Points and lines of the celestial sphere - how to find the almucantarat where the celestial equator passes, which is the celestial meridian.

What is the celestial sphere

Celestial sphere - an abstract concept, an imaginary sphere of infinitely large radius, the center of which is the observer. At the same time, the center of the celestial sphere is, as it were, at the level of the observer's eyes (in other words, everything that you see above your head from horizon to horizon is this very sphere). However, for ease of perception, we can consider the center of the celestial sphere and the center of the Earth, there is no mistake in this. The positions of the stars, planets, the Sun and the Moon are plotted on the sphere in the position in which they are visible in the sky at a certain point in time from a given point of the observer's location.

In other words, although observing the position of the luminaries in the celestial sphere, we, being in different places on the planet, will constantly see a slightly different picture, knowing the principles of the "work" of the celestial sphere, looking at the night sky, we can easily orient ourselves on the ground using a simple technique. Knowing the view overhead at point A, we will compare it with the view of the sky at point B, and by the deviations of familiar landmarks, we can understand exactly where we are now.

People have long come up with a number of tools to facilitate our task. If you navigate the "earthly" globe simply with the help of latitude and longitude, then a number of similar elements - points and lines, are also provided for the "heavenly" globe - the celestial sphere.

Celestial sphere and position of the observer. If the observer moves, then the whole sphere visible to him will move.

Elements of the celestial sphere

The celestial sphere has a number of characteristic points, lines and circles, let's consider the main elements of the celestial sphere.

Observer vertical

Observer vertical- a straight line passing through the center of the celestial sphere and coinciding with the direction of the plumb line at the point of the observer. Zenith- the point of intersection of the observer's vertical with the celestial sphere, located above the observer's head. Nadir- the point of intersection of the vertical of the observer with the celestial sphere, opposite to the zenith.

True horizon- a large circle on the celestial sphere, the plane of which is perpendicular to the vertical of the observer. The true horizon divides the celestial sphere into two parts: suprahorizontal hemisphere where the zenith is located, and subhorizontal hemisphere, in which the nadir is located.

world axis ( earth axis) - a straight line around which the visible daily rotation of the celestial sphere occurs. The axis of the world is parallel to the axis of rotation of the Earth, and for an observer located at one of the poles of the Earth, it coincides with the axis of rotation of the Earth. The apparent daily rotation of the celestial sphere is a reflection of the actual daily rotation of the Earth around its axis. The poles of the world are the points of intersection of the axis of the world with the celestial sphere. The pole of the world, located in the constellation Ursa Minor, is called north pole world, and the opposite pole is called south pole.

A large circle on the celestial sphere, the plane of which is perpendicular to the axis of the world. The plane of the celestial equator divides the celestial sphere into northern hemisphere, in which the North Pole of the World is located, and southern hemisphere where the South Pole of the World is located.

Or the meridian of the observer - a large circle on the celestial sphere, passing through the poles of the world, zenith and nadir. It coincides with the plane of the earth meridian of the observer and divides the celestial sphere into eastern And western hemisphere.

Points north and south- points of intersection of the celestial meridian with the true horizon. The point closest to the North Pole of the world is called the north point of the true horizon C, and the point closest to the South Pole of the world is called the south point Yu. The points of east and west are the points of intersection of the celestial equator with the true horizon.

noon line- a straight line in the plane of the true horizon, connecting the points of north and south. This line is called noon because at noon, local true solar time, the shadow from the vertical pole coincides with this line, that is, with the true meridian of this point.

Points of intersection of the celestial meridian with the celestial equator. The point closest to the southern point of the horizon is called point south of the celestial equator, and the point closest to northern point horizon - point north of the celestial equator.

Vertical luminaries

Vertical luminaries, or height circle, - a large circle on the celestial sphere, passing through the zenith, nadir and luminary. The first vertical is the vertical passing through the points of east and west.

Declension circle, or , - a large circle on the celestial sphere, passing through the poles of the world and the luminary.

A small circle on the celestial sphere, drawn through the luminary parallel to the plane of the celestial equator. The visible daily movement of the luminaries occurs along the daily parallels.

Almukantarat luminaries

Almukantarat luminaries- a small circle on the celestial sphere, drawn through the luminary parallel to the plane of the true horizon.

All the elements of the celestial sphere noted above are actively used to solve practical problems of orientation in space and determining the position of the stars. Depending on the purposes and conditions of measurement, two different systems are used. spherical celestial coordinates.

In one system, the luminary is oriented relative to the true horizon and is called this system, and in the other, relative to the celestial equator and is called.

In each of these systems, the position of the luminary on the celestial sphere is determined by two angular values, just as the position of points on the surface of the Earth is determined using latitude and longitude.

The celestial sphere is an imaginary spherical surface of arbitrary radius, in the center of which is the observer. Celestial bodies are projected on celestial sphere.

