Solstices and Equinoxes

At the Christmas and New Year holiday season Earth residents celebrate significant astronomical milestones.

Annual calendars reference seasons and lesser known events such as solstices and equinoxes. We become aware of environmental phenomena such as seasonal changes resulting in changing temperatures. Each year about December 22, Northern Hemisphere residents experience the winter solstice. Events such as solstices and equinoxes, important topics in the study of astronomy, occur twice per year.  

The winter solstice locates the season of the longest daily period of darkness and the shortest daily period of daylight during the calendar year. At about 40º N latitude, longest darkness translates to approximately fifteen hours of darkness and nine hours of daylight. The solstice also roughly locates the dates on which earliest sunset times pair with latest sunrise times.

We are aware of long and short daylight as well as early sunsets and late sunrises depending on our latitude. On a summer visit to Anchorage, Alaska, we visited stores open for business at 10:30 PM. Farther north we could have experienced “the land of the midnight sun.” Variations in day/night lengths and sunrise/sunset times depend on latitude and the time of year. In the northern hemisphere we experience the winter solstice in December. Our southern hemisphere friends experience their summer solstice in December, because their seasons are reversed.          

Some may not be familiar with the effect of changing angles of sunlight striking the earth’s surface from season to season. Summer’s direct rays provide more heat than winter’s indirect rays. In winter, the same amount of heat energy is spread over a larger area. Diluted heat energy results in lower temperatures. All of these effects contribute to seasonal temperature variation. These temperature phenomena provide topics of conversation for friends and neighbors.

Meteorology and astronomy are innately related. Those sciences provide plentiful opportunities for studying and understanding cause and effect relationships. Astronomy is considered an earth science because what occurs in space affects events on earth. There are fascinating interconnections between the different sciences. For example, the solstices mentioned above are astronomical phenomena, but they significantly relate to weather and climate.  

Teachers of astronomy use a number of devices to clarify astronomical concepts. One device was a celestial sphere consisting of a transparent plastic globe. Inside was a small earth globe students could manually rotate. On the plastic sphere prominent stars and constellations of the sky were imprinted. Students were able to visualize that they were located on the small earth globe insidethe large sphere. In real life, earth residents gaze at the night sky and imagine individual stars and groupings of stars called constellations located on an imaginary sphere surrounding our planet. In reality, all celestial objects are located at various distances from our planet—some close, some light years away.

The celestial sphere model helps us understand some of the most important astronomical concepts. On our classroom celestial sphere model there were two great circles. One great circle was the “celestial equator” formed by extending the plane of the earth’s equator out to the plastic sphere. Another great circle was formed by extending the plane of the earth’s orbit out to the celestial sphere. (We cannot visualize the plane of the earth’s orbit from our plastic celestial globe model. But we can see where the plane of earth’s orbit crosses the model’s celestial equator.) The two great circles cross each other twice at an angle of 23.5º because earth’s axis of rotation is tilted 23.5º from the plane of the earth’s equator.

On our celestial sphere model, astronomers have plotted celestial coordinates of two great circles—the celestial equator and the ecliptic. The eclipticis the great circle formed by the path traced by the position of the sun throughout the year. We link a post from 10/18/2011 which explains the concept of the great circle in our skies called the ecliptic:  The term “zodiac” helps us understand the concept of the ecliptic.

The ecliptic locates an infinite number of sun positions. It also locates several solar positions EXACTLY ON, at the HIGHEST POINT, and at the LOWEST POINT in relation to the celestial equator. When the sun is located exactly ON the celestial equator we experience an equinox—equal night and day. When the winter solstice occurs, the sun apparently stands still for a few days at its lowest point above the horizon, neither rising nor falling. The sun is close to the horizon and at many latitudes the temperature is COLD. Six months later when the summer solstice occurs, the sun again stands still for a few days at it highest location above the horizon. The sun is highest in the sky and the temperature is WARM.

If the axis of the earth were not tilted there would be no equinoxes, no solstices, and no seasons. We close our discussion of solstices and equinoxes with a link to our 2017 post—Earth Without Seasons. There is much to learn about God’s master plan to provide for the physical welfare of teeming billions of Earth residents. God provides for not only man’s spiritual welfare but also man’s physical welfare.

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