Celebrate the Seasons

Celebrate the Seasons:

Solstice

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Seasonal variations in sunlight and moisture drive the growth and reproductive cycles of plants and animals. Some animals migrate or hibernate as adaptations to global and seasonal variations in sunlight and temperature. Ancient civilizations watched the sky closely for agricultural purposes and built monuments aligned to solstices and equinoxes.

Solstice means the Sun stands still

Because the Earth's axis of rotation is tilted 23.45°, the Sun appears to move north or south of the celestial equator during the year. The solstices are the points when the Sun appears to stop moving north or south. The summer solstice is the longest day of the year because the Sun rises and sets very far north. The summer solstice Sun is very high overhead at midday and the potential total daily sunlight is at its maximum. The spring and fall equinoxes are when night and day are equal in length.

The seasonal variation in the distance from the Sun is almost insignificant. The higher the Sun is overhead, the more potential sunlight strikes a horizontal area on the Earth's surface. When the rays are oblique, the light is spread over a larger horizontal area. The word potential is used as a warning because atmospheric losses reduce the amount of sunlight reaching the surface.

Apart from light scattered by clouds, a clear sky also absorbs and scatters sunlight. The Sun is low in the sky in the early morning and the late afternoon. At these times, the light rays traverse a longer atmospheric path and losses due to absorption and scattering are greater.

San Luis Obispo County is north of 35° N. latitude. On the summer solstice, sunrise is early and sunset is late and the day lasts over 14 hours. The total sunlight affects plant growth and the entire food web of the animal world. Of course, cloud cover may produce additional variations in sunlight and dry seasons may reduce the benefits of sunlight.

At 35° N. latitude, the Sun is 35° below vertical at mid-day, or 55° above the horizon on the two annual equinoxes. At the two annual solstices, the Sun is shifted higher or lower by 23.45°. It is high in the sky on the summer solstice because the Earth's axis is tilted toward the Sun, so the rays are incident at steeper angles.

This produces a higher flux on a horizontal surface. Conversely, the winter solstice has a ten hour day and a lower mid-day Sun elevation because the Earth's axis is tilted away from the Sun, so the Sun rises late, sets early, and deposits less flux in mid-day. The seasonal variation in the orientation of the Earth's axis relative to the Sun is a consequence of the fact that the Earth's axis of rotation is constant in space as the Earth orbits the Sun during the year. This is an example of conservation of angular momentum, which also explains the properties of spinning tops and gyroscopes.

The Sun's elevation is changing quickly around the equinoxes whereas the change decreases around the solstices. This is a manifestation of the word solstice: the Sun stands still. In the 16 days after solstice, the length of day changes by only six minutes, whereas the length of day changes by 35 minutes in the 16 days before equinox. The change is about six times greater. Notice the symmetry between spring and summer. Notice the difference between spring and fall, increasing vs. decreasing sunlight. There is less sunlight in the fall than in the spring. Yet September generally is warmer than April is. Weather changes slowly and lags the solstices and equinoxes because of heat retention by the Earth and flow patterns in the oceans and atmosphere.

Origin of the Moon and the tilt of the Earth's Axis

Recent research has convinced most astronomers that the Moon was created 4.5 billion years ago after the recently formed Earth was struck by a planetesimal larger than Mars. Both the Earth and the planetesimal melted. Ultimately, the dense core of the planetesimal merged with the melted Earth and the cooling Earth formed layers of crust, mantle, and core that are highly layered. The Moon formed from vaporized portions of the mantles of the two colliding worlds. This explains the lower density of the Moon, its remarkably large size compared to Earth, and its great similarity in composition to the Earth's mantle. The impact theory may also explain the tilt of the Earth's axis of rotation.

