By Dr. Steve Kipp
Minnesota State University
ST PETER — Happy new year! That’s easy to say but what exactly does it mean to have a new year and why does it begin Jan. 1?
Today we know that a year is more or less the time it takes the Earth to circle the sun once. But in the ancient world a year was the seasonal interval from the beginning of one spring to the beginning of the next. That seasonal time interval, called the tropical year, is the time period calendars were designed to measure. If you are curious, the tropical year length is about 365.2422 days. The Gregorian calendar—the one we use—with its leap year rules comes close to matching this tropical year length.
The year could start anytime although a good time for it to begin would be the first day of spring, around March 20, as in the very early Roman calendar. It is suggested that it was the legendary Roman king Numa Popilius in about 713 BCE who added January and February to the calendar and who decided to start the year in January.
The Earth’s closest approach to the sun, called perihelion, actually does occur early in January but Numa Popilius probably didn’t know that. This year perihelion occurs January 3. In any event January is a good name for the month that marks the beginning of the year. It is named for the Roman god Janus, the god of doorways, who looks both into the past and into the future.
Unfortunately clouds here in Mankato hid the winter solstice total lunar eclipse last month but you will have another chance to see an eclipse (sort of) in January. Eclipses usually occur in pairs and there is a partial solar eclipse happening January 4. Unfortunately, to see it you need to be in Europe, Asia or North Africa. The new year will bring six eclipses: four partial solar and two total lunar eclipses.
January brings us the brilliant winter sky. By the time it gets nice and dark the big, bright, beautiful constellation Orion the hunter is seen rising on his side in the east. Orion is a huge stick figure man. He is instantly recognizable by the three equally bright belt stars in a tilted line. Above the belt stars are two bright stars representing Orion’s arms and below the belt are two bright stars representing Orion’s feet.
Following Orion into the sky at Orion’s heels is his hunting dog, Canis Major. Canis Major doesn’t show a clear pattern of stars but it does contain the brightest star in the night sky, Sirius, appropriately called the “dog star.” If the name “Sirius” sounds familiar remember that Harry Potter’s godfather was Sirius Black.
Sirius was long suspected to have a faint companion star. That companion, Sirius B, was first observed by Alvin Clark January 31, 1862. Sirius B turned out to be a white dwarf, a stellar corpse made of super-dense material. Sirius B is the size of the Earth but it has the mass of the sun! Because Sirius A—the bright star we see in the sky—is the “dog star,” Sirius B is sometimes known as the “pup.”
Send comments and questions to Steve Kipp, at email@example.com