The northern limit graze of Aldebaran this evening at Osoyoos, British Columbia far exceeded my expectations. We had about twenty events over a period of 2 minutes and 12 seconds. The reason that I say “about 20” is that sometimes the blinks were coming so fast that I could not keep up while shouting into my tape recorder. In the March Sky&Telescope, page 49, David Dunham says that Aldebaran’s large angular diameter “is equivalent to 40 meters at the Moon’s distance.” Thinking about it, I must have been about 15 metres from Jim Failes. And there were two cases when he saw one blink while I saw a double blink each time. We were using identical telescopes at nearly the same power.
The early part of the graze featured four slow fades. At the first two fades the star did not completely disappear before returning to full brightness! This constitutes resolution of the disk of the K5 giant star Aldebaran with the aid of the lunar limb.
The latter part of the graze had four blinks when Aldebaran just reappeared for a mere fraction of a second in narrow gaps in the lunar terrain.
The first few events were on the lunar dark limb, the rest on the bright limb. But, having observed many occultations of Aldebaran on the bright limb, I knew that the bright limb would not be a problem visually because Aldebaran is much brighter than any adjacent tiny part of the Moon, plus there is a strong colour contrast between the orange star and the light yellow Moon. But a video camera would likely have had a problem on the bright limb, judging by the over-exposed Moon on the dark limb videos that I have seen.
Many, many thanks to Brad Timerson of IOTA who calculated a lunar limb profile specifically for the longitude of Osoyoos. We had nine observers at longitude 119d 28.466m West, latitude 49d 02.551m North, elevation 304m. All of us are people who enjoy this hobby as visual observers, so nobody gave up their eyepiece to a video camera.
At the Antares graze near Olympia, Washington state in July, 2005 we had a North-South fence of observers because lunar grazes still had scientific value at that time. But those days are gone, at least for grazes of first magnitude stars. So, instead of the traditional North-South line of observers in which some were lucky and saw multiple events while others saw only two events (one disappearance and one reappearance), we were all gathered together where Brad Timerson had indicated that the sweet spot should be, and it is difficult to imagine that anywhere else around Osoyoos could have been blessed with as many slow fades.
I had eight events at the Antares graze in 2005, so “about 20” tonight was far beyond my hopes.
It is always wonderful when you know that you have just had an experience that will form a lifetime memory. Tonight’s grazing occultation of Aldebaran was one. I used my 80mm apo refractor at 67 power. The sky was clear (as the Clear Sky Clock had forecast eleven hours earlier); the transparency was excellent; the seeing was above average; and the Moon’s altitude was 50d. The semi-arid climate of Osoyoos has one of Canada’s largest collections of private observatories, and it came through for us this evening.