Looking at a myriad of stars in the night sky, a person thinks about his place in the Universe. The popularization of astronomical knowledge and the general interest in astrology do not fill the gaps in knowledge about the structure of the universe. And although everyone taught astronomy at school, in this article we recall some fundamental concepts: these are the celestial meridian, the celestial equator, and the coordinates of the celestial sphere.
Foreword
When you look at the starry sky, it seems that all celestial objects are located on the surface of a sphere - giant and rotating in the direction of the West. Even the first Museyon astronomers known to us (Ancient Egypt, 296-270 years before our era) Timoharis and Aristillus designated the concepts of the celestial sphere and began to refine the position of the stars on it. It was then that the science of the sphere began, which deals with the study of the apparent position of stars and the compilation of star catalogs and maps. And although we know that the idea of the heavenly arch of the ancients is erroneous, but even today it is such a model that is most convenient for describing the starry sky.
Academic concepts
The celestial sphere is an imaginary giant dome of arbitrary radius, the inner surface of which is strewn with stars that the observer sees from the Earth. This imaginary sphere has a number of points and lines that help to understand the movement and location of celestial bodies. Like the geographic coordinate lines on the surface of the planet, the celestial sphere is also divided by the corresponding lines. The highest point of the sphere above the observer’s head is called the zenith (the opposite point is the nadir), and the fixed axis of this sphere is called the axis of the world. It passes through the north and south poles and through the eye of the observer, and remains completely motionless. The equator divides the celestial sphere into two hemispheres perpendicular to the axis, and the circle passing through the poles and zenith is the celestial meridian. The circle of the celestial sphere, which is formed at the intersection of it and the horizontal plane that passes through the observer’s eye perpendicular to the vertical line at the observation point, is the true (mathematical) horizon.
Observer and stars
The type of sky depends on the location of the observer on the surface of the planet. At the South and North Poles he will see completely different pictures of the starry sky. Stars and the celestial sphere revolve around the pole of the world: in the case of the North Pole of the planet, this is the zenith of the North Star, but at the South Pole there is no bright star. The observer, located at the equator of the planet, theoretically sees both poles of the world and all stars. But wherever we observe the stars, the main line on the celestial sphere is the celestial meridian.
Plane relationship
The meridian, or circle drawn through the poles and zenith with nadir, intersects with the horizon at points in the north and south. The line of intersection of the plane of the sky and the meridian is a midday line. If you draw a line through the center of the sphere perpendicular to midday, then it will cross the horizon at the points of west and east. The celestial equator intersects with the horizon precisely at these points and divides the sphere into the northern and southern hemispheres.
Climaxes
Any star passes the celestial meridian twice a day. In the highest position it will be located south of the pole, and in the lowest - to the north. The phenomenon of the passage of the celestial meridian by the luminary is called the climax, upper and lower, respectively. To observe the climaxes, passive instruments are used - telescopes, which are installed in its plane. The axis of the world relative to the plane of the celestial meridian is parallel and tilted to the local horizon at an angle corresponding to the geographical latitude of the observation area. Planets culminate twice a day, which served as guidelines for the development of cartography of the celestial sphere.
Celestial coordinate systems
Why does humanity need all these lines of the celestial meridian? They are used in astronomy to determine the position of celestial bodies. There are many coordinate systems that differ in the choice of the fundamental plane and the reference point. But all of them involve setting the position in the form of two angular quantities or arcs and are called spherical coordinate systems, namely:
- Horizontal topocentric - the center is the location of the observer, fundamental - the plane of the true horizon.
- The first and second equatorial, fundamental plane in this case is the equator plane.
- Ecliptic - the ecliptic plane (the large circle of the celestial sphere along which the annual movement of the Sun passes) is chosen as the fundamental plane.
- Galactic - in this system, the main is the plane of our Galaxy.
The most commonly used are the horizontal and equatorial systems.
Use and application of coordinate systems
Most modern computer programs use a horizontal topocentric system to determine the position of the stars, the data for which can be obtained using goniometric instruments and when observed through a telescope in an azimuthal setup. Equatorial systems are more often used in determining the exact time and when observing with a telescope on an equatorial setup. The ecliptic system is used to calculate the orbits of the planets or the moon. However, most often you have to use several systems. For example, to calculate the coordinates of the moon, first carry out calculations in the ecliptic system, then recalculate the data in the equatorial, and then go to the horizontal topocentric coordinate system.
If you are an amateur astronomer
Even without special instruments, the distance between celestial bodies can be estimated, although very approximately. Since the observer is always in the center, and the sky is a sphere, all distances between objects are measured in corners. A full circle corresponds to 360 degrees. You can approximately determine the angle by comparing it with the distance between the fingers of the outstretched arm. If more accurate data are needed to determine the location of a celestial object, an astrocompass with a rectangular coordinate system is used.
Celestial Meridians and Astrology
That the stars change their position in space, even the ancient inhabitants of our planet noticed. However, the most noticeable movement can be observed in the Sun, the Moon, planets relative to distant stars, the movement of which can be seen when observing them for centuries. Ancient observers in a strip of about 16 degrees along the ecliptic identified twelve constellations called zodiacal. They were given great importance by astrologers. But today, after centuries, the precession of the Earth has changed the main points of the ecliptic. And the thirteenth sign, where the Sun and planets may appear, was the constellation Ophiuchus (Ophiuchus), which is not the zodiac. All skeptical remarks towards astrology owe this. But the zodiac signs are still used in astronomy today to denote the totality of stars in the sky. For example, one of the two most important points in the celestial sphere, where the solar ecliptic crosses the equator, is indicated by the sign of the constellation Aries (Ophiuchus).
Summarizing
Observations of the position of stars in the celestial sphere played a role in the greatest discoveries of mankind. The precession and nutation of the Earth, aberrations and parallax of stars still continue to play a crucial role in astronautics and astronomy. Knowledge of the celestial coordinates and the celestial meridian is a solution to the problems of measuring time, determining the geographical coordinates on Earth, catalogs of stars. In addition, this knowledge provides information on the spatial distribution of stars, which is used to study the motion of natural and artificial celestial bodies and to pose problems in stellar astronomy and astrophysics. Knowing where the celestial meridian intersects with the horizon line has largely determined the development of the scientific and technological progress of mankind since the earliest times of its existence.