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Excellent SF Bay Area graze April 15 of Upsilon Geminorum

Walt Morgan via Tinka Ross


The best grazing occultation for the S.F. Bay Area this year will be the evening of Friday April 15, 2005.

The star, Upsilon Geminorum, at magnitude 4.1 is almost bright enough to qualify this graze as Spectacular. This graze is included in the map on page 80 of the January 2005 Sky and Telescope Occultation Highlights article.

Features. This fine graze is especially suitable for beginners. The 46% sunlit waxing moon will be west at a high elevation (56 degrees), and there will be a large cusp angle (12 degrees on the dark side of the south cusp). It should even be possible to observe this graze with binoculars, and the time of the event, immediately after 9:35 p.m. PDT, near the end of the workweek for most of us, is unusually convenient. The Universal Time (UT) and date are: 0435 hours, April 16.

Location. The graze path of particular interest for this event is a little more than one kilometer wide. A location on the west side of I-880 in south Fremont, near the intersection with Fremont Blvd., has been chosen for an expedition. There are good views to the west, and there should be very little vehicular traffic.

Techniques. I will be video-recording this graze with an audio timing signal from WWV superimposed, a technique that has become very affordable, and one that nearly eliminates timing inaccuracies. Scientifically useful data can also be obtained using the traditional method of recording the observer's voice and a timing signal onto an audiotape. It is also quite acceptable for a person to join the group just to observe - disappearances and reappearances of a bright star on the dark lunar limb are never-forgotten experiences.

Profile. The predicted lunar profile is a single large peak, with a fairly steep slope on the leading edge, and a long ramp on the trailing edge. The steep leading edge means that the first Disappearance for all observers will happen within a span of about 30 seconds. Then, depending on the observer's location, the star will be hidden for one to three minutes. The subsequent Reappearances for the group will be spread over about two minutes, and during that time many observers might see multiple events as the star passes briefly behind small irregularities on the lunar limb.

Alternate locations. The graze path also crosses San Francisco, from the Presidio to the Portrero area. Further south it can be observed near I-5 south of Los Banos, and near Fresno, Bakersfield, Mojave, Victorville and Yuma. Maps and other assistance can be provided for anyone who would like to organize an expedition for any of these locations by contacting me at, a virus-protected address dedicated to this graze.

More information. Persons wishing to join me in Fremont may obtain details, including a JPEG map of the meeting site and a plot of the predicted profile, by emailing requests to the same address.


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