by Carol A Westbrook: It all began about 4.5 billion years ago, give or take a few millennia. The earth was still young, having been formed by the accretion of material that orbited around the sun. It was a young, moderate-sized planet, and, like the other planets in the solar system, it spun like a top as it rotated around the sun, with an axis that was parallel to the sun’s rotational axis. This configuration was the most stable for the planets in the solar system, though today only Mercury and Jupiter still have an axis that is parallel to the sun’s.
On that day, 4.5 billion years ago, an erratic planet about the size of Mars, called Theia, came crashing through the solar system and collided with the young Earth. This was not the same collision that killed most of the dinosaurs, which happened about 66 million years ago. The collision of the two planets resulted in the formation of the moon. But there was another significant result of this collision – the Earth’s axis shifted, from a position parallel to the sun’s, to one that is off by 23°! This tilt is the reason we have seasons.
The change in the earth’s axis away from parallel to the sun means that during the year, with Earth’s rotation around the Sun, the northern hemisphere will be pointing toward the Sun for about half the year, warming the earth and giving us summer. In the other half of the year it will be pointing away from the Sun, giving us cooler temperatures, or winter. The transitional seasons, spring and fall, are accompanied by more neutral temperatures.
Now, fast forward 4.8 billion years. No longer hunter-gatherers, humans now live a more settled existence. With it came civilization. They became more settled, relying heavily on agriculture and animal husbandry for sustenance. Farmers looked for reproducible patterns to help their ventures succeed. It was important to know when spring would arrive; when was lambing season; when were the rains going to start, and so on. They relied on their priests and holy men to give them this information, and priests used calendars derived from astronomy to make their predictions.
Often large, complex monuments were built to help follow the movement of the sun and predict the important days in the calendar. The most familiar, and elaborate, of such monuments is Stonehenge, located on the Salisbury plain near the town of…