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Seasons of the Year

In the northern hemisphere, winters are cool and summers dry and hot. As already noted, twice a year, halfway between summer and winter, the Sun rises exactly in the east, and sets exactly in the west. We now know that on the days when this happens, day and night are equal in length, and that time of year is therefore called "equinox." One equinox happens in the fall ("autumnal equinox") and one in the spring ("vernal equinox," "ver" is Latin for spring). As fall advances towards winter, the location of sunrise moves south, as does the location of sunset. The steepness of the curve traced by the Sun does not change, nor does the rate ("speed") with which the Sun appears to move along it, but the length of the curve changes, it becomes shorter. Around December 21 --the "winter solstice" halfway between the equinox dates (typically, September 23 and March 21) sunrise and sunset are as far south as they can go (at any one location). As a result, the Sun has its shortest path for the year, the day is at its shortest and night is at its longest. Other days of that season are short, too, which is one reason for the colder weather in winter.

The apparent path of the Sun across the sky. In summer, the Sun's path is longest, and so are the days. In winter, the Sun's path is shortest, and so are the days.

After that the points of sunrise and sunset migrate northward again, and days get longer. This migration continues past equinox (when it is at its fastest), and the Sun crosses the horizon furthest northwards around June 21, the "summer solstice", longest day of the year with the shortest night. After that days get shorter again as sunset and sunrise migrate south again. The long days of summer, of course, match the warmer summer weather.

Elevation of the Sun

The length of the day is not the only reason summers are hot and winters cold. Another is the elevation of the Sun above the horizon. When the Sun is near the horizon, not only are the shadows which it casts stretched to greater length, so is its illumination. Any beam of sunlight then spreads out along a greater distance on the ground, diluting the heat given to any area. The noontime Sun in winter is low in the sky, and its heating is less pronounced, while the summer Sun can be almost overhead, heating the ground much more effectively.

Planets and the Zodiac

Not all stars keep fixed positions on the sphere of the heavens. Even early sky-watchers noted that a few moved about: the ancient Greeks called them "planets", wanderers. The names we use today came from the Romans, who named them after their chief gods--Mercury, Venus, Mars, Jupiter and Saturn. Mercury and Venus are always close to the Sun and can only be seen shortly after sunset or before sunrise: Mercury is so close that most of the year it cannot be seen at all, because the bright sky drowns out its light. Venus is brighter than any other star (with appropriate conditions and looking right at it, you can see it even in the daytime) and Jupiter takes second place.

Finding the Pole Star - The Big Dipper and Little Dipper

The Big Dipper consists of 7 bright stars, forming a dipper, a small pot with a long handle. Astronomers name it "Ursa Major," Latin for "the big she-bear," and some other languages also refer to it as the Big Bear. Imagine a line connecting the two stars at the front of the "dipper", continue it on the side where the dipper is "open" to a distance 5 times that between the two stars, and you will arrive at (or very close to) the pole star. Because of their role in locating Polaris, these two stars are often called "the guides." Ursa Minor, the "Small Bear" or "Little Dipper" is a constellation somewhat resembling the Big Dipper, and Polaris is the last star in its tail. The "dipper" itself faces the tail of the Big Dipper, so that the two "tails" (or "handles") point in opposite directions. The two front stars of the "little dipper" (quite smaller and more square than the big one) are fairly bright, but other stars are rather dim and require good eyes and a dark sky.

The Path of the Sun, the Ecliptic

The orbit of the Earth around the Sun. This is a perspective view, the shape of the actual orbit is very close to a circle.

Seasons

If the Earth's axis were perpendicular to the ecliptic, as in the drawings here, the Sun's position in the sky would be halfway between the celestial poles, and its daily path, seen from any point on Earth, would stay exactly the same, day after day. Each point on Earth would be carried around the axis AB once a day. On the equator (point C) the sun would always rise until it was overhead, then again descend to the horizon. At the poles (A and B) it would always graze the horizon and never get away from it. Except at the pole, every point would be in the shadow half the time, when on the right of the line AB, and would experience night; the other half it would be in the sunlight, experiencing day. Because the motion is symmetric with respect to the line AB, day and night anywhere on Earth are always equal. Actually, the axis of rotation makes an angle of about 23. 5 degrees with the direction perpendicular to the ecliptic. That makes life a lot more interesting.

Seasons of the Year

Equinox and Solstice

In particular (drawing above), the angle between the Earth's axis and the Earth-Sun line changes throughout the year. Twice a year, at the spring and fall equinox (around March 21 and September 22--the exact date may vary a bit) the two directions are perpendicular. Twice a year, the angle is as big as it can get, at the summer and winter solstices, when it reaches 23.5 degrees. In the summer solstice (around June 21) the north pole is inclined towards the Sun, in the winter solstice (around December 21) it faces away from it. Let us look at the summer solstice first, with the Sun on the left.