Due to the small size of the Earth, in comparison with the distances to the stars, observers located in different places on the earth's surface can be considered as being in the center of the celestial sphere. In fact, no material sphere surrounding the Earth exists in nature. Celestial bodies move in the boundless space of the world at various distances from the Earth. These distances are unimaginably great, our vision is not able to assess them, therefore, to a person, all celestial bodies appear to be equally distant.

During the year, the Sun describes a large circle against the background of the starry sky. The annual path of the Sun in the celestial sphere is called the ecliptic. Moving across ecliptic. The sun crosses the celestial equator twice at the equinoxes. This happens on March 21 and September 23.

The point of the celestial sphere, which remains motionless during the daily movement of stars, is conventionally called north pole peace. The opposite point of the celestial sphere is called south pole peace. Inhabitants northern hemisphere they do not see him, because he is under the horizon. A plumb line passing through the observer crosses the sky overhead at the zenith and at the diametrically opposite point, called the nadir.

The axis of visible rotation of the celestial sphere, connecting both poles of the world and passing through the observer, is called the axis of the world. On the horizon below the north pole of the world lies north point, its diametrically opposite point - south point. East and West Points lie on the horizon line and are 90° apart from north and south points.

The plane passing through the center of the sphere perpendicular to the axis of the world forms plane of the celestial equator, parallel to the plane earth's equator. The plane of the celestial meridian passes through the poles of the world, points of north and south, zenith and nadir.

Celestial coordinates

The coordinate system in which the reference is made from the plane of the equator is called equatorial. The angular distance of the star from the celestial equator is called , which varies from -90 ° to + 90 °. declination considered positive north of the equator and negative south. is measured by the angle between the planes of great circles, one of which passes through the poles of the world and the given luminary, the second through the poles of the world and the vernal equinox point lying on the equator.

Horizontal coordinates

Angular distance is the distance between objects in the sky, measured by the angle that is formed by the rays going to the object from the point of observation. The angular distance of the star from the horizon is called the height of the star above the horizon. The position of the sun relative to the sides of the horizon is called the azimuth. The countdown is from the south clockwise. Azimuth and the height of the star above the horizon is measured with a theodolite. In angular units, not only the distances between celestial objects are expressed, but also the sizes of the objects themselves. The angular distance of the celestial pole from the horizon is equal to the geographic latitude of the area.

The height of the luminaries at the climax

The phenomena of the passage of luminaries through the celestial meridian are called climaxes. The lower climax is the passage of the luminaries through the northern half of the celestial meridian. The phenomenon of the passage of the luminary of the southern half of the celestial meridian is called the upper climax. The moment of the upper culmination of the center of the Sun is called true noon, and the moment of the lower climax is called true midnight. The time interval between climaxes - half a day.

For non-setting luminaries, both climaxes are visible above the horizon, for ascending and setting lower climax occurs below the horizon, below the north point. Every star culminates in a given area is always at the same height above the horizon, because its angular distance from the celestial pole and from the celestial equator does not change. The sun and moon change height
which they culminate.

Origin

Word zenith came from an inaccurate reading of the Arabic expression سمت الرأس ( samt ar-ra's), meaning "direction to the head" or "way overhead". In the Middle Ages, during the 14th century, this word came to Europe through Latin and possibly through Old Spanish. It was shortened to samt("direction") - samt and with spelling errors transformed into senite - senit. Through the Old French and Middle English word senite finally developed in the 17th century into modern word zenith .

Relevance and use

The concept of "zenith" is used in the following scientific contexts:

• It serves as a direction for measuring zenith angle, which is angular distance between the direction to the object of interest to us (for example, to a star) and the local zenith relative to the point for which the zenith is determined.
• It defines one of the axes of the horizontal coordinate system in astronomy.

Thus, it is connected with the concepts of the elements of the celestial sphere - a plumb line and a circle of the height of the luminary.

Strictly speaking, the zenith is only approximately associated with the local plane of the meridian, since the latter is defined in terms of the rotational characteristics of the celestial body, and not in terms of its gravitational field. They coincide only for an ideal symmetric body of revolution. For the Earth, the axis of rotation does not have a fixed position (for example, due to constant movements of ocean water and other water resources), and the local vertical direction, determined through the gravity field, itself changes direction in time (for example, due to lunar and solar tides and ebbs).

Sometimes the term zenith belong to highest point achieved celestial body(Sun, Moon, etc.) in the process of its apparent movement in orbit relative to a given observation point. However, the Great Astronomical Dictionary gives the following definition of zenith:

A point on the celestial sphere directly above the observer's head. The astronomical zenith is formally defined as the intersection plumb line with the celestial sphere. Geocentric zenith - the intersection with the celestial sphere of a line running from the center of the Earth through the position of the observer. The geodesic zenith is on a line normal to the geodesic ellipsoid or spheroid at the observer's position.

Thus, in relation to, for example, the Sun, the zenith can only be reached at low latitudes.