# Time and Calendar

In very ancient times, people did not have a correct idea of the shape and size of our planet and what place it occupies in space. Now we know that the physical surface of the Earth, which is a combination of land and water, is geometrically very complex; it cannot be represented by any of the well-known and mathematically studied geometric figures. On the surface of the Earth, seas and oceans occupy about 71%, and land – about 29%; the highest mountains and the greatest depths of the oceans are negligible compared to the size of the entire earth. So, for example, on a globe with a diameter of 60 cm, Mount Everest with a height of approximately 8840 m appears as just a grain of 0.25 mm. Therefore, the body, limited by the surface of the oceans, in a calm state mentally continued under all continents, is taken for the general – theoretical – form of the Earth. This surface is called a geoid (geo – in Greek “earth”). In the first approximation, the figure of the Earth is considered an ellipsoid of revolution (spheroid) – a surface formed as a result of the rotation of an ellipse around its axis.

The dimensions of the terrestrial spheroid were determined repeatedly, but the most fundamental of them were established in 1940. Soviet scientists F.N. Krasovsky and A.A. Izotov. In 1964 by the decision of the International Astronomical Union (MAC) for the terrestrial spheroid, the major semi-axis was adopted, perpendicular to the minor axis and lying in the plane of the Earth’s equator a = 6378.16 km, the minor axis of the Earth’s spheroid, coinciding with the Earth’s rotation axis at = 6356.78 km. The rotation of the Earth around its axis can be proved in various ways.

In ancient times, people believed that the Sun, moving relative to the stars, circled our planet in a circle for one year, while the Earth seemed to be motionless and located in the center of the universe. Such a system is called geocentric. A new stage in the development of astronomy begins with the publication in 1543. books of N. Copernicus “On the rotation of celestial bodies”, which sets out the heliocentric (Helios- “sun”) system of the world, reflecting the actual structure of the solar system. According to the theory of N. Copernicus, the center of the world is the Sun, around which the spherical Earth and all similar planets move, and moreover in the same direction, each rotating relative to one of its diameters, and that only the Moon rotates around the Earth, being its constant satellite, and together with the latter moves around the Sun, while approximately in the same plane.

To determine the position of various bodies in the celestial sphere, it is necessary to have “reference” points and lines. And here, first of all, a vertical line is used, the direction of which coincides with the direction of gravity. Continued up and down, this line crosses the celestial sphere at points Z and Z ’, called zenith and nadir, respectively.

A large circle of the celestial sphere, the plane of which is perpendicular to the ZZ ’line, is called a mathematical or true horizon. The diameter of the PP ’, around which the celestial sphere rotates in its visible movement (this rotation is a reflection of the rotation of the Earth), is called the axis of the world: it intersects the surface of the celestial sphere at two points – the northern P and the southern P’ poles of the world. The large circle of the celestial sphere QLQ’F, whose plane is perpendicular to the axis of the world PP, is the celestial equator; he divides the celestial sphere into the northern and southern hemispheres. The Earth rotating around its axis moves around the Sun along a path lying in the plane of the Earth’s orbit VLWF. Its historical name is the ecliptic plane. The ecliptic is the apparent annual motion of the sun. The ecliptic is inclined to the plane of the celestial equator at an angle of 23 ^ 27 ’~ 23.5 ^; it crosses it at two points: at the point of the spring and the point of the autumnal equinox. At these points, the Sun in its visible movement passes respectively from the southern celestial hemisphere to the northern (March 20 and 21) and from the northern hemisphere to the southern (September 22 or 23).

Only on these days of the equinoxes (twice a year) the rays of the Sun fall on the Earth at right angles to the axis of its rotation and therefore only twice a year, day and night last 12 hours (equinox), and the rest of the year or day is shorter than night or vice versa. The reason for this is that the Earth’s axis of rotation is not perpendicular to the ecliptic plane, but tilted toward it at an angle of 66.5 ^.

The motion of the moon around the earth for a number of reasons is very complex. If the Earth is taken as the center, then the moon’s orbit, as a first approximation, can be considered an ellipse. When the Moon is in the closest proximity to the Earth at perigee, its distance from the Earth’s surface is 356,400 km, at the apogee this distance increases to 406,700 km. Its average distance from Earth is 384,000 km. The moon’s orbital plane is tilted at an angle of 5 ^ 09 ’; the points of intersection of the orbit with the ecliptic are called nodes, and the line connecting them is called the line of nodes. The line of nodes moves towards the movement of the moon, making a complete revolution in 6793 days, which is about 18.6 years.