With the slow march of the stars to the west each evening and each season, the starry patterns form a celestial calendar. In the northern hemisphere, winter begins on December 21 at 4:44 a.m. CST. At this time the sun’s celestial coordinates are 18 hours of right ascension and -23.5 degrees declination. The sun passes overhead at Tropic of Capricorn, 23.5 degrees south of the equator. On ancient star maps, the sun appeared in front of the stars of Capricornus. Over time the earth has pivoted from the sun’s and moon’s gravitational forces so that the astronomical solstice point appears in front of Sagittarius. See more about this below.
The group of stars that signal the beginning of winter is the informal group known as the Winter Triangle that are made of Sirius, Procyon and Sirius. This giant sidereal shape appears complete in the southeastern sky by 9:30 p.m. CST in late December. Sirius, the brightest star in the night sky, is distinctly blue-white through binoculars. It may twinkle vividly when viewed near the horizon on a clear, cold night from heat rising from your terrestrial surroundings. Procyon — known as “Before the Dog” because it rises a little before Sirius, the Dog Star — appears farther north along the eastern horizon. Procyon is not as blue as Sirius. The third corner of the triangle is Betelgeuse, the reddish star at the shoulder or armpit of Orion. Its colors are distinct through binoculars and a small telescope as well.
The change of seasons brings us a procession of new celestial signs. The Winter Triangle and other bright stars in the eastern sky tell us that Winter is here.
The constellations are represented by multiple definitions. The familiar stick figures made by connecting the stars is the most common fashion. Another way is to divide the sky into patches like those on a quilt, although each is not equal in size. The chart above shows the sun at solstice noon. (The green line represents the sun’s apparent annual path on the sky that is caused by the earth’s revolution. The moon and planets appear to move closely to that line, not on it but within a few degrees. The red line at the top is the celestial equator; it is directly above our planet’s equator in the sky. The vertical white line is the meridian, an imaginary line that starts at the southern horizon, goes through the overhead – zenith — generally through the North Star — and to the northern horizon.) The meridian divides the rising stars from the setting stars. It marks the highest point a celestial object can reach during that day. For the sun, it is noon. When the sun is east of the meridian, that’s morning; we use the letters “a.m.” (ante meridiem) to designate that the sun is before the meridian. When it is west of the meridian, that’s afternoon and we use the letters “p.m.” (post meridiem) to designate that the sun has past the meridian. For me in that moment of time when the sun is south, it is neither 12 a.m. nor 12 p.m. It is Noon. So next time you meet somebody for lunch, great them with a “Good Noon!”
Earlier this year the Internet’s heart fluttered when a NASA expert stated there were 13 “signs” (for example see) in the zodiac. This is not news. The zodiac is the constellations that form the background for the real and apparent movement of the sun, moon and planets. Notice on the chart above that the ecliptic, the plane of our solar system, goes through Ophiuchus, the Snake Handler, which may have been a predecessor to the modern physician. The time that the sun spends in front of the stars of Ophiuchus is longer than the time in Scorpius. This is the constellation that made the Internet’s heart skip a beat. And there are occasions when the moon moves through part of Orion. So there are 14 constellations that form the zodiac. It’s not news, though.
Have a happy holiday season. For me, it’s Christmas, so I wish you “A Merry Christmas” and a “Joyous New Year.” For my readers south of the equator, it’s summer. Have a great time at the beach!