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Physics LibreTexts

2.2 Motions in the Sky

Imagine that it is 40,000 years ago and that you are a nomadic hunter-gatherer. Much of your life is spent in the search for food and shelter. You are familiar with the slanting path of the Sun as it crosses the sky. You know the steady shift of the length of day and night throughout the year. The slow change of the climate gives you clues to the appearance of ripening berries and the migration of herds of animals. The Sun and the stars give you the tools for navigation. At night, by the safety of a fire, you and your tribe weave stories around the changing shapes of the Moon and the patterns of the stars.

We have lost touch with the sky. Most people are only vaguely aware of the cycles of Sun, Moon and stars. Our lives are regulated by watches, and by artificial heat and light. Most of us live in urban areas where the stars are barely visible. However, for much of history this information was essential for human survival.

Here is what someone at northern latitudes would observe over the course of a year:

•In summer, the Sun rises north of due east and the day is longer than the night. In winter, the Sun rises south of due east and the day is shorter than the night.

•The Sun, Moon and planets traverse the same strip of sky from east to west.

•Stars rise in the east and set in the west. They all appear to slowly rotate about a fixed point in the northern sky.
 


Classical constellations of the Zodiac. Click here for original source URL.

• The pattern of stars in the constellations dos not change from year to year.

• Any particular star rises and sets slightly earlier each night. The constellations migrate through the sky completely in one cycle of the seasons.

•Some of the planets move irregularly among the constellations, occasionally reversing their direction of motion.
 


Series of images of different lunar phases animated. Click here for original source URL.

• The Moon changes its phase on a regular cycle. A full moon is high in the sky around midnight and a new moon is high in the sky around midday.
 


Total solar eclipse, revealing the Sun's corona. Click here for original source URL.

• Lunar eclipses are more frequent and last longer than solar eclipses. Neither occurs every month.

The Sun rises in the east and sets in the west, traveling on a slanting path across the sky. In summer, the Sun rises north of due east and the day is longer than the night. In winter, the Sun rises south of due east and the day is shorter than the night. The Sun rides higher in the sky in summer than in winter. Using a distant horizon as a marker, you see that the shifting path of the Sun repeats each time there is a cycle of the seasons. You celebrate the return of the Sun to its higher trajectory because it means that winter is easing its icy grip.

The Moon also orbits the sky along the same path as the Sun but it appears to move at a different rate. When it is close to the Sun, the Moon appears as a crescent with the lit side pointing towards the Sun. When it is on the opposite side of the sky to the Sun it is a fully lit disk. The changing face of the Moon also follows a regular pattern. The Moon is a mysterious object — it is remote yet the level of the oceans responds to it.
 


Photograph of the night sky that captures the movement of the stars in the northern hemisphere. Click here for original source URL.

At night the stars also rise in the east and set in the west. Over the course of a night they rotate about a fixed point in the northern sky. As the seasons progress, different groups of stars are seen above horizon at sunset. The stars migrate completely around the sky during one cycle of the seasons. You note that the colors of different stars vary from a dull red to a brilliant blue-white. Your imagination is captured by the delicate band of light that arcs across the winter sky.

You can notice five bright points of light that move from night tonight among the fixed star patterns. You can locate these "wanderers" in the strip of sky traversed by the Sun and Moon; they do not twinkle. They also have the peculiar habit of occasionally reversing their nightly motion.

You know the sky offers surprises. Every so often the full Moon is darkened to a blood red color and then slowly brightens again. You have been told of a rare but fearsome event when the midday Sun darkens inexplicably. You can vividly imagine the disquiet of animals as they respond to the false night and the sudden chilling of the air.

You need no telescope or advanced timekeeping to measure the cycles of the sky. Humans have made a slow march towards understanding these phenomena. There is a natural reluctance of humans throughout history to accept that the Earth might not be the center and main feature of the universe. Until a few centuries ago, people thought that the celestial objects occupied an "ethereal realm" where the laws were different than on Earth. More recently comes the beginning of the idea that the same physical laws apply to the untouchable objects of space as to mundane terrestrial objects. This is followed by the application of logic and mathematics to the universe. Astronomy is "the first science."