September 23, 2019

Astro-Event of the Week: 1st-7th December, 2008: A dusk tri-conjunction.

This week’s astro-event requires absolutely no gear to observe, just a clear sky. Look westward into the dusk, just after sunset the evening of Monday, December 1st. Venus and Jupiter will be joined by a thin, waxing crescent Moon for a spectacular tri-conjunction . The three will span a circle of less than four degrees of arc. Observers from European longitudes will actually see the Moon occult Venus… North American observers will see a very close pass. Note how the configuration of the three changes from night to night.

This week’s astro-term of the week is Angular Separation. In astrometry, this is the apparent, as opposed to the actual, spacing between astronomical objects. It is measured in degrees (360 in a full circle), minutes (60 in a degree), and seconds (60 in a minute) of arc. Usually heard when discussing double stars, this term can also be used to discuss separation during conjunctions.

Cool tip; you can measure angular separation using nothing but…yourself! A flat hand at arm’s length is about ten degrees of arc; each finger (assuming you have five!) is about two degrees. Try it; this “arm & hand method” is accurate about to a degree!

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