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Pondering the Measure of a Year: Why do we celebrate the New Year on January 1?
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Pondering the Measure of a Year: Why do we celebrate the New Year on January 1? A question I get quite often when I am speaking of subjects astronomical is, why do we celebrate the New Year on January 1? Is there any particular significance particular date that lends itself starting our year then, or is the matter arbitrary? There is a simple answer these questions, and then there are several complicated answers. I can start simply answering the question in the simple way and stating, No, there is no astronomical significance our celebration of the New Year on January 1, and yes, it is sort of arbitrary. But I have never been stop at answering questions in a simple way when I get the opportunity to delve into more complex discussions. So let’s start with a somewhat more basic question. Just what is a year? Ask this question to most people and you will generally get a puzzled as if they are pondering just what sort of trap you are setting for them, then they will indignantly answer, a year is the time it takes for the Earth complete an orbit around the sun. Of course, that is the right answer. Well, it is sort of the right answer anyway. It is also a very wrong answer in an important way, because the reality is, from a human perspective, it isn’t the way we measure a year. For most of our history on the planet, the time it took for the Earth to complete an orbit around the sun hasn’t been a terribly important piece of information to our survival as a species. But let’s consider answer for a moment. If you want to measure the time it takes for the Earth to complete an orbit around the sun, how do you do it reliably? The best clock we have is the stars. So, let’s say there is a particularly bright star which, at some point during the year, appears directly at zenith, or in other words, directly overhead. It would then be a relatively straightforward matter to construct a small structure with a very small hole at the top. When bright star is directly overhead, it will then, and then, shine directly into the very small hole in your structure. So you just keep track of the star during the course of the year and when it once again shines through the little hole in your structure, you know the Earth has completed an orbit of the sun with respect to the stars. In other words a year has passed. But that isn’t how we humans measure a year because, as I noted above, it isn’t important us. The time it takes for the Earth complete an orbit of the sun with respect the stars is called the sidereal year. Through most of human history, it has been important for us to keep track of the Earth’s seasonal cycle. When we were primarily hunter-gatherers, animal migration patterns and times when various plants produced fruit or other edibles are tied to the seasonal cycle. When we invented agriculture, it became critical for us to track the seasons so we knew when to plant crops. In earlier times, keeping track of the north and south movements of the sun became a reliable way to track the seasons, so humans a lot of attention apparent solar movements. Of course, it wasn’t the sun that was moving, but Earth’s approximately 23 degree axial tilt and the fact the north pole always points in essentially the direction provides the illusion it is. So the earliest human calendars typically charted a year that began with the Vernal Equinox, the day when the sun crosses the celestial equator moving north. It was the first day of spring, and the time when the weather would generally cooperate with planting the new year’s crops. Our modern calendars still reflect the bias of a year which began with the Vernal Equinox in March. The month of September literally means 7th month, even though in modern times, it is the . Similarly, October means 8th month, November means month and December, 10th month. March was, of course, the first month of the new year. The months of January and February were typically so economically unimportant many ancient civilizations didn’t give them names. Some of the summer months originally had names reflected a numerical progression, but they were eventually appropriated politicians renamed them promote their own immortality. The point here is ancient civilizations didn’t care much how long it too for the Earth to orbit the sun, nor did they care much it did. The question of whether the Earth orbited the Sun or the Sun orbited the Earth led some intense theological debates a few centuries ago, but our ancestors, the question was more or less irrelevant. What they were concerned about was the length of time from Vernal Equinox to Vernal Equinox. And right now most of you are probably asking, isn’t time a year? The answer is, yes, But it isn’t the as the sidereal year. As Earth rotates on its axis and revolves around the sun in its orbit, there are important forces acting upon it; the gravitation pull of the moon and the gravitational