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Physics LibreTexts

2.11: Dividing Time

In mid-northern latitudes, such as in the United States, Canada,or Europe, the Sun rides much higher above the horizon in the summer than in the winter. In the summer, it rises in the northeast, crosses the meridian nearly overhead, and sets in the northwest. But in the winter, it rises in the southeast, crosses the meridian low in the south, and then sets in the southwest. You can mark the passage of the seasons by tracking the position of sunrise or sunset on the horizon. A distant feature like a rugged mountain range makes a simple but effective measuring device.

A calendar is a means of counting the days in a year. Ancient people did this by counting the days until the sunrise (or sunset) position moved back to its extreme northerly (or southerly) position after a cycle of the seasons — called a solar year. The earliest records we have show a count of 360 days in the year. The Egyptians had revised the count to 365 days and they added a leap year — a year of 366 days inserted every fourth year — for an average calendar 365¼ days long. By 2700 B.C., the Babylonians had refined this to a calendar of 365.26 days which was accurate to about 30 minutes in a year. This is an impressive calendar. Nearly 5000 years ago, the length of the year was known to an accuracy of better than one part in ten thousand!
 


Illustration of the position of the earth relative to the sun during specific times of the year, indicating seasons. Click here for original source URL.

Ancient observers divided the year into seasons using four special dates that we still recognize. Winter solstice is the first day of winter, around December 22nd. On this day the Sun rises and sets farthest to the south. This day has the shortest period of daylight of any day in the year in the northern hemisphere. The pre-Christian pagan cultures of England and France began the year on this date, to celebrate the return of the Sun toward the northern sky. Since the seasons vary smoothly throughout the year, it is quite arbitrary when we choose to begin the calendar. Spring equinox is the first day of spring, around March 21st. On this day the Sun rises due east and sets due west. Day and night are equal in length on this day (in the word equinox, equi- means equal and -nox means night). Other pagan cultures, such as those that worshipped the goddess Maia, began the year on this date because it marked the beginning of the cycle of new growth. Summer solstice is the first day of summer, around June 22nd. On this day the Sun rises and sets farthest to the north. It has the longest daylight period of the year. In many ancient calendars, it marked a day of celebration, when the days were long and the weather pleasant. Autumn equinox is the first day of fall, around September 22nd. On this day the Sun rises due east and sets due west and day and night are again equal.

Primitive cultures also marked the midpoints of the calendar between the solstices and the equinoxes. These dates are February 1st, May 1st, August 1st, and November 1st. In Ireland, these festivals are all still celebrated and are known by their Gaelic names: Imbolic, Beltane, Lughnasa, and Samhain. May Day was originally a fertility festival in the pagan world, and it is still a folk festival in England. And of course the eve of November 1st is celebrated in many parts of the world, as All Saints' Day in England, as the Day of the Dead in Mexico, and as Halloween in the United States.

You might think that knowledge of solstices and sunrise positions is useless knowledge from the ancient past. However, it has applications in the present, especially in an environmentally conscious age. Consider the layout of windows in your home or apartment. Windows facing northeast or northwest allow sunlight to enter on summer mornings or afternoons, when you are likely not to want extra heat in the house. Draperies or shades on these windows in the summer will block sunlight and save on energy bills. Windows facing southeast or southwest, on the other hand, will let in the low-angled light of the winter Sun, giving a free input of extra heat. In a house specifically designed to take advantage of these ideas, windows are placed to take advantage of the midwinter morning and afternoon Sun on the southeast and southwest side. Trees might be planted to shade the northeast, east, west, and northwest sides of the house during the summer mornings and afternoons. Roof overhangs shade southern exposures from the high summer midday Sun, while letting the low winter midday Sun warm the interior and exterior of the home.

Most of the major divisions of time have astronomical origins. The illumination cycle of the Moon gives rise to the month. Every 29½ days there is a full Moon or a new Moon, and this cycle divides approximately 12 times into the solar year. We have found carved animal bones and other artifacts dating back 20,000 to 30,000 years in France and other parts of Europe. Some of these objects have numbers of notches that indicate that the cave dwellers were using them to count months. These portable calendar sticks are among the oldest human relics.

