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Reged: 07/12/04

Loc: Chicago
Moon Occults Star - Boston Graze - Dec 18-19
      #6250179 - 12/13/13 12:59 AM

The 96% illuminated waning gibbous Moon will occult (cover) the magnitude +3.6 star Lambda Geminorum for observers in New England, upstate New York, Alaska and a large portion of Canada during the night of 2013 DEC 18-19. Immersion will occur on the lunar bright limb and emersion on the dark limb. Observers on the only graze path will witness the star winking on and off as lunar hills and valleys pass by. Observers north of the graze path will see a full occultation.

Ive created a North American graze map, preview graphic with data for Boston, and a set of Besselian elements. They can be seen at

Photos and descriptions of the event would be welcome additions to his thread. If you would like specific occultation or graze predictions for your location, please reply to this post with your longitude, latitude, elevation and time zone.

The graze path will pass immediately west and south of Boston before heading over Cape Cod. Below is my algorithm for calculating the graze path in that vicinity. It is only valid between western longitudes 6950 and 7210. Zenith Angle is the position of the star on the lunar limb as measured counterclockwise from the zenith. The only input required is a western longitude entered as a positive decimal value. The output is also decimal. Of course if desired, the output can be converted to sexagesimal.

W = Western Longitude as a positive decimal value
L = W 71 (L may be positive or negative, but L is always positive)

Northern Latitude = 42.25730 + 0.46477 L 0. 00190 L
Hour EST = 4.49141 0.03056 L 0.00027 L
Zenith Angle = 149.4662 + 0.9327 L + 0.170 L
Refracted Star Altitude = 40.5062 + 0.9133 L - 0.0108 L

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