pull of the sun. These gravitational forces cause the Earth wobble as it spins on its axis, much as a top wobbles as you watch it spin in a table. This wobble causes the Earth to complete its seasonal cycle about 20 minutes faster than it takes for the Earth to complete an orbit around the sun with respect to the stars. Thus, the time it takes for the Earth to complete its seasonal cycle is called the tropical year. The sidereal year is 365 days, 6 hours 9 minutes and 9.76 seconds. The Tropical year is 365 days, 5 hours, 48 minutes and 45 seconds. There are other ways to measure the year as well, for example the anomalistic year, which is the time it takes for the Earth to complete a revolution around the sun with respect to the apsides. This is a rather important consideration in astronomical calculations, but not important to our discussion here. The difference between the time it takes for the Earth to complete its seasonal cycle, the tropical year and the time it takes to complete an orbit of the sun with respect to the stars, the sidereal year has some interesting consequences. Our human measure of a year is, for obvious important economic considerations, based on the tropical year. Back when we began the year with the Vernal Equinox, not was the sun crossing the celestial equator, it was also entering the constellation Aries, the ram. That is why Aries is listed as the first constellation of the zodiac. But the difference in the tropical and sidereal years causes an eastward shift of the stars annually. Over the course of a year or even a decade, this shift isn’t terribly important. Over the course of centuries it becomes huge. Since the time the zodiac was first mapped, the sun has dropped a full constellation behind. It won’t cycle back to the point where it enters the constellation of Aries on the date of the Vernal Equinox for about another 23,000 years. This demonstrates why we can’t rely on the sidereal year for agricultural purposes. So, why do we start the year on January 1, when there is obviously an elegant and pretty rational reason to start with with a date which has astronomical and agricultural significance? Wouldn’t it make more sense to start it on the Vernal or Autumnal Equinox, or even the Summer or Winter Solstice? Well, yes, it would. But now we enter the realm of politics, which rarely has a rational basis. Prior to 45 B.C., the calendars most commonly were based on lunar rather than solar cycles. These were messy and complicated and almost nobody was able keep accurate track of the year. So Julius Caesar enlisted the of an Alexandrian astronomer, Sosigenes, do away with the lunar calendar and implement a more accurate solar calendar. In order reconcile the change as seamlessly as possible, Caesar added 67 days 46 B.C. which caused 45 B.C. begin on January 1. Caesar’s calendar was based on a 365 day year, with 1 day added the month of February every 4th year reconcile the fact Sosigenes calculated the year be 365.25 days long. All was hunky dory 1.5.7.0, when the Julian calendar had fallen 10 days of sync with the observed seasonal cycle due the fact Sosigenes calculation of the length of the year was a little off. So Pope Gregory XIII commissioned the astronomer Christopher Clavius to construct a more accurate calendar. Clavius new Gregorian made the correction which 1 of every 4 centennial years would be a leap year, cleaning up the error of the Julian calendar. It also became necessary delete 10 days from the year 1.5. 8.2, which caused serious discontent among the peasants believed their lives had been shortened 10 days by Papal edict. Not everyone adopted the Gregorian calendar right away. Great Britain and the colonies didn’t adopt it 1.7.5.2, which created some interesting problems for us later on. By the time England and what is now the U.S. adopted the calendar it was now days of sync with the seasonal cycle. Once again, there was discontent among some less educated folk regarding the loss of days of their life. Further, the change messed up a holiday we now celebrate. According the old Julian calendar, which was still in effect the time of Washington’s birth, George was born on February ,1.7. 3.1. But after the change, which deleted days from the old Julian calendar, advanced the year by 1 year, and moved the date of the new year January 1 instead of March 25, we now recognize his birth date as February 22, 1.7.3.2. But the calendar is now in reasonably good accord with the tropical year, the New Year is celebrated on January 1 and all is right with world...well almost. But next time you are in a museum or planetarium and some smart ass astronomer asks you tell him or her what a year is, I recommend this response: “Well, most people would tell you it is the time it takes for the Earth orbit the sun, but in reality, what we the year is the time it takes for the Earth complete its seasonal cycle.” I guarantee you will get a sly<b> wink </font></b>from the astronomer and a puzzled from the others gathered. |
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