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Here’s an oddity for those who were taught that noon is the middle of the day, and the sun is due south at 12:00. Neither is true.
Noon this time of year is not 12:00. On Tuesday and Wednesday it is at 12:15 p.m., and it’s getting steadily later, as measured by the time when the sun is at its highest, halfway between sunrise and sunset.
Midnight similarly is at 12:15 a.m. Blame something called the analemma.
Style guides — books that tell how to write for a general audience — warn writers of all kinds to stay away from using a lot of numbers. Fine, so here we go anyway.
Noon reaches its latest point of the year on Feb. 2, coincidentally Groundhog Day, when noon will be at 12:17 p.m.
Like a pendulum, it pauses there before swinging back the other way. It remains at 12:17 until Feb. 19.
Although it would appear from this that nothing much is happening to the sun’s (apparent) position, there is in fact all kinds of movement of the sun in the sky through that period. On Feb. 2, it is 28 degrees above the horizon at high noon. That’s not very high.
But by Feb. 19 the sun at noon will climb to 33.6 degrees, a rise of more than five degrees. That’s still only half as high as it climbs at the beginning of summer, but still enough to feel warmth on your face, or to warm up a brick wall.
Then the solar noon begins swinging back in the other direction. It would reach 12:00 again on April 27, only we’ll be on Daylight Time by then, so it’s actually called 1:00 p.m.
From there you would think it would swing the other way, and it does, but only briefly. For a few days in early May noon is one minute earlier, and then it starts moving later again until autumn. Bizarre, but there it is. There’s no nice, neat pendulum movement involved.
So, about that “analemma” pattern of the sun’s movement. The not-so-helpful definition is that the analemma “can be considered as a graph in which the sun’s declination and the equation of time are plotted against each other.” Oh joy.
More simply, the sun just sits there and Earth has a constantly changing speed around it, which gives us an imperfect viewing platform.
Earth is not moving around the sun in a circle. Johannes Kepler in the early 1600s first realized our orbit has the shape of an ellipse, a sort of stretched-out circle.
As a consequence of that, we are moving at different speeds — faster when close to the sun (in our winter), more slowly in July when we are farther from the sun.
With the Earth moving faster on some days than on others, our eyes tell us that we are cruising along steadily and the sun is shifting in our sky. Earth’s tilt further complicates the apparent position of the sun. It all makes the sun’s position at 12:00 (or any other time of day you pick) appear to shift slightly east and west throughout the year.
The elliptical orbit and the axis tilt have another consequence: Winter is shorter than summer by about three days in the northern hemisphere. The length of seasons is determined by when the sun reaches its farthest points north and south, and when it crosses the equator. Based on that, the lengths are: Spring, 92.8 days; summer, 93.6 days; fall, 89.8 days; and winter, 89.0 days. Those are rounded a bit.
A further oddity: Earth is about five million kilometres closer to the sun in our winter, but we don’t benefit because our hemisphere is tilted away from the sun. The #@$#% Aussies get all that extra warmth.
tspears@postmedia.com
twitter.com/TomSpears1
查看原文...
Noon this time of year is not 12:00. On Tuesday and Wednesday it is at 12:15 p.m., and it’s getting steadily later, as measured by the time when the sun is at its highest, halfway between sunrise and sunset.
Midnight similarly is at 12:15 a.m. Blame something called the analemma.
Style guides — books that tell how to write for a general audience — warn writers of all kinds to stay away from using a lot of numbers. Fine, so here we go anyway.
Noon reaches its latest point of the year on Feb. 2, coincidentally Groundhog Day, when noon will be at 12:17 p.m.
Like a pendulum, it pauses there before swinging back the other way. It remains at 12:17 until Feb. 19.
Although it would appear from this that nothing much is happening to the sun’s (apparent) position, there is in fact all kinds of movement of the sun in the sky through that period. On Feb. 2, it is 28 degrees above the horizon at high noon. That’s not very high.
But by Feb. 19 the sun at noon will climb to 33.6 degrees, a rise of more than five degrees. That’s still only half as high as it climbs at the beginning of summer, but still enough to feel warmth on your face, or to warm up a brick wall.
Then the solar noon begins swinging back in the other direction. It would reach 12:00 again on April 27, only we’ll be on Daylight Time by then, so it’s actually called 1:00 p.m.
From there you would think it would swing the other way, and it does, but only briefly. For a few days in early May noon is one minute earlier, and then it starts moving later again until autumn. Bizarre, but there it is. There’s no nice, neat pendulum movement involved.
So, about that “analemma” pattern of the sun’s movement. The not-so-helpful definition is that the analemma “can be considered as a graph in which the sun’s declination and the equation of time are plotted against each other.” Oh joy.
More simply, the sun just sits there and Earth has a constantly changing speed around it, which gives us an imperfect viewing platform.
Earth is not moving around the sun in a circle. Johannes Kepler in the early 1600s first realized our orbit has the shape of an ellipse, a sort of stretched-out circle.
As a consequence of that, we are moving at different speeds — faster when close to the sun (in our winter), more slowly in July when we are farther from the sun.
With the Earth moving faster on some days than on others, our eyes tell us that we are cruising along steadily and the sun is shifting in our sky. Earth’s tilt further complicates the apparent position of the sun. It all makes the sun’s position at 12:00 (or any other time of day you pick) appear to shift slightly east and west throughout the year.
The elliptical orbit and the axis tilt have another consequence: Winter is shorter than summer by about three days in the northern hemisphere. The length of seasons is determined by when the sun reaches its farthest points north and south, and when it crosses the equator. Based on that, the lengths are: Spring, 92.8 days; summer, 93.6 days; fall, 89.8 days; and winter, 89.0 days. Those are rounded a bit.
A further oddity: Earth is about five million kilometres closer to the sun in our winter, but we don’t benefit because our hemisphere is tilted away from the sun. The #@$#% Aussies get all that extra warmth.
tspears@postmedia.com
twitter.com/TomSpears1
查看原文...
