The star dome for the earthly observer is in continuous rotation. If, being in the Northern Hemisphere of the planet, on a moonless and cloudless night, look into the northern part of the sky for a long time, it becomes noticeable that the entire diamond scattering of stars revolves around one inconspicuous dim star (this is only ignorant people say that the Polar Star is the brightest). Some of the luminaries are hidden behind the horizon in the western part of the sky, others take their place.
The carousel lasts until morning. But the next day, at the same time, each star is again in its place. The coordinates of the stars relative to each other change so slowly that for people they seem eternal and motionless. It is no coincidence that our ancestors imagined the sky as a solid dome, and the stars as holes in it.
Strange star - starting point
Once upon a time ourancestors drew attention to one strange asterisk. Its peculiarity is immobility on the heavenly slope. It seemed to hover at one point above the northern edge of the horizon. All other celestial bodies describe regular concentric circles around it.
In what images this star did not appear in the imagination of ancient astronomers. For example, among the Arabs, it was considered a golden stake driven into the firmament. Around this stake, a golden stallion gallops (we call this constellation Ursa Major), tied to it with a golden lasso (constellation Ursa Minor).
It is from these observations that the celestial coordinates originate. Quite naturally and logically, the fixed star, which we call the Polaris, has become the starting point for astronomers to determine the location of objects on the celestial sphere.
By the way, we, the inhabitants of the Northern Hemisphere, are very lucky with the star compass. By chance, of those that are one in a million, our Polar Star is exactly on the line of the axis of rotation of the planet, thanks to which, anywhere in the hemisphere, you can easily determine the exact position relative to the cardinal points.
First star coordinates
Technical means for accurate measurement of angles and distances did not immediately appear, but people have been striving to somehow systematize and sort the stars for a long time. And even though the instruments owned by ancient astronomy did not allow us to determine the coordinates of the stars in the digitized form familiar to us, this was more than compensatedimagination.
Since ancient times, the inhabitants of all parts of the world divided the stars into groups called constellations. Most often, constellations were given names based on external resemblance to certain objects. So the Slavs called the constellation Ursa Major just a bucket.
But the most widespread are the names of the constellations given in honor of the characters of the ancient Greek epic. It is possible, albeit with some stretch, to say that the names of the constellations and stars in the sky are their first primitive coordinates.
Pearls of the sky
Astronomers did not ignore the most beautiful bright stars. They were also named after Hellenic gods and heroes. So the alpha and beta constellations of Gemini are named respectively Castor and Pollux after the names of the sons of Zeus, the Thunderer, born after his next love adventure.
The star Algol, the alpha of the constellation Perseus, deserves special attention. According to legend, this hero, having defeated the fiend of the gloomy Tartarus in a mortal battle - the Gorgon Medusa, who turns all living things into stone with a glance, took her head with him as a kind of weapon (the eyes of even a severed head continued to "work"). So, the star Algol is in the constellation the eye of this very head of Medusa, and this is not entirely accidental. Ancient Greek observers drew attention to periodic changes in the brightness of Algol (a binary star system whose components periodically overlap each other for an earthly observer).
Of course, the “winking” star became the eye of the fairy-tale monster. Coordinates of the star Algol in the sky: right ascension - 3 hours 8 minutes, declination + 40 °.
Heavenly calendar
But we should not forget that the Earth rotates not only around its axis. Every 6 months the planet is on the other side of the Sun. The picture of the night sky naturally changes in this case. This has long been used by astrologers to accurately determine the seasons. For example, in ancient Rome, students were impatiently waiting for Sirius (the Romans called it Vacation) to appear in the morning sky, because these days they were allowed to go home to rest. As you can see, the stellar name of these student holidays has survived to this day.
Besides school holidays, the position of objects in the sky determined the beginning and end of sea and river navigation, gave rise to military campaigns, agricultural activities. The authors of the first detailed calendars in different parts of the world were precisely astrologers, astrologers, priests of temples, who learned to accurately determine the coordinates of the stars. On all continents where the remains of ancient civilizations are found, entire stone complexes built for astronomical observations and measurements are found.