Summer and Winter

The boundary AB between sunlight and shadow--between day and night--is always perpendicular to the Earth-Sun line, as it was in the example shown at the beginning. But because of the tilted axis, as each point on Earth is carried on its daily trip around the rotating Earth, the part of the trip spent in daylight (unshaded part of the drawing) and in the shadow (shaded) are usually not equal. North of the equator, day is longer than night, and when we get close enough to the north pole, there is no night at all. The Sun is then always above the horizon and it just makes a 360-degree circuit around it. That part of Earth enjoys summer. A mirror-image situation exists south of the equator. Nights are longer than days, and the further one gets from the equator, the larger is the imbalance--until one gets so close to the pole that the sun never rises. That is the famous polar night, with 24 hours of darkness each day. In that half of the Earth, it is winter time. Half a year later, the Earth is on the other side of the Sun, that is, the Sun's position in the above drawing should be on the right, and the shaded part of the Earth should now be on the left (light and dark portions in the drawing switch places). The Earth's axis however has not moved, it is still pointed to the same patch of sky, near the star Polaris. Now the south pole is bathed in constant sunshine and the north one is dark. Summer and winter have switched hemispheres.

 
A big difference between summer and winter is thus the length of the days: note that on the equator that length does not change, and hence Spring, Summer, Fall, and Winter do not exist there (depending on weather patterns, however, there may exist a "wet season" and a "dry season"). In addition (as the drawing makes clear), the Sun's rays hit the summer hemisphere more vertically than the winter one. That, too, helps heat the ground, as explained further in section #4, "The Angle of the Sun's Rays." At equinox, the situation is as in the first drawing, and night and day are equal (that is where the word "equinox" comes from) 

Some interesting facts If June 21 is the day when we receive the most sunshine, why is it regarded as the beginning of summer and not its peak? And similarly, why is December 21, the day of least sunshine, the beginning of winter and not mid-winter day? Blame the oceans, which heat up and cool down only slowly. By June 21 they are still cool from the winter time, and that delays the peak heat by about a month and a half. Similarly, in December the water still holds warmth from the summer, and the coldest days are still (on the average--not always! ) a month and a half ahead. And what about our distance from the Sun? It, too, varies, because the Earth's orbit around the Sun isn't an exact circle. We are closest to the Sun--would you believe it? --in the cold wintertime, around January 3-5. This may have an interesting implication for the origin of ice ages, as will be explained later. It also ties to an interesting story of the unusually bright Moon of December 22, 1999.

The Angle of the Sun's Rays 

In the US and in other mid-latitude countries north of the equator (e.g those of Europe), the sun's daily trip (as it appears to us) is an arc across the southern sky. (Of course, it's really the Earth that does the moving.) The sun's greatest height above the horizon occurs at noon, and how high the sun then gets depends on the season of the year--it is highest in mid-summer, lowest in mid-winter. Boy scouts used to be taught (perhaps still are) that someone lost in the woods can often tell the north direction by checking on which side of tree-trunks lichens grew best. Lichens avoid direct sunlight, and with the sun's path curving across the southern sky, the north side of a tree-trunk is the one most shaded.

For a similar reason--but to collect sunlight rather than avoid it--solar collectors for heating water or generating electricity always face south. In addition, they are invariably tilted at an angle around 45°, to make sure that the arrival of the sun's rays is as close to perpendicular as possible. The collector is then exposed to the highest concentration of sunlight: as the drawing shows, if the sun is 45 degrees above the horizon, a collector 0.7 meters wide perpendicular to its rays intercepts as much sunlight as a 1-meter collector flat on the ground. It therefore heats its water faster and reaches a higher temperature. French wine producers, too, have for centuries preferred southward-facing hillsides, on which ripening grapes get the most sunlight. The same also holds for the Earth. The rays of the summer sun, high in the sky, arrive at a steep angle and heat the land much more than those of the winter sun, which hit at a shallow angle. Although the length of the day is an important factor in explaining why summers are hot and winter cold, the angle of sunlight is probably more important. In the arctic summer, even though the sun shines 24 hours a day, it produces only moderate warmth, because it skims around the horizon and its light arrives at a low angle. The apparent motion of the sun can be important in designing a building, in particular in the placing of windows, which trap the sun's heat. In a hot sunny climate such as that of Texas or Arizona, it is best to have the largest windows face north, avoiding the sun. The south-facing walls, on the other hand, should be well insulated and their windows should be small, allowing cross-ventilation when needed but not admitting much sunlight (wooden shutters on the outside of the windows also help). In Canada the opposite directions might be chosen, to trap as much heat as possible from the winter sun. Overhangs above south-facing windows also help. In summer, with the noontime Sun high in the sky, such an overhang casts a shadow on the window and keeps the house cool. In the winter, however, when the Sun stays close to the horizon, the overhang allows it to shine through the window and warm the rooms inside.