Marking time in years
Published on Oct 11, 2006 at 11:10 am.
1 Comment.
Filed under Uncategorized, calendars.
“Well, another year has gone by.” You hear that a lot. What does it mean? Can’t you say that every day? What makes that day so special?Â
You go to the doctor and they ask how old you are. You say about 45. They say, “But according to your chart you are only 44.” (If they already knew, why did they bother asking???)  ”You won’t be 45 until next week.” Huh. If I am only a few days from being 45, doesn’t that make me “about 45″? Why do you jump an entire integer in one day? How is one day so different from another?
Some people look at their anniversaries and it means a lot to them. Well, shouldn’t their marriage mean just as much to them the day before and the day after? If not, then that’s really sad. Well, I don’t have that luxury, anyway. For me, when a particular date comes along, it’s just another year gone by alone. I still haven’t found anyone to share my life with. But, why the fascination with “years”? What is a year anyway?
We define a year as the time that it takes the Earth to orbit the Sun. OK. That turns out to be about 365.256 days. That is the sidereal year. But, Earth also wobbles its axis in a manner that physicists call precession. The effect of precession is that the Earth’s north pole sweeps out a cone in the sky every 26,000 years. If you change which way the Earth’s north pole is pointing, though, then you also change when in its orbit the north pole is pointed most directly at the Sun or away from the Sun (the solstices), and you also change the times that the Sun is most directly over the equator (the equinoxes). Combining the orbital motion of the Earth with the precessional motion, we find that the equinoxes and solstices (and thus seasons) repeat about every 356.242 days. We call this period the tropical year. Earth’s orbit is a bit elliptical. The closest point that the Earth comes to the Sun, called the perihelion, occurs in early January. But, as Earth swings around the Sun, its orbit precesses — that is the whole orbit swings around a bit. It isn’t much, but the effect does mean that the time between one perihelion and another perhelion isn’t exactly the same as either a tropical year or a sidereal year. The time between perihelions, called the anomalistic year, is 365.260 days. And, just to confuse the issue, there is also something called the draconic year, which is the time for the Earth to go around the Sun enough for the ascending node of the Moon’s orbit to line up with the Sun until it does so again. This is even shorter than the other years because the Moon’s orbit precesses quite a bit. The draconic year (sometimes also called the eclipse year) is about 346.62 days.Â
The Gregorian Calendar, which the vast majority of the world uses, has leap days added in now and then (every four years, with three skipped per 400 years, the last being in 1900 and the next in 2100), averages 365.2425 days (very close to the actual tropical year 365.2424 days) so it keeps pretty much in track with the seasons.
But why is this time so special? It is really an arbitrary time period that we’ve picked to measure long periods of time with because such a period of time is important in agrarian societies.   But, most of us don’t live in anything approximating an agrarian lifestyle, so why do we keep time this way? Even the terms “decade” and “century” simply mean 10 and 100. If we were to redefine the “year”, by default we’d redefine decade and century.
But, I am willing to concede on the point of measuring time by the tropical year, or even the sidereal year. After all, it is useful to have a longer time interval. But, I don’t get all the getting hung up on a particular date in that time interval. After all, after you have gone 364 days, going one more day isn’t all that big of a deal. That’s less than 0.3% longer! Why increment by an entire year? And why dwell on that day as something really special?
Of course, it is a cultural thing, and I do it, too. A particular date comes along, and you think back over the last 365 days (or 366 days if it were leap year). You look at the hopes and dreams that you had a year ago, and you look at how most of them didn’t come true, and you are pretty much just where you were a year ago. I guess that the moral is to try to be happy where you are in life, because it probably isn’t going to be getting any better.
-Astroprof
Astroprof’s Page » Leap Day on February 29, 2008 at 11:51 am: 1
[…] I’ve written about the calendar a number of times before, including one posting about years, and even a post on why the New Year starts January 1. But, since this is the first leap year since I started Astroprof’s Page, it seemed natural that I blog about Leap Day. However, I looked at some of the blogs that I read a lot and found that Professor Astronomy and the Bad Astronomer had already written about leap year. So, you should check out what they have to say, too. […]