Updated June 01, 2013

Astronomy and time

Anguish of the next day

Human societies in the past have always sought to locate in time, perhaps driven by the anguish of the next day but also by the need to understand the world in which they lived. Like today, there are thousands of years, they already raised the question of their environment and without astronomy they were lost in the immensity of the Universe, astronomy is a need for men and it was vital for the human civilizations of the past. Today the notion of time is included in our civilization, no longer arises the question of date, season, month, day and time are always displayed before our eyes, these data are no longer to calculate, the clocks do for us. Our ancestors studied the heavens to find recurring events to establish a clock and it is by observing the sky they have do. In fact, the sky is a reference, the seasons are marked by the cyclic appearance of stars, the sun marks the day, the Moon marks the month and week. With the cycle of the moon, our ancestors landmark first repetitive pattern, month, besides the word "mens" in Greek in its primitive sense means "moon." Our ancestors noticed that the seasons are connected to the stars and they draw the constellations seeing imaginary figures reassuring, corresponding to familiar images, especially those of animals they know. The configuration of stars changes with the seasons, because inclination of the Earth exposes to us a different sky throughout his journey of 365 days in its orbit. This cyclic journey is a wonderful clock of one year on which we organize. Understand its environment is an advantage for civilizations then they build gigantic monuments marking time (Stonehenge, Pyramids, Chaco Canyon,...). Solstices, the longest day and the shortest of the year, are a reference, a marker for the season and the year.

The date of the solstice varies very slowly, it does not change significantly over several hundred years, Earth finds itself always in the same place in its orbit at the same time. The celestial vault still shows the same figures at the same time of the year. This precision "divine" of the celestial clocks helped past civilizations (Inca, Mayan, Egyptian and Greek) to organize and plan their tasks largely agrarian.
Observing the sky, the first civilizations to explore the concept of time, the sun, moon and stars have played an important role for them, they were able to measure the time and explain the mysteries of their world. With knowledge of the solstices, two centuries BC, Eratosthenes of Cyrene (-276 -194 BC. JC) shows that the Earth is huge and it is more round. With a piece of wood he remarkably measured the diameter of the Earth showing that it was a sphere. He planted the stick in the ground of Siena in southern Egypt and he noticed that the summer solstice when the sun was at its zenith, the wooden stick had no shade while at his home in Alexandria at the same time the stick had a shadow. By measuring the size of the shadow he gets the circumference of the Earth to a few hundred kilometers near. Indeed, depending on the length of the shadow it calculates the angle (7.2°) with the vertical line and obtains the fiftieth of a circle, knowing the distance between Siena and Alexandria, he multiplied by 50 and obtained the equatorial circumference of the Earth. This measure is extraordinarily precise fort this period it is of 39689 km instead 40075 km.

NB: Time is linked to the apparent movement of the Sun, it is midday when the sun is highest in the sky, the hour is different at each point of the Earth.

Image: Through knowledge of the solstices, two centuries BC, Eratosthenes of Cyrene (-276 -194 BC) shows that the Earth is enormous and in addition, it is round. It measures the circumference of the Earth with extraordinary precision for this epoch, its measurement gives 39689 km instead of 40075 km. Image taken by Apollo 8, the first mission to be carried men beyond earth's orbit between 21 December and 27 December 1968.

Meaning of the days of the week

The division of the week into seven days is approximately a quarter of a lunar month lasts 28 days, in fact the sidereal revolution of the moon is 27 days 7 h 43 min 11.5 s. According to the International Organization for Standardization, Monday is considered the first day of the week (ISO 8601). Weeks in a year are numbered from 01 to 52 or 53 depending on the number of Thursday, the week number 01 is the one that contains the first Thursday of January. A week has seven days whose names are connected with astronomy. Astronomers from the Roman period only see six bright objects that moved in the sky. These objects were, Saturn, Jupiter, Mars, Venus, Mercury and Moon. This is one explanation for the origin of the days of the week. Monday is the day of the moon, Tuesday, the day of Mars, Wednesday, the day of Mercury, Thursday, day of Jupiter, Friday, the day of Venus, Saturday, day of Saturn, and Sunday is the day of the Sun. It was discovered planets Uranus and Neptune, and later in the 18th and 19th century, with the invention of the telescope. Other celestial objects seen from Earth does not move, by cons observers can see, month by month, the planets move through the sky.

Moon is moving fastest. On Sunday, the seventh day of the week was added to the week by the Emperor Constantine I in 321. In France, Sunday is not linked to the name of a celestial object, but it is the God's day for Catholics.
The division of the week into seven days is roughly a quarter of a lunar month which lasts 28 days, actually 27 days 7 h 43 min 11.5 s.