Seasonal changes in sunlight happen because the Earth's axis of daily rotation is fixed in space and remains tilted at 23.5 degrees as the Earth circles the Sun annually.


The Equinox: A Time of Change and Challenge

Bob Field 2000

Today September 22, 2000 is the Autumnal Equinox. The summer and winter solstices get all the publicity, but this is a special time of year, too. It is a time of great change and provides great challenges for the survival of the plants, animals, and human inhabitants of the Earth.

Locally, we barely notice it as we go about our busy lives. Summer is over and school schedules affect the lives of students and parents alike. Yet, the coastal weather is often the best of the year, as it is warm, dry and sunny and inland temperatures are mild.

Many animals migrate in the fall or winter. The Monarch butterfly is here from October to February. Many birds leave the northern latitudes for our mild winters. Marine mammals like the elephant seal and the humpback whale migrate for feeding and breeding purposes.

Ancient civilizations around the world labored hard and long to build monuments aligned to seasonal positions of the sun. What is the sun doing to us or for us during the year that produces so many seasonal changes?

Actually the sun is not doing anything different from one season to the next. Rather, the Earth orbits the sun once a year. Since the orbit is nearly a circle, the distance to the sun does not change much seasonally.

To understand the seasons, you have to understand that the Earth rotates daily on an axis tilted about 23.5 degrees relative to the plane of the Earth's orbit. The axis remains fixed in space relative to the stars as the Earth orbits the sun. That is why the North Pole points toward Polaris, the North Star, throughout the year.

As the Earth circles the sun, the North Pole is tilted toward the sun in June and away from the sun in December. The days are very long in summer and very short in winter. Above the Arctic Circle, the sun does not set on the summer solstice and does not rise on the winter solstice.

The Central Coast gets over 14 hours of daylight in late June but less than ten in late December. Throughout the spring and summer, the days are long and the sun is high in the sky because the Earth's axis is tilted toward the sun. The sun rises in the northeast and sets in the northwest.

Because the Earth's axis is tilted, the sun appears to move north or south of the celestial equator during the year. It is directly over the Tropic of Cancer on our summer solstice and the Tropic of Capricorn on our winter solstice. The solstices are the points when the sun appears to stop moving north or south. Solstice means the sun stands still.

On the Equinoxes, the sun rises in the east and sets in the west. The Earth's axis is tilted at right angles to the direction of the sun. Equinox refers to equal night and day because the sun is above the horizon for twelve hours at all latitudes in both the Northern and Southern Hemisphere.

The sun's elevation changes quickly around the equinoxes whereas the change decreases around the solstices. This expresses the essence of the word solstice, which means the sun stands still.

Locally, in the 16 days after solstice, the length of day changes by only six minutes, whereas the length of day changes by35 minutes in the 16 days after an equinox. The change is about six times greater. Thus the equinoxes are times of rapid change and many challenges.

You can track the changes in sunrise and sunset times in the newspaper for the next three months. If you look today, you will notice that the sun is up slightly more than twelve hours on the equinox. Why? Because two optical effects add about eight minutes of light to the equinox day.

Sunrise occurs when the leading edge of the sun appears over the horizon whereas sunset occurs when the trailing edge of the sun disappears below the horizon. This adds two minutes.

Because the atmosphere refracts or bends the rays of the sun, the sun appears above the horizon a few minutes before it rises and after it sets. This adds about five minutes.

Seasonal changes in weather depend on more factors than just the length of day and the elevation of the sun. Late April and late August have the same amount of daylight and the sun's height at noon is the same. Yet August is much warmer than April.

One reason is that weather changes slowly and lags about six weeks behind the solstices and equinoxes because of heat retention and circulation in the oceans and atmosphere.

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