Physics Photo of the Week
February 7, 2014
The Drinking Gourd
The Big Dipper - the prominent star formation in the
northern skies - had been called "The Drinking Gourd" due to
its resemblance to a natural utilitarian "dipper" made from a
hollowed gourd. This was the name given by mountain and
plantation cultures in the pre-Civil War American South.
Other cultures have called it "The Plow" and "The Great
Bear". Click on the image for a larger photo.
The Drinking Gourd asterism is close to the north celestial
pole, it never sets below the horizon, and is always viewable
in the northern skies at all times of the year. It is
reputed that the highly recognizable star formation, visible
at all times of the year, served to provide directions for
slaves escaping to freedom via the Underground Railroad.
"Follow the Drinking Gourd" eventually became a popular
folksong - "discovered" in 1912 by folklorist H. B. Banks and
published by Banks in Publications of the Texas Folklore
Society, Number VII, Austin, 1928. The song has
become popular during the civil rights movement of the 20th
century and has enriched all our lives with the rich music of
African American culture and reminds us of the tremendous debt
we owe to African Americans for the inhuman treatment of these
Scientifically the Big Dipper/Drinking Gourd asterism has
played an important part in measuring the atmospheric
extinction of starlight when measuring the brightness of
stars. The stars of the Drinking Gourd asterism are
well-known to possess standard brightness values - or
magnitudes. In February (and again in September) the
Drinking Gourd spans a large elevation difference.
Dubhe, the topmost star in the photo, is about the same
brightness as Alkaid, the lowest star at the end of the
handle. But because of the thicker atmosphere at low
elevations, Alkaid actually looks fainter in the image.
By measuring the apparent brightness of the well-known stars
at different elevations, we can measure the effect of the
atmosphere and account for the atmosphere effect in
stellar photometry measurements.
The Drinking Gourd asterism is also one of the few star
formations in which the stars are physically related to each
other. It is believed that the seven bright stars are
actually members of an open cluster of stars that were formed
at the same time out of a giant hydrogen cloud - similar to
the Pleiades cluster (PPOW Nov.
22, 2013). The Drinking Gourd stars are relatively
close - less than about 100 light years compared to over 400
light years for the Pleiades and the seven stars are all
moving relative to the other stars in relatively the same
directions in space.
Physics Photo of the Week
weekly during the academic year on Fridays by the Warren Wilson
. These photos feature interesting phenomena in
the world around us. Students, faculty, and others are
invited to submit digital (or film) photographs for publication
and explanation. Atmospheric phenomena are especially welcome.
Please send any photos to email@example.com.
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