The starry dome for the Earth observer is in continuous rotation. If, being in the northern hemisphere of the planet, on a moonless and cloudless night you look into the northern part of the sky for a long time, it becomes noticeable that the entire diamond cluster of stars revolves around one inconspicuous dim star (it is only ignoramuses telling us that the North Star is the brightest). Part of the luminaries hides behind the horizon in the western part of the sky, others take their place.
The carousel lasts until the morning. But the next day, at the same time, each asterisk again appears 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 accident that our ancestors imagined the sky as a solid dome, and the stars as holes in it.
Strange Star - reference point
Once upon a time, our ancestors drew attention to one strange little star. Its peculiarity is immobility on the heavenly slope. It seemed to hover at one point above the northern edge of the horizon. Nevertheless, the rest of the heavenly bodies describe regular concentric circles around her.
In what images did this star not imagine in the imagination of ancient astronomers. For example, among the Arabs, it was considered a golden stake driven into the firmament of heaven. 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 heavenly coordinates originate. Quite naturally and logically, a fixed star, which we call the Polar Star, has become for astronomers the starting point for determining the location of objects in the celestial sphere.
By the way, we, the inhabitants of the Northern Hemisphere, were very lucky with a star compass. By chance, of those that are one in a million, our Polar Star is located exactly on the line of the axis of rotation of the planet, thanks to which it is easy to determine the exact position relative to the cardinal points anywhere in the hemisphere.
First stellar coordinates
Technical means for accurate measurement of angles and distances did not immediately appear, but people have long sought to systematize and sort stars, for a long time. And even though the instruments that ancient astronomy owned did not allow us to determine the coordinates of stars in the usual digitized form, this was more than compensated by imagination.
From ancient times, residents of all parts of the world divided stars into groups called constellations. Most often, constellations were given names based on external similarities with certain objects. So the constellation Ursa Major the Slