have stuck with it for many decades: Dunham, whom I met in the National Capital Astronomers in the 1960s, still leads it now, more than half a century after observing his first occultation!
Here in Northern California, Walt Morgan has organized expeditions for several decades. Walt's alert commented: "For many years IOTA classified grazes as Marginal, Favorable, or SPECTACULAR! There were mighty few of the latter, and now those labels are not used at all. Nevertheless, I think you will agree with me that it would be appropriate to apply the classification to the following:
- star: magnitude 3.5 eta Geminorum
- moon: 36% sunlit
- limb: northern
- cusp angle: +14 degrees (dark)
- lunar elevation: 42 degrees
- lunar azimuth: 266 degrees (west)
- when: 9:43 p.m. Saturday, April 9, 2011
- where: Vacaville, Dixon, Davis area
"If you have been waiting for just the right opportunity to break out your occultation tools, this is the one: as grazes go, it has everything going for it, including the time of day and day of week."
One of the brightest stars to occult (binoculars would suffice), the Moon not glaringly bright, in its best, easiest situation, at a convenient hour, on a weekend, in a place easy to reach from a freeway - wow!
It's been more than 20 years since the last graze I observed - Regulus, November 30, 1988, Fremont California. So I'm not exactly in practice. Devotees measure many per year.
But the timing was perfect, the weather was clear (though windy), I had most of the equipment I needed and could easily borrow the rest. So I went. Walt's long experience has led to immaculate preparations. I checked in with him at the appointed spot, south of Dixon, at dusk, a good 2 hours in advance, and also said hello to Derek Breit and several other veteran "grazers" I'd met at various astronomical gatherings. Our observing line was well populated with 9 observing stations, and another, near Stockton, had 11 more.
I brought my Astroscan telescope, 2 eyepieces, binoculars, a small portable table, a larger card table, and a few ways to keep warm. There was a streetlight just north of my assigned position, and wind coming from the southeast. I positioned my van to block both streetlight and wind, and set the telescope on the floor inside to look out the side door. I could close the door for warmth when I wasn't observing. That wasn't very much, since the whole glorious Winter Oval was there, deserving long looks with the Astroscan. With the car radio tuned to the right station, and the borrowed voice recorder turned on, there were no hassles, and I even kept pretty warm! I ended up not needing the binoculars, either table, or most of the warmth gear - but experience has long proven that it's better to bring too much than too little.
The star winked at me! 3 times!
Walt timed my voice on the audio. First, a peak hid the star for 3 seconds. Then the star was visible again through a valley for 15 seconds. A big mountain hid the star for 1 minutes and 42 seconds. Then the star shone through a valley for just 1 second till it was hidden by another peak for 3 seconds. After that the star was no longer occulted from my location.
Others took videos, whose results can be timed to individual frames, with no "reaction time" delay from going through a human. Derek Breit, who has way more experience and way better equipment than I have, has posted this page about the event, and at the bottom you can click on his spectacular video. You can easily see the result of wind shaking his telescope. But you can also see the star not merely blinking off and on, but dimming as edges of hills barely blocked part of the star! The timing is in "Universal Time" (Greenwich, with a few small corrections). His location was a few hundred meters north of me, closer to the Moon's edge, and obviously in the perfect position to take advantage of his experience and equipment.
One really neat effect I remember from that graze a couple decades ago has been outmoded by technology. These days, most observers make videos rather than voice timings. Back then, almost everybody used voice, and at that event we had so many observers we were very closely spaced. So as a disappearance or reappearance neared me, I could hear observers from up or down the line saying so into their recorders, then I saw it, then others farther down or up the line. I heard the profile of the mountains and valleys live, in stereo!