There is a difference between time kept using the Sun and Moon, and time kept using the stars. The time taken for the Sun to pass through the meridian on successive days is a solar day. The time taken for a star to pass through the meridian on successive nights is a sidereal day. Since every star rises and sets a little bit earlier every day, a solar day is about 4 minutes longer than a sidereal day.

Similarly, the time taken for the same phase of the moon to recur is the Moon’s synodic period — 29.5 days. This is longer than the time taken for the Moon to pass the same place among the stars — the Moon’s sidereal period is 27.3 days. These differences occur because the stars that would be seen in the direction of the Sun shift gradually as the Earth spins and as the Moon orbits the Earth.

Once ancient people recognized the fixed pattern of the constellations, they discovered that five bright "stars" were different from all the others. These "stars" moved from week to week relative to the pattern of the fixed stars. They became known as planets, from the Greek word for wanderer. The planets had unusual attributes. For example, they were not found any place in the sky but always in the strip of sky occupied by the Sun and Moon. Some planets were only seen close to the Sun, others were seen far from the Sun. Some planets would even reverse their direction in the sky when viewed from week to week. Our names for the planets come from the Roman gods Mercury, Venus, Mars, Jupiter, and Saturn. (The other planets — Uranus, Neptune, and Pluto — were not discovered until the invention of the telescope, and the Earth was not yet recognized as one of the planets.)

Every culture has felt the need to create a chunk of time like a week. Ancient Egyptians used 10 days, the Babylonians used 7 days, the Assyrians used 5 days, and some West African tribes have used 4 days in the week. Our calendar is based on Roman tradition, which named the seven days of the week after the seven "moving" objects in the sky — the Sun, the Moon, and five planets. Some of these names are obvious: Saturn-day, Sun-day, Moon-day. The connections to other planets are clearer in languages that are derived from Latin such as Spanish, Italian, and French. Tuesday is Mars-day for example (Mardi in French, and Martes in Spanish). Wednesday is Mercury-day (Mercredi in French, and Miercoles in Spanish), Thursday is Jupiter-day (Jeudi in French, and Jueves in Spanish), and Friday is Venus-day (Vendredi in French, and Viernes in Spanish). So what happened to the names of these four days in English? They were named after gods from the Anglo-Saxon culture of a thousand years ago. Tuesday comes from Tiw, the Norse god of war. Wednesday comes from Woden, the supreme deity. Thursday comes from Thor, the god of thunder. And Friday comes from Frigg, the wife of Woden and goddess of love and beauty.

Even the division of the day into hours has an astronomical origin. Egyptian astronomers used a sequence of bright stars across the sky for timekeeping at night. Twelve timekeeping stars were visible during the critical midsummer period when the Nile would flood so the night (and later the day) was divided into twelve hours. Timekeeping was very primitive until the last 250 years. The Greeks used sundials and the Romans perfected the water clock, where water could drip at a regulated rate though a small hole in a hard stone or jewel. The sand hourglass dates from 8th century Europe. Nobody could divide hours into minutes and seconds until the pendulum clock was invented in the 17th century. We can thank the Babylonians of 5000 years ago for the division of the hour into 60 minutes and the minute into 60 seconds.

Many other features of our calendar and timekeeping spring from the pagan cult of Sun worship. Stonehenge and other great prehistoric structures were built to measure and celebrate the motions of the Sun. Many pagan traditions were borrowed by the early Christian calendar that we still follow. The year starts on January 1st. This copies the pagan cultures which began their calendar when they could detect the Sun beginning to move further north in its rising and setting position. Our rest day of Sunday follows the pagan day of worship of the Sun. Why do clocks move clockwise? In northern Europe clocks were designed to mimic the arcing motion of the Sun from left to right across the southern sky. Our habits of timekeeping are a rich brew of astronomical ideas taken from earlier cultures.