The calendar

Astronomical observations have probably been made since the Stone Age. People were looking for ways to find their way in the world. After the rising and setting of the sun and moon had been connected, it was not long before this constant movement was also mentally transferred to the stars. All the stars seem to orbit a certain point in the night sky, the Celestial North Pole. Depending on the season, the celestial bodies rise on their orbit to a certain height above the horizon line. Using the sun as an example, its highest point during the day is fixed as noon. Depending on how high the sun rises, it rises earlier and sets later. As early as the Neolithic Age, astronomers recognized that there are four distinctive points in the year:

The summer solstice, when the sun, after rising higher and higher, begins to sink again.

The winter solstice, when the sun, after sinking further and further, begins to rise again.

The two equinoxes, when the sun is so high in the sky that day and night are the same length.

Many cultures aligned their most important religious festivals with these days. This cycle was also suitable for describing the seasons. Since the invention of agriculture and animal husbandry, it has been very important to be able to predict the right day for sowing and harvesting, regardless of the weather. For shorter periods of time, the phases of the moon were used, which only took about 28 days for a cycle. In many calendar systems, attempts were made to reconcile the solar cycle and the lunar cycle. To achieve this, it was important to measure the exact length of the year, i.e. the time it takes for the Earth to orbit the Sun. With these calendars, early advanced civilizations were able to pinpoint religious ceremonies to specific days of the year.

 

The sundial

For most people, timekeeping accurate to the hour was important in everyday life. The main aim was to measure working hours and to observe prayer times in the temple. The sundial has been known since about 3500 BC in ancient Egypt. It is based on the principle of letting the shadow, which is cast by a rod (gnomon), for example, hit a surface with markings and thus indicate the time of day. Over the day, the shadow of the rod moves across the dial with the sun. Only 12 hours are displayed on a sundial. At night, she can't tell the time. However, the length of these hours varies with the length of the day. In June, an hour can be up to 75 minutes long, and in December only 45 minutes. Depending on how the sundial is built, the length of the shadow can also be used to indicate the day of the year relatively accurately. Today, sundials are mainly decorative elements in architecture.

For a sundial to work, you need to know the place where it will be placed. The shadow cast by the rod, which later shows the time, not only changes over the course of a year, but the latitude at which the sundial stands determines how short or long it can be. The higher the latitude, the less high the sun is above the horizon on the shortest day of the year. If you cross the 60th parallel, the sun does not rise above the horizon at all on this day.

Longitude also plays an important role. Two sundials in different cities never match. That's because the sundial shows noon when the sun is at its highest in that place. As soon as you travel with a pocket watch, however, you must set your watch back to the local solar time at your destination. When trains began to run in the 19th century, the need for uniform timekeeping became apparent. It was agreed to set up 24 time zones, at the transition of which the clock must be changed by one hour at a time. Today, the time zones are divided into zones of 15° longitude each around the equator from Greenwich near London. However, the exact affiliation to a time zone is often determined by states by law, so that the regular division of time zones only applies nationwide on the high seas. It was also determined that outside the Earth – in space – Greenwich time applies.

 

Timekeeping in astronomy

In addition to the sun and moon, many stars can be seen in the night sky. As early as 4,000 years ago, the Babylonians realized that not all points of light in the night sky move along seemingly fixed paths. These wandering stars were called planets. Some of them can only be seen just before sunrise or after sunset. Others move across the sky all night. Sometimes they come closer to each other, then they move away from each other again and Mars moves backwards in a loop every now and then. The Babylonians have already tried to calculate when there would be encounters between the planets again, as they attached great importance to these events. At night, the stars do not stand still. They seem to revolve around the north celestial pole, and if astronomers wanted to record their position, they had to record not only their height above the horizon, but also the time.

 

The Water Clock

To measure time without the sun, ancient astronomers used a clepsydra – a water clock. It consists of a vessel filled with water and a pointer mounted on a float. If you let the water run off constantly, the pointer moves down on a scale and thus shows the time. Other ancient timekeeping techniques included hourglasses, oil lamps with marked oil levels, and even incense clocks in China, where the burning time of incense sticks was used to measure time. Even in ancient times, hours of the same length were always used in “scientific timekeeping”, and these were then divided into minutes and seconds.

 

Mean Solar Day and Sidereal Day

Astronomers still use a different day length for calendar calculation. For an everyday calendar, it is sufficient to measure the length of the day from the time of the sun's highest position to the time when the sun is at its highest again. This period was divided into 24 equal hours and still forms the basis of timekeeping in everyday life today. In astronomy, the "sidereal day" is used instead of the "mean solar day". It does not measure the time between two peaks of the Sun, but between two peaks of a star. Since the earth revolves around the sun, the earth must rotate a little more than 360 degrees every solar day until the sun is at its highest again, as it has moved on in the meantime. The stars, on the other hand, are much further away than the Sun and the effect that leads to a longer solar day is hardly measurable. By the way, a sidereal year is always exactly one day longer than a solar year.

With the invention of the telescope and the wheel clock, astronomers were able to measure the positions of stars much more accurately. New theories such as Kepler's laws and Isaac Newton's law of gravity also made it possible to predict the positions of celestial bodies. It was now possible to predict the positions of planets so precisely that astronomical events, such as the transit of Venus in front of the Sun, could be predicted. To observe such a passage and thus calculate the distance between the Earth and the Sun, Captain James Cook and a team of astronomers were sent on a research trip to Tahiti in 1768. This measurement took a decisive step forward in understanding our solar system.

 

The sculpture

Time moves along the arrow of time only in one direction. Humans have always tried to describe this process and have developed different ways of measuring time. The sculpture below artistically describes how sun and lunar calendars, sundials and mechanical clocks, hourglasses and atomic clocks are used for this purpose.

 

500 m to the last station

212 m to Pluto