On a hot summer day, when the weather is clear and we are exhausted from high temperatures, we often hear the phrase “the sun is at its zenith”. In our understanding, we are talking about the fact that the celestial body is at the highest point and warms as much as possible, one might even say, scorching the earth. Let's try to plunge a little into astronomy and understand in more detail this expression and how true our understanding of this statement is.
Earth Parallels
Even from the school curriculum, we know that on our planet there are so-called parallels, which are invisible (imaginary) lines. Their existence is due to the elementary laws of geometry and physics, and knowledge of where these parallels come from is necessary in order to understand the entire course of geography. It is customary to highlight the three most important lines - the equator, the Arctic Circle and the tropics.
Equator
The equator is called the invisible (conditional) line dividing our Earth into two identical hemispheres - Southern and Northern. It has long been known that the Earth does not stand on three pillars, as was believed in antiquity, but has a spherical shape and, in addition to moving around the Sun, rotates around its axis. So it turns out that the longest parallel on Earth, with a length of about 40 thousand km, is the equator. In principle, everything is clear from a mathematical point of view, but does it matter for geography? And here, upon closer inspection, it turns out that the part of the planet that is located between the tropics gets the most solar heat and light. This is due to the fact that this area of the Earth is always turned towards the Sun, so the rays here fall almost vertically. From this it follows that on the equatorial parts of the planet the highest air temperature is observed, and air-saturated air masses create strong evaporation. The sun at its zenith at the equator happens twice a year, that is, it shines absolutely vertically down. For example, in Russia such a phenomenon never occurs.
Tropics
On the globe there are Southern and Northern tropics. It is noteworthy that the sun at its zenith is here only once a year - on the day of the solstice. When the so-called winter solstice occurs - December 22, the Southern Hemisphere turns out to be maximally turned towards the Sun, and vice versa on June 22.
Sometimes the Southern and Northern tropics are named after the zodiac constellation that appears on the path of the Sun these days. So, for example, the South is conventionally called the tropic of Capricorn, and the North - Cancer (December and June, respectively).
Arctic circles
The polar circle is considered to be a parallel above which a phenomenon such as a polar night or day is observed. The location of the latitude at which the polar circles are located also has a completely mathematical explanation, it is 90 ° minus the inclination of the planet axis. For the Earth, this value of the polar circles is 66.5 °. Unfortunately, people in temperate latitudes cannot observe these phenomena. But the sun at its zenith on the parallel corresponding to the Arctic Circle, the event is absolutely logical.
Well-known facts
The Earth does not stand still and, in addition to moving around the Sun, rotates around its axis every day. Throughout the year, we observe how the longitude of the day, the temperature of the air outside the window change, and the most attentive can note a change in the position of the stars in the sky. In 364 days, the Earth travels a full path around the Sun.
Day and night
When it is dark with us, that is, night, this suggests that the Sun in a given period of time illuminates the other hemisphere. A logical question arises why the day is not equal to the length of the night. The fact is that the plane of the trajectory is not at right angles to the earth's axis. Indeed, in this case, we would not have the seasons at which the ratio of the longitude of day to night changes.
In the 20th of March, the North Pole leans towards the Sun. Then at about noon on the equator line you can absolutely say that the sun is at its zenith. Then follow the days when a similar phenomenon is observed in the more northern points. Already on June 22 the sun at its zenith is located on the tropic of Cancer, in the northern hemisphere this day is considered the middle of summer and has a maximum longitude. For us, the most familiar is the definition - the solstice phenomenon.
It is interesting that after this day everything happens anew, only in the reverse order, and continues until the sun is at its zenith again at the equator line at noon - this happens on September 23. At this time, midsummer begins in the southern hemisphere.
From all this it follows that when the sun is at its zenith at the equator, the duration of the night is 12 hours all over the globe, and the same length of time is day. We are used to calling this phenomenon the day of the autumn or spring equinox.
Despite the fact that we have figured out the correct explanation of the concept of “sun at its zenith”, it’s still more usual for us to have a wording that simply means finding the sun as high as possible on a given day.