A property of nature is time. We can become aware of the passage of time by observing repetitive phenomena, such as the movement of a pendulum or the succession of day and night. Perhaps one of man's oldest concerns has been to understand what time is and to try to measure it. To this end, throughout history, he has built an immense variety of clocks, from sand clocks to those we know today, clocks that have become more and more precise, capable of measuring tiny fractions of a second, something that was unthinkable in ancient times.

Since ancient times, hourglasses and clepsydra invented by the Egyptian and Chinese cultures have been used, the time lapses corresponding to the time it took for a quantity of sand or water to pass from one container to another through a small orifice. In fact, a coffee pot where a fixed amount of water drains from a funnel-shaped container into a pitcher is a clepsydra. Another form that was widely used consisted in the manufacture of candles of a certain length that when consumed measured a certain time.

Hourglasses, clepsydras, and candlesticks all measure time intervals imprecisely.

One phenomenon that gives us a notion of time is the succession of day and night. Observing the position of the Sun in the sky, we can have an idea of the time that is near the Equator, the Sun rises at dawn on the eastern horizon, at about 6 am, at noon it is near the zenith and at sunset, it goes into the western horizon, at about 6 pm. It has been determined that this lapse is a unit of measurement of time and we call it day, we have divided it into 24 hours approximately 12 hours of light, and 12 of darkness, using dozens, units used by the Chaldeans to count.

The ancient Egyptians believed that every day the sun god Ra was born and sailed along with the celestial vault in a sacred boat. This myth dates back some 4,000 years. Every morning the boat sailed through the sky, which for them was an ocean, leaving the eastern horizon. At night, the boat would enter the western horizon and continue its journey through the underworld, where it would die. The Greeks had a similar idea. For them, Apollo drove a cart pulled by three horses through the sky, and thus managed to travel the sky in one day.

As man organized himself into communities and had favorable conditions to develop a civilization, he constructed large buildings and structures designed to provide information about the passage of time. The Egyptians, for example, used sundials of two types - obelisks, which are huge sculptures, and others, of a personal type, in the shape of a T. The latter were placed facing east so that in the morning the shadow of the horizontal bar was large and as the sun rose, the shadow was shortened until it disappeared at noon; then the instrument had to be turned so that the T now pointed west so that the shadow gradually grew until sunset.

It is very probable that the invention of the sundial constituted by a rod nailed somewhere, was a Chaldean invention (300 B.C.); later this clock was popularized in Greece and Rome. Furthermore, the Romans brought Egyptian obelisks to Europe, which they then continued to use as sundials and popularized their portable use.

It is worth mentioning that if the Earth were flat, all sundials would produce, at the same time, an equal shadow, regardless of the latitude where they are located. Eratosthenes, who lived in Egypt before our era, noted that the Earth is round because, at noon, sundials at different latitudes cast shadows of different lengths.

So far we have described the operation of sundials placed at latitudes near the equator. What would happen with a sundial placed near one of the poles? During the six months when it is practically only night, it would be useless! Since the Sun seems to move around the horizon throughout the day and never sets, the shadow of the sundial would circle around it, like the hands of the clock. Now, a clock parallel to the ground, like the T-bar, would cast a shadow so far away that it would be of little use.

Despite their precise construction, sundials have two major problems: they are useless at night or when it is cloudy.

Seasons

In this section, we will discuss the seasons in some detail. To do so, we will analyze what happens on other worlds and on Earth due to the different inclinations of their respective rotation axes. Let's look at some explanations of simple but necessary things to understand what happens.

All planets have two movements: the first one is the rotation movement, which consists of the planets spinning around themselves like spinning tops; the second one is the translation movement, that is, the planet revolves around the Sun. Both movements happen in the same plane, it is something like being in the "cups" of a fair. The only exceptions are Uranus and Pluto, which rotate in a different way from the rest of the planets; this makes them look as if they were moving on the platform of the horses: these planets "roll on their axes".

The rotational motion is made by the planets on an imaginary line called the axis of rotation. The direction of the axis of rotation remains the same as the planet moves around the Sun. In other words, the direction of the axis of rotation does not change.

The direction in which the axis of rotation of each planet points, combined with the translational motion, gives rise to the seasons. To understand how they are produced, let's see what the seasons are like on different planets. First let's try to explain two extreme cases: Jupiter, where the axis of rotation is not tilted, and Pluto, where the axis of rotation is practically lying down.

During its path around the Sun, Jupiter's axis of rotation is always perpendicular to the plane of translation, as if its axis were "stationary". Because of this, when Jupiter rotates, the amount of light falling on each point of its surface is always the same, no matter where it is in its orbit. That is, day and night last the same throughout the year. However, on Jupiter, as on the rest of the planets, the insolation is greater at the equator than at the poles because it depends on the angle at which the Sun's rays enter. When the Sun is at the zenith, its rays are hotter than when they are at the horizon.

In Pluto the situation is different; its axis of rotation is completely tilted. Let's see Pluto along its orbit when Pluto is in a position where its north pole points almost toward the Sun. Although it is rotating, sunlight falls almost always on the same half of the planet, and the other half is always dark. On the sunlit side, it is summer, and on the other side it is very cold, it is winter.

When Pluto is in a position where the south pole is the one pointing towards the Sun, now this side is always illuminated, while the north side is always dark. That is, it is now summer at the south pole and winter at the north. During its orbit Pluto has intermediate positions; due to Pluto's rotational motion, throughout the day the entire surface is illuminated.

As it can be seen, in Jupiter there are no seasons and in Pluto there are, and they are extreme. What happens on Earth? Well, it happens that the Earth's axis of rotation is not as inclined as Pluto's, but it is more inclined than Jupiter's. This is why there are seasons in Pluto. This is why there are seasons on Earth. In other words, on Earth, the situation is neither as boring as on Jupiter, where there are no seasons nor as extreme as on Pluto, an all-or-nothing situation. The Earth's axis of rotation is tilted about 23 degrees so that alternately the northern and southern hemispheres receive more solar radiation.

Sunrise and sunset

Although in reality, it is the Earth that revolves around the Sun, we feel that the Sun revolves around us, so that we can follow the Sun's path through the skies of our planet or imagine what it would be like if we were on the surface of other worlds. We can think about where the Sun would be positioned at midday on other planets during the different seasons, first if we were an inhabitant of the equator and then if we were at the poles. Let us begin with Jupiter: an inhabitant of the equator would always see the Sun overhead at noon, that is, at the zenith. Moreover, he would invariably see that the Sun rises exactly in the east and sets in the west. An inhabitant of the pole would always see the Sun near the horizon, no matter what day of the year.

On Pluto, an inhabitant of the north pole would see, in summer, that the Sun is above his head; even if the planet rotates, the Sun would still be above his head. In the spring the Sun would be on the horizon, but during the winter the Sun would not rise at all, it would be in the opposite direction to his feet, no matter how much the planet rotates. An inhabitant of the equator, in spring, would see that the Sun is at its zenith at noon and that it would rise from the east to set in the west. As his year progresses, he would notice that at noon the Sun approaches the horizon. The Sun would rise and set further and further to the north or south depending on the seasons.

On Earth, an inhabitant of the equator has the Sun at the zenith at noon during spring and autumn and the Sun rises in the east and sets in the west. In summer and winter, at noon, the Sun is +23° and -23 5°.

In conclusion, observing repetitive phenomena such as the succession of day and night has allowed us to measure what time is. This has been done since ancient times using various instruments. One of the repetitive events is the passing of the seasons, produced by the tilt of the Earth's axis of rotation.