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.


Comet NEOWISE, July 13, 2020

2022, May 30-31:  Sky Watcher’s Alert!

May 30-31, 2022:  A fragmented comet may put on a display in the southwestern sky around midnight on this evening.   Earth may encounter debris from a comet informally known as SW3.

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Crescent Moon, Venus, and Aldebaran, July 17, 2020

2022, June 30: Planet Racetrack, Green Star

June 30, 2022: The gap between the four morning planets continues to widen.  In the evening sky a green star may lie among the stars of Scorpius that is in the south as twilight ends.

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An image like this shows that our galaxy is always "partly cloudy." Not unlike Earthly clouds that block parts of the sky (say on a starry night), tremendous clouds of gas and dust obscure the things that are beyond them.

2022, June 29:  Last Call, Mercury, Night Sky, Black Hole

2022, June 29: Sagittarius A star, the Milky Way’s suspected black hole, is in the south during the midnight hour.

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Categories: Astronomy, Sky Watching

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