One of the well-known questions in meteorology is “What is the cause of seasons?” Many related secondary questions occur. Are any facts about the motions of Planet Earth, its speed in orbit, its path through space, and the precise location it occupies at any given time, truly trivial? These questions connect with the cause of seasons.
Before one proceeds very far with formal education, teachers introduce scientific lessons on astronomy and weather. With respect to the cause of seasons, weather and astronomy are integrally related. Simple concepts are discussed first, such as the basic facts of our solar system—our Earth as a sphere and its motions of rotation and revolution. These facts and motions are not perceptible to observers on Earth. But our lives depend primarily on perception of real motion—motions that actually occur. In astronomy students must become aware of the difference between real and apparent motion—not always an easy distinction. They must learn that most perceived motions of the Sun, Moon, and stars are not real. Rather, they are apparent, caused by the real motion of Earth’s rotation on its axis and the real revolution of Earth in orbit around the Sun.
How do educators deal with the challenge? They use examples of the distinction between real and apparent in real life: One of the best examples is a video of how occupants on a train interpret motion of passengers waiting on the platform to board. When people outside the train change positions, is their motion real? or is it apparent? If outside passengers actually move, their motion is real. If the platform passengers do not actually change positions when the train starts to move imperceptibly, their motion is only apparent. An added challenge arises if both train passengers and platform occupants actually move.
Similar problems of distinguishing real from apparent motion occur in determining seasonal beginnings and endings of astronomical spring, summer, winter, and autumn. (Readers may review the precise relationship of “points, times, speeds, motions, and paths in space” related to Planet Earth as discussed in our previous 3/6/19 post.) Why is astronomical summer 94 days long while astronomical winter is only 89 days long? This curious fact relates to the speed of revolution of Earth in its orbit. Since our planet possesses an elliptical orbit, there is a position where the Earth travels slightly slower. This is called aphelion, the point of maximum distance. There is also perihelion, the point of minimum distance, when Earth travels faster by 3.34%. At aphelion, the point of maximum distance, Earth travels somewhat slower.
At aphelion the Sun’s gravity is slightly weaker so the Earth does not “fall” as quickly. Earth’s orbital speed is slightly diminished. When Earth approaches the Sun a little closer at perihelion, the planet “falls” more quickly. Its orbital speed increases slightly. The difference in speed in units of miles/sec is 18.21 mi/sec vs 18.82 mi/sec. Earth hurtles along in its orbit, on average, at 67,000 mph. These statistics fascinate most students of astronomy and weather trivia.
Our home planet is actually somewhat closer to the sun in winter, 91 million miles compared with 94.5 million miles. This seems counterintuitive. Perihelion occurs in early January and aphelion in early July. The effects of Earth’s 23.4º axis tilt are far more significant in terms of the onset of summer and winter environments. Likewise, astronomical summer’s 94 days compared with astronomical winter’s 89 days is essentially insignificant because few residents are counting the days.
When one first learns that we live on a spherical planet, that fact is difficult to discover from a personal physical point of view. We may be overcome with wonder. “How do we know?” some may inquire. Similar questions may be generated as they discover most motions of the Sun, Moon, and stars are not real, but apparent. Skilled teachers should use these occasions to teach young scholars modern methods of science discovery.
Most planets in our Solar System and the thousands of extrasolar planets discovered so far have axis tilts. Earth’s axis tilt makes life for 7.5 billion residents possible. As of March 1, 2019, 3999 confirmed exoplanets in 2987 planetary systems have been discovered in our Milky Way Galaxy. A Wikipedia entry states there are 200 billion stars in our galaxy of which 11 billion may possess habitable planets. This figure is incredibly optimistic. Of 3999 confirmed planets not one has come close to Earth conditions which support complex, wondrous life of millions of different species. There are hundreds of unique, fine-tuned, life-supporting parameters on Earth. Absence of any of these conditions would preclude the possibility of life.