Astronomical experiments

Celestial sphere

For many centuries, the "terrestrial firmament" was considered a model of inviolability and immobility. There is nothing surprising that this mistake lasted so long, because all our senses speak of the immobility of the Earth and the rotation of the "vault of heaven" around it with the stars, the Sun and the Moon. But even now in astronomy, as a memory of those ancient times, the concept of the celestial sphere is used - an imaginary infinitely large sphere, in the center of which the observer is located and on the surface of which the movements of celestial bodies are displayed.

Of course, the most noticeable is the daily rotation of the sky - the Sun rises in the morning, passes through the sky and sinks below the horizon, the stars that are visible in the east in the evening rise high in the south by midnight and then sink to the west, the Sun rises again ... It seems that the sky rotates around an invisible axis located near the North Star.

Movement of stars around the Pole of Peace. Photo by A. Mironov

But the daily rotation of the sky is very dependent on our position on the globe - if we find ourselves in the southern hemisphere, it will be very unusual for us that the Sun moves across the sky in the opposite direction - from right to left. Let's take a closer look at how the apparent rotation of the firmament changes in different parts of the Earth.

To begin with, it should be remembered that the height of the Pole of the World (the point around which the sky rotates) above the horizon is always equal to the geographical latitude of the observation site. This means that at the north pole, the North Star will be at its zenith, and all the luminaries will move in daily rotation from left to right parallel to the horizon, never rising or setting. Being at the pole, we could see the stars of only one hemisphere, but on any night.

On the contrary, for an observer at the equator, there are no non-rising stars (however, as well as non-setting ones) - all the stars of the sky are available for observation, they rise vertically in the eastern part of the horizon and set exactly 12 hours later in the western part of the sky.

In mid-latitudes, some of the stars in the vicinity of the pole never fall below the horizon, but the same region of the sky around the opposite pole is never available for observation, while the rest of the stars, located in a strip on both sides of the celestial equator, rise and set during the day.

The movement of the luminaries in the middle latitudes of the southern hemisphere will look approximately the same, with the only difference being that the South Pole of the World will be visible above the horizon, around which the stars rotate clockwise, and the equatorial constellations familiar to us, turned upside down, rise above everything in the northern part of the sky and move from right to left.

The movement of the sun and the day

Speaking about the movement of the stars, we were not interested in the distance to them and the movement of the Earth around the Sun - the distances to the stars are huge and the changes in their positions due to the annual movement of the Earth are very small and can only be measured with very accurate instruments. Quite another matter is the Sun. The movement of the Earth in its orbit results in the apparent movement of the Sun among the stars. The path that the Sun takes in the sky during the year is called the ecliptic. Since the earth's axis is tilted by 23.5 °, when the Earth revolves around the Sun, either the northern or the southern hemisphere turns to it - this explains the change of seasons on our planet.

When the northern hemisphere is turned towards the Sun, summer comes there, the Sun in its visible path along the ecliptic turns out to be in its northern part and in our northern hemisphere it rises higher above the horizon. At the North Pole, for half a year, the Sun becomes a non-setting luminary - there comes the polar day. A little further south, the polar day lasts less and at the latitude of the polar circle (66.5 ° - the polar circle is 23.5 ° from the pole) the Sun does not set for only a few days in the middle of summer, near the day of the summer solstice (June 22). In winter, the Sun does not rise at the Pole for almost half a year (a little less due to refraction), to the south the polar night becomes shorter and outside the Arctic Circle, the Sun rises above the horizon even in the middle of winter.

In the middle and equatorial latitudes, the Sun always rises and sets, the length of the day strongly depends not only on the time of year, but also on the latitude - the closer to the equator, the less the length of the day differs in winter and summer, and the closer the length of day and night to 12 hours. But only at the equator the length of day and night is always constant. The duration of twilight also depends on latitude - in the equatorial latitudes the Sun sets perpendicular to the horizon and twilight is the shortest, and at the latitude of St. Petersburg in the middle of summer they last from sunset to sunrise - these are the famous white nights.

It depends on latitude how high the Sun can rise above the horizon - on the day of the solstice, this height will be 90 ° -φ + 23.5 °.

By the way, the erroneous opinion is very widespread that at the equator the Sun is always at its zenith at noon - this is not so, at any point on the Earth lying between the lines of the tropics (from 23.5 ° S to 23.5 ° N). ) the Sun passes exactly through the zenith only twice a year, at the equator - on the equinoxes, and on the lines of the tropics - only once a year, on the day of the summer solstice on the northern tropic and on the day of the winter solstice - on the south.

The movement of the Earth around the Sun leads to another important phenomenon - the duration of a solar day (the time interval between two noons) does not coincide with the sidereal days (the time interval between the passage of a star through the meridian). The fact is that the Earth needs additional time to turn by the angle that it passes in a day in its orbit. Moreover, the duration of a solar day is not constant (see the article Equation of Time). It is easy to make a rough estimate - in a day the earth passes 1/365 of its orbit, or a little less than 1 °, and if the Earth rotates around its axis (360 °) in about 24 hours, then it will turn 1 ° in about 4 minutes. Indeed, a sidereal day is 23 hours 56 minutes 4 seconds.

moon

Since ancient times, our satellite has served people to count time, and this is no coincidence - the change in the phases of the moon is easy to observe and the duration of the month is not difficult to determine, besides, the month has become a very convenient intermediate unit for measuring time between a day and a year. By the way, the usual seven-day week is also associated with the moon - 7 days is about a quarter of a month (and the phases of the moon are also measured in quarters). Most ancient calendars were lunar and lunisolar.

Of course, the first thing that catches your eye when observing the Moon is the change in its appearance during the month from a thin crescent, which can be seen immediately after sunset, 2-3 days after the new moon, to the phase of the first quarter (in the northern hemisphere, the right half of the disk is illuminated Moon), further to the full moon, the last quarter (the left half of the disk is illuminated) and, finally, to the new moon, when the Moon approaches the Sun and disappears in its rays. The change of phases is explained by the change in the position of the Moon relative to the Sun when it revolves around the Earth, a full cycle of phase change - a revolution relative to the Sun or a synodic month lasts about 29.5 days. The period of revolution relative to the stars (sidereal month) is slightly less and is 27.3 days. As you can see, a year contains a non-integer number of months, so lunisolar calendars use special rules for alternating 12-month and 13-month years, because of this they are quite complicated and are now supplanted in most countries by the Gregorian calendar, which has nothing to do with the Moon - only months (albeit longer than lunar months) and weeks remained in memory of his predecessors...

There is another interesting feature in the motion of the Moon - the period of its rotation around its axis coincides with the period of revolution around the Earth, so our satellite is always turned to the Earth with one hemisphere. But it cannot be said that we can only see half of the Moon's surface - due to the uneven orbital motion of the Moon and the inclination of its orbit to the Earth's equator, relative to the Earth observer, the Moon rotates slightly both in latitude and in longitude (this phenomenon is called libration) and we can to see the edge zones of the disk - in total, about 60% of the lunar surface is available for observations.

Jean Effel, Creation of the World
- It's not easy to start the universe!