Imagine an Eternal Spring (no tilted axis)

What would our world be like if the Earth's axis were not tilted or were barely tilted? I have never seen a discussion of this question. The length of night and day would be equal everywhere on Earth throughout the year. I call this Eternal Spring because every day is an equinox. We probably would hardly notice the passage of years if the seasons didn't produce variations in weather or in the angle of the Sun and Moon. The monthly variations of the Moon would certainly draw our attention. The small variation in the distance from the Sun increases sunlight slightly in January at all latitudes. Elevation, latitude, and surface water would be dominant factors in determining local climates. Animals would probably have no reason to migrate between latitudes and there would be no solar cues on when to anyway! Migration of young plants and animals would still be important to avoiding overcrowding a territory.

Plant and animal reproductive cycles would not be synchronized with seasons. There would be more latitudinal stratification in evolution patterns and in plant and animal adaptations. Would there be more or less biodiversity? The Torrid Zone would still have a wide variety of species and they probably would have spread out over the latitudes. Without the stresses related to seasonal weather changes, species would not have had to develop adaptations to so many variables, so I would expect fewer dramatic variations in species. We could guess that evolutionary progress would have been slower. Perhaps humans would not yet have evolved without the driving force of seasons on the animal world.

Perhaps human civilization would have advanced faster or slower. Would agriculture have advanced without well-defined planting seasons? Would there be any incentive for early civilization to understand the sky (archaeoastronomy)? How would this have affected the development of cultures, social structure, and religious institutions? Would biological cycles be more synchronized with monthly full Moons and fortnightly high tides? The Frigid Zone would not get long days of sunlight so the Arctic would be a region of scarcity rather than vast quantities of nutrients for migrating birds and mammals. Without enough sunlight to melt the snow accumulating at the Poles, it is likely that ice would keep growing and the Earth would experience an endless ice age. Perhaps the oceans would be dry as all water formed ice. Ice reflects a lot of sunlight and would cool the Earth further. More like eternal winter than eternal spring. On the other hand with fewer photosynthesizing plants, the atmosphere would have a lot of carbon dioxide to produce the greenhouse effect of trapping heat from the Sun. Therefore...?

Seasonal Sunlight, Latitudes, Growth and Migration

On the Equinox, the total daily sunlight peaks at the equator where the Sun's rays are most direct and drops to zero at the poles where the surface does not intersect any of the Sun's rays during the day. The winter solstice in the Southern Hemisphere and the summer solstice curve of the Northern Hemisphere occur on the same date.

There is no sunlight in the Frigid Zone on the winter solstice. Sunlight is at a minimum at the Equator on the summer solstice and is slightly lower than on the Equinox there because the rays are less direct and the length of day is still 12 hours. The long days in the Temperate Zone and Frigid Zone make up for the decreased Sun altitude so that the Frigid Zone has more sunlight than anywhere else on the summer solstice.

Long summer days allow plants more time to utilize the sunlight efficiently with photosynthesis and allow animals long days for feeding. Mild summer Arctic temperatures are less stressful and dehydrating than the extreme heat of the Torrid and Temperate Zones. No wonder so many birds and marine mammals migrate so far north in summer. If you were a plant or animal, where would you want to be? If you were near the Arctic Circle in August, you might think about heading south to follow the food supply or to keep warm. Would you need to go as far as the Tropic of Cancer? Notice the enormous change in sunlight from summer to winter in the mid-latitude Temperate Zone such as 35° N. latitude.

In general plants and animals have different lifestyles in the Temperate and Torrid Zones as a result of the differences in seasonal factors. Migrations, daily commutes, hibernation, dormancy, growth cycles, breeding seasons of animals and growth and fruiting and flowering of plants are examples of adaptations that have been favored by the seasonal variations of climate on Earth. The interactions between plants and animals, as well as animals and animals and the life cycles of insects as plant pollinators and animal food sources are all correlated with the seasons. These also are influenced by seasonal and latitudinal variations in precipitation, temperature, winds, and ocean currents.

