Seasons
Published on Jul 1, 2006 at 4:39 pm.
1 Comment.
Filed under Earth, calendars.
This is a followup to yesterday’s post. Here in the United States, the calendar says that Summer starts at the Summer Solstice (June 21, this year), Spring started at the Vernal Equinox (March 20, this year), Autumn starts at the Autumnal Equinox (September 22, this year), and Winter will begin at the Winter Solstice (December21, this year). Note, my disclaimer “this year” for each of these dates. They move around slightly. There are several reasons for that. The biggest reason is that the Earth does not take exactly an integer number of days to orbit the Sun. So, the times for these events will slowly shift about 6 hours or so each year, until after four years they are a day off. But, we toss in an extra year in the calendar every four years to take care of that. There are other factors that shift the times a bit, but they are much smaller factors, so we won’t worry about them here. Oh, and the equinoxes and solstices are specific points in time, not just days. The dates that I gave are for the United States. For my readers in Europe or Asia, a couple of these will change. These are the calendar seasons. Sometimes, they are called the astronomical seasons. However, I have always thought that these were dumb dates to start the seasons. After all, it is hot before late June, and the heat normally breaks by early to mid September. Around here, in dry years, the trees start to shed leaves are early as September, in fact. Definitely, it gets cold well before Christmas. It is not uncommon to have a freeze in November. And, normally, it is pretty darn nice out (spring like) by early March. What gives? We pick these dates for the seasons because they correspond to things happening in the sky. As the Earth goes around the Sun, it is tilted 23.5 degrees. The northern hemisphere it tilted most towards the Sun at the Summer Solstice. At the same time, the southern hemisphere is tilted most away from the Sun. This means that you have the longest days in the northern hemisphere and the longest nights in the southern hemisphere. It is summer in the north, and winter in the south. The opposite happens at the Winter Solstice. At the equinoxes, the Sun is directly overhead at the Earth’s equator. Technically, we are supposed to get equal lenght night and day at the equinox, but that doesn’t really happen. If you look at tables of sunrise or sunset, there is slightly more than 12 hours of daylight on the equinox. The reason for this is twofold. First, the Sun is not a point source. So, even when the center of the Sun is on the horizon, half of the Sun is above the horizon. This effectively makes sunrise a about a minute early and sunset a minute or so late. Also, atmospheric refraction tends to raise the Sun’s image a bit, again making sunrise early and sunset late. The actual dates of equal day and night are a few days on the winter side of the equinoxes. But, since these dates (solstices and equinoxes) mark well defined astronomical happenings, that are related to the seasons, then they are what the calendar says as the season divisions. But, as I said, the weather does not exactly correspond to this. Hmm. Another way of defining seasons goes back thousands of years. In this traditional method, the solstices and equinoxes, rather than marking the beggining and ends of seasons, mark the middle of the seasons. So, June 21, being the longest day of the year, would the be the midpoint of the Summer. There would be a mid-summer festival held then. The middle of spring would be the Vernal Equinox, and the middle of Winter would be the Winter Solstice. This way of looking at seasons has something to be said for it. After all, that makes all winter days short and all summer days long. The way we do it now, the Summer days work as an exact mirror image of the Spring days. One month before the solstice has the same length day as one month after. In the traditional method, both would be summer. Hmm. Interesting. However, that puts summer beginning in early May (say May 1?) and ending about early August. Winter would begin in early November and end in early February. But, for anyone living around here, you know that August is as hot as July, and there isn’t much difference normally between October and November, and February is only slightly warmer than January. Both April and May are about the same climate here, as well, with both normally sort of mild. So, this is about as far off as the calendar method. However, there is a British custom that seems to work very nicely. Here, the seasons are three months each. March, April, May are Spring. June, July and August are Summer. September, October, and November are Autumn. And, that leaves December, January, and February for Winter. This has the effect of skewing the begginings of the seasons only about three weeks earlier than the astronomical method. It uses whole calendar months, which is convenient to keep track of things. And, it seems to fit the weather patterns nicely. Hmm. This method also seems to fit how we conduct ourselves in the US, as well. We normally think of Summer as starting at the Memorial Day weekend (last of May) and ending about the time that we go back to class in the Fall Semester (late August). The travel and retail industries seem to consider summer to be from Memorial Day to Labor Day (early September). Winter is generally from Thanksgiving to Spring Break (that is late November to early March). So, again, the British system seems to fit very well. Personally, this is how I’ve always thought of it anyway, despite what the calendar says. Anyway, I would support a move to redefine the seasons according to a more rational method. The way we do it now basically just sort of evolved. There is nothing really that says that we have to define the seasons as starting precisely at the moment of the solstices and equinoxes. That is pretty silly, anyway, and it doesn’t really fit with how we live our lives. The three month block method seems to work just fine from a planning, retail, and travel perspective. So, does anyone out there want to figure out how to go about changing things?
-Astroprof
Astroprof’s Page » Late sunrise on December 22, 2009 at 2:10 pm: 1
[…] Equinox. Autumn starts at the autumnal equinox and ends at the winter solstice. I’ve written about the seasons before, and I think that this is perhaps not the best way to declare the seasons, but I suppose […]