Spring 1999 Looking Up with Dr. Arthur Babcock,President, ASCC Tales of the Occult Amateurs Use the Weaver Student Observatory to Observe the Moon and Aldebaran |
| Occultations, when an
astronomical body such as the Moon or a planet blocks our view of a more distant body,
such as a star, offer astronomers a chance to make observations that would be otherwise
impossible or very difficult to make. For example, occultations of stars by the Moon
revealed, long before the space program, that the Moon has no appreciable atmosphere, for
a star occulted by the Moon winks out instantaneously; if there were a lunar atmosphere,
the star would undergo a process of gradual dimming before disappearing entirely. Lunar
occultations, when timed with great precision, are also useful in calculating the mass and
size of the Moon, and refining the details of the Moon’s orbit. I remember "observing" Encke’s Division in the rings of Saturn—very difficult to see directly in amateur telescopes—in July of 1989, when the planet passed over a bright star, 28 Sagittarii. At first, the star disappeared as the outermost ring covered it up, but then it popped back into view as the Encke gap in the rings passed over it. The brightening was so dramatic that I was sure that it was the wider Cassini division that I was seeing, but I was wrong. Occultations are also one of the classic areas of amateur contributions to astronomical research. Some widely available features of modern technology enable observers without expensive professional equipment to make highly precise timings. By recording the Moon’s passage over a bright star on videotape, for example, we can take advantage of the fact that video is composed of a succession of frames each lasting only 1/30 of a second to determine within .03 second the moment the star disappears. All one needs is to place a highly accurate time reference on the tape as well, by recording the time signals broadcast on short wave radio by WWV.
MIRA’s new Weaver Student Observatory has exactly the equipment required: a computer-controlled telescope, a CCD video camera and a TV/VCR combination. When the January, 1999, meeting of the ASCC coincided with an occultation of bright Aldebaran by the Moon, I decided that the opportunity was too good to pass up. Occultation day was quite rainy, but the weather was predicted to clear in the evening. Several ASCC members (Robin Casady, Ileene and Paul Franks, Bob Trubell, Zena Zeres and I) gathered in the observatory dome. We got the video on the 4" refractor going, running it to the TV/VCR combination. The weather improved rapidly, and at occultation time, it was almost completely clear. Unfortunately, the cheap short wave radio I had bought to provide time signals wouldn’t receive WWV inside the observatory, so I was reduced to putting visual time signals on the tape by looking at my watch (synchronized to WWV) and flipping on the electronic crosshairs generator at one-minute intervals. This put a split-second crosshair on the videotape, but was limited by the accuracy—probably not high—with which I could read my watch and flip the switch. Fortunately, Kim Cohan arrived and cobbled up a long-range audio cable so the blasted radio could stay outside and still connect to the audio input on the VCR. Reception was still marginal, but we got two clear signals, one at 11:41:00 and another at 11:42:00. This was after Aldebaran was covered, but you can work backwards as easily as forwards, so I calculate that Aldebaran was covered by the Moon at 11:37:38pm PST. Aldebaran was drifting a little as we tracked it approaching the occultation, so for the reappearance, we guessed what part of the Moon to point at, based on a drawing in Sky and Telescope. Sure enough, the star turned up there, almost exactly one hour after disappearing. There are some asteroid occultations later in the year that might be visible from MIRA. These won’t be quite as easy to record, but the video camera ought to see them if mounted on the 14". Watch this space for further developments. |