The most important factor affecting climates and seasons is the amount of sunlight incident on horizontal surfaces on the curved globe. This depends on latitude, rotation and tilt of the Earth, and the Earth's location in orbit about the Sun. The atmosphere absorbs and scatters sunlight and transfers heat and moisture via clouds, fog, winds and rain. Atmospheric absorption and scattering depend on the cloud cover and the slant angle of incident sunlight. Losses are greater at latitudes and times of day when the Sun is low in the sky. Surface waters (oceans, large lakes, and rivers) affect climate by reflecting and absorbing sunlight, evaporating water, moderating heat, and responding to winds and currents. They also can redistribute organic and inorganic material as well as living plants and animals. Polar ice caps reflect sunlight and remove liquid water from the Earth's surface. Landforms such as mountains, plains, and canyons redistribute materials due to erosion by wind and water and deposition of soil. Volcanoes distribute materials on the land and in the air, affecting the optical properties of each. Soil, rock, and vegetation absorb and reflect sunlight depending on color and brightness. Plants retain moisture and soil and moderate temperatures.

Archaeoastronomy

In the Old World, the great Egyptian pyramids and England's Stonehenge (50 ton stones transported over 100 miles) were built about 4000 to 5000 years ago. The pyramids are oriented to the Sun. Egyptians worshipped a Sun deity. The summer solstice Sun rises over the heel stone at Stonehenge. There are now alignment problems because the Earth's axis precesses once every 26000 years. The Pantheon (120 AD) in Rome has a circular aperture in the dome to let in a shaft of light that hits certain spots at the solstices and equinoxes. The North portal is aligned to the rising morning star, Venus, on her day, April 1, which is 8 days after the equinox. (Caesar claimed to be descended from Venus.) Venus is the goddess of the garden and spring. The Roman or Julian calendar is essentially the one still in use.

The New World also has pyramids and a Woodhenge oriented to the Sun, and a site where a shaft of light hits a mark inside a chamber. Most New World sites date from 1000-1500 AD. The Chichen Itza pyramid is tied to the calendar and has 4 x 90 steps. It is oriented so that the stone serpent along the steps appears to undulate at the equinox because of the interplay of sunlight and shadow. The Caracol Tower at Chichen Itza is oriented toward the solstice and looks like an observatory. The Sun Dagger in Chaco Canyon in New Mexico and the Big Horn Medicine Wheel in Wyoming are also oriented to dramatize and observe seasonal variations in solar angle. What is the significance of these great monuments? Can you tell seasons from the weather without tracking the Sun precisely? No, because weather is too variable from day to day and year to year. If you decide to plant a crop based on weather you may be way off and suffer significant penalties.

The Chumash celebrated the fall harvest with ceremonies. They celebrated the Winter Solstice with the sunstick made of a steatite disk on an 18" stick. Window Cave located on Vandenberg AFB is a prehistoric site of Chumash winter solstice ceremonies. The cave happens to be oriented so that sunlight falls onto ancient rock art symbols in the interior on the winter solstice. The solstice is critical to survival: if the Sun does not turn around, it may be lost. Tragedy follows if the rains don't return also. Would the summer solstice have been important in their lifestyle? How often did they even see the Sun in summer along the foggy coast?

Additional Reading Related to Changing Seasons

1. Tjeerd van Andel, New views on an old planet, 2nd ed. Cambridge 1994.

2. Edward Tarbuck & Frederick Lutgens, Earth Science, Prentice-Hall 9th Ed. 2000.

3. Simon Lamb & David Sington, Earth Story, Princeton 1998. BBC Series

4. S. Berthon & A. Robinson, The Shape of the World, Rand McNally 1991. PBS Series

5. World Book, Science Year 2000, Annual Science Supplement, 1999.

6. Robin Baker, The Mystery of Migration, Viking 1981.

7. Anthony Aveni, Ancient Astronomers, Smithsonian 1993.

8. Georgia Lee, The Chumash Cosmos, Bear Flag 1997.

Go to www.calpoly.edu/~rfield for more pages on science and natural history.