2021: March Evenings, Winter Triangle

March Evenings - The Winter Triangle - Sirius, Procyon, and Betelgeuse - are in the south after sunset.
March Evenings – The Winter Triangle – Sirius, Procyon, and Betelgeuse – are in the south after sunset.

Winter’s brightest – Sirius, Procyon, Betelgeuse, Rigel, Capella, Aldebaran, Castor, and Pollux – are shifting farther west as the final days of winter click off. During February we encouraged readers to compare the star colors of the Winter Sampler.

Sirius, nighttime’s brightest star, is “at the meridian,” in the southern sky.

The meridian is an imaginary celestial line that cuts the sky in half.  The line starts in the south, arches upward through overhead (zenith), cuts down through the celestial pole near Polaris and into the horizon at the north cardinal point. 

Stars east of the meridian are getting higher in the sky, while those west of the meridian are getting lower.

The star is highest when it is at the meridian – in other words south for readers in the northern hemisphere.

The sun is highest when it is south.  This occurs at noon or midday.  When the sun is east of the meridian we use the letters “a.m.” for ante meridiem, before noon for our clock times.  Post meridiem, p.m., is used for times after midday.

As the time shown on the chart above, one hour after sunset, on March 11, Sirius is slightly east of that celestial dividing line.  Betelgeuse is west of the meridian, while Procyon is east of the line.

The stellar Trio is frequently known as the Winter Triangle.  Each night until Sirius disappears into bright sunlight during May, the pattern is farther west each evening after sunset.

Sirius (“the scorching one”) belongs to the constellation Canis Major. To our distant ancestors, Sirius first appeared in the morning sky – helical rising – during the hottest time of the year, naming them “dog days.”   Procyon (“before the dog”) is part of Canis Minor. It rises before the Dog Star (Sirius).  In mythology, the two dogs belonged to Orion the Hunter.  Betelgeuse (“the armpit of the white-belted sheep”) makes the third vertex of winter’s triangle.

The triangle’s westerly shift tells us that winter’s days are short and the longer days of spring are approaching.



Categories: Astronomy, Sky Watching

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