Horizontal coordinate system
Shows the coordinates of stars and other objects on the celestial sphere in the "here and now" mode relative to the horizon. The first coordinate is the height of the object above the horizon. An angular value is measured in degrees. The maximum value is +90° (zenith). The luminaries have a zero value of the coordinate,located on the horizon line. And finally, the minimum height value -90° is for objects located at the nadir point or at the observer's feet - the zenith is vice versa.
The second coordinate is the azimuth - the angle between the horizontal lines directed to the object and to the north. This system is also called topocentric because of the binding of coordinates to a certain point on the globe.
The system is not without flaws. Both coordinates of each star in it change every second. Therefore, it is not very suitable for describing, say, the location of stars in constellations.
Star GLONASS and GPS
How is such a system used? If you move around the planet at sufficiently large distances, the stellar picture will certainly change. This was noticed by ancient navigators. For an observer standing at the very North Pole, the North Star will be at its zenith, directly overhead. But a resident of the equator will be able to see the Polar only lying on the horizon. Moving along the parallels (from east to west), the traveler will notice that the points and times of sunrise and sunset of certain celestial objects will also change.
This is what sailors have learned to use to determine their location in the oceans. By measuring the angle of elevation above the horizon of the North Star, the navigator of the ship received the value of the latitude. Using an accurate chronometer, the sailors compared the time of the local noon with the reference (Greenwich) and received the longitude. Both terrestrial coordinates, apparently, could not be obtained without calculatingcoordinates of stars and other celestial bodies.
For all its complexity and approximateness, the described system for determining the location in space has faithfully served travelers for more than two centuries.
Equatorial first stellar coordinate system
In it, celestial coordinates are tied both to the surface of the earth and to landmarks in the sky. The first coordinate is the declination. The angle between the line directed to the luminary and the plane of the equator (the plane perpendicular to the axis of the world - the line of direction to the North Star) is measured. Thus, for stationary objects in the sky, such as stars, this coordinate always remains the same.
The second coordinate in the system will be the angle between the direction to the star and the celestial meridian (the plane in which the axis of the world and the plumb line cross). Thus, the second coordinate depends on the position of the observer on the planet, as well as the moment in time.
The use of this system is very specific. It is used when installing and debugging the mechanisms of telescopes mounted on turntables. Such a device can "follow" objects rotating together with the celestial dome. This is done to increase the exposure time when photographing areas of the sky.
Equatorial 2 starry
And how are the coordinates of stars determined on the celestial sphere? For this, there is a second equatorial system. Its axes are fixed relative to distant space objects.
First coordinate,like the first equatorial system, is the angle between the luminary and the plane of the celestial equator.
The second coordinate is called right ascension. This is the angle between two lines lying on the plane of the celestial equator and intersecting at the point of its intersection with the axis of the world. The first line is laid to the point of the vernal equinox, the second - to the point of projection of the luminary on the celestial equator.
The right ascension angle is plotted along the arc of the celestial equator clockwise. It can be measured both in degrees from 0° to 360°, and in the "hours: minutes" system. Every hour equals 15 degrees.
How to measure the right ascension of a star, the diagram shows.
What else are the coordinates of stars?
To determine our place among other stars, none of the above systems is suitable. Scientists fix the position of the nearest luminaries in the ecliptic coordinate system. It differs from the second equatorial one in that the base plane is the plane of the ecliptic (the plane in which the earth's orbit around the Sun lies).
And finally, to determine the location of even more distant objects, such as galaxies, nebulae, the galactic coordinate system is used. It is easy to guess that it is based on the plane of the Milky Way galaxy (this is the name of our native spiral galaxy).
Is everything perfect?
Not really. The coordinates of the polar star, namely the declination, is 89 degrees 15 minutes. This means that it is almost a degree away frompoles. For navigating the terrain, if a lost person is looking for a way, this location is ideal, but for planning the course of a ship that will have to travel thousands of miles, an adjustment had to be made.
Yes, and the immobility of the stars is an apparent phenomenon. A thousand years ago (very little by cosmic standards), the constellations had a completely different shape.
So scientists could not determine for a long time why in the pyramid of Cheops an inclined tunnel leaves the burial chamber to the surface of one of the faces. Astronomy came to the rescue. The coordinates of the brightest stars in different periods of time were calculated thoroughly, and astronomers suggested that during the construction of the pyramid, exactly on the line where this tunnel "looks", there was the star Sirius - a symbol of the god Osiris, a sign of eternal life.
