Under the celestial sphere is understood 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. For example, the eye of the observer, the center of the Earth, the center of the instrument, etc. are taken as the center. Each of the celestial bodies has a corresponding point on 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.
Directly overhead
It is there that there is such a point in the sky as the zenith. This is such a direction, which indicates an indication of the top, directly located above a specific place. More precisely, the concept of "top" is defined in astronomy, meteorology, geophysics. They understand it as a direction that is opposite to the acting force of gravity in a given place.
When the sun is inthe zenith of a vertical tree trunk will not cast a shadow on the ground. This 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 "samt ar-ra's". Its meaning is "the way overhead". There is another translation option - "direction to the head." In the 14th century, through the Latin language, this term came to Europe. It was shortened to "samt" - "direction" and transformed into "senit", and in the 18th century it became "zenith".
Understanding the concept of "points of the celestial sphere", you 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, such as the Sun, Moon. This occurs when their apparent orbital movement is observed, viewed from a specific observation point.
But in the Big Astronomical Dictionary, regarding the term under study, it is said that this is the highest point of the celestial sphere, which is located directly above the head of an observing person.
As for the astronomical zenith, it is formally defined as the intersection of a plumb 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 at the center of the Earth, they mean the geocentric zenith. Zenith is opposed to such a point on the celestial sphere as nadir.
Nadir
This is the same direction as gravity. The word "nadir" comes from the Arabic "Nazir", meaning "opposite", that is, it is opposite to the zenith. This is exactly the direction that points to the bottom, located under a certain place.
Because the concept of "below" is perceived as rather vague, the nadir is defined by scientists in a more strict framework. Namely, in geophysics, astronomy, meteorology it sounds as follows. Nadir is the direction that coincides with that in which the gravitational force acts at a particular point. The lowest point of the celestial sphere - the nadir - is opposed to the zenith.
Use of term
Such a concept as nadir is also used in the imaging geometry, which is directed downwards in relation to the orbiting satellite. It is used, for example, in remote sensing of the atmosphere. And also in a situation in which the orientation of the astronaut to the Earth occurs during spacewalks.
When a word is used figuratively, it refers to the lowest point observed in a person's state of mind. Or they can talk about the low quality of someone's professional activity.
The term can also be used to denote the lowest point reached by a celestial body as it moves along its apparent 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, exactly in technical terms, it can bereach only at a certain moment and only at low latitudes.
Next, other lines and points of the celestial sphere will be considered.
Axis of the world and 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. During the Earth 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 the point opposite the center around which the Northern hemisphere of the sky rotates, there is a similar center of rotation belonging to the Southern celestial hemisphere. But after all, 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 the daily rotation of the sky is called the axis of the world.
There is also such a thing as "the poles of the world". They are called 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 around it.
Celestial Equator
A plane perpendicular to the world axis (it intersects the celestial sphere at its center) is a planecelestial 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 North and the other is 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 that are listed have a connection with the observed rotation of the celestial sphere. Whereas it itself follows from the actual rotation of the globe.
