A celestial sphere is understood as an imaginary sphere of an arbitrarily given radius, and its center is located at any point in space. The location of its center depends on what task is set. The center is taken, for example, the eye of the observer, the center of the Earth, the center of the instrument, etc. Each of the celestial bodies has a corresponding point in the celestial sphere, which is crossed by a straight line. It connects two centers - spheres and luminaries. Next, some points and lines of the celestial sphere will be considered.
Right above your head
It is there that there is such a point in the sky as the zenith. This is the direction that indicates the top directly located above a specific place. More precisely, the concept of "top" is defined in astronomy, meteorology, geophysics. They understand him as such a direction that is opposite to the acting force of gravity in a given place.
When the sun is at its zenith, a vertically located tree trunk will not cast a shadow on the ground. Such a phenomenon can be observed in the tropics twice a year, in sunny weather, at noon.
The word “zenith” is an inaccurate reading of one of the Arabic expressions, which sounds like “sem ar-ra's." Its meaning is "the path above the head." There is another translation option - “head-on direction”. In the 14th century, through the Latin language, this term came to Europe. It was reduced to “sem” - “direction” and transformed into “senit”, and in the 18th century became the “zenith”.
Understanding the concept of “points of the celestial sphere”, one should consider what types of zenith are.
Several definitions
In some cases, this term refers to the highest point that is reached by a celestial body, for example, the Sun, the Moon. This occurs when their apparent orbital movement is observed, considered relative to a specific observation point.
But in the Big Astronomical Dictionary regarding the term studied, it is said that this is the highest point of the celestial sphere, which is located directly above the head of the observing person.
As for the astronomical zenith, it is formally defined as the intersection of a vertical line with such an “object” as the celestial sphere.
When it comes to the intersection of the celestial sphere and the line that goes through the observer's location, starting in the center of the earth, we mean the geocentric zenith. Zenith is opposed to such a point in the celestial sphere as nadir.
Nadir
This direction is identical to that in which the force of gravity acts. The word "nadir" comes from the Arabic "nazir", meaning "opposite", that is, it is opposite to the zenith. This is exactly the direction that indicates the bottom, which is under a certain place.
Since the concept of “below” is perceived as rather vague, nadir is defined by scientists in a more stringent framework. Namely, in geophysics, astronomy, meteorology, this sounds as follows. Nadir is the direction coinciding with that in which the gravitational force acts at a specific point. The lowest point of the celestial sphere - nadir - is opposed to the zenith.
Use of the term
Such a concept as nadir is also used in the geometry of visualization, which is facing downward with respect to the orbiting satellite. It is used, for example, in remote sensing of the atmosphere. And also in a situation in which the astronaut is oriented toward the Earth during spacewalks.
When the word is used in a figurative sense, then they designate the lowest point observed in the mental state of a person. Or they can talk about the low quality of one's professional activity.
The term can also be used to denote the lowest point reached by a celestial body when it moves along a visible orbit with respect to a given observation point. So, for example, the word “nadir” is used when determining the position of the Sun, but at the same time it can be achieved technically only at a certain moment and only at low latitudes.
Next, other lines and points of the celestial sphere will be considered.
The axis of the world and the poles of the world
If you look closely at the night sky, you can see that during the day the circles described by the stars, the more the farther they are from the North Star. Throughout the Earth's day, the North Star describes a very small circle, and it can always be seen above the horizon at the same height. It is located in the northern part of the celestial sphere.
At a point opposite the center around which the northern hemisphere of the sky rotates, there is a similar center of rotation related to the southern celestial hemisphere. But the location of our eye is the center of the celestial sphere. This means that this sphere rotates around a certain axis, and this is the rotation of a single whole. And this axis passes through the eye of the observer. The axis of daily rotation of the sky is called the axis of the world.
There is such a thing as "poles of the world." They are called the points at which the intersection of such imaginary objects is observed, which are both the celestial sphere and the axis of the world. Close to the North Pole of the world is the North Star. The distance between them is approximately 1 °. In the southern hemisphere of the sky is the South Pole of the world. There are no bright stars near it.
Celestial equator
A plane that is perpendicular to the world axis (it intersects the celestial sphere at its center) is the plane of the celestial equator. Whereas the line of intersection of the latter with the celestial sphere is the celestial equator.
This equator divides the sky into two hemispheres. One of them is the North, and the other is the South. You can see the following pattern. And the celestial equator, and the poles of the world, and the axis of the world are similar to the equator, poles and axis of the Earth. But this is natural, since all the names listed are related to the observed rotation of the celestial sphere. Whereas it itself follows from the actual rotation of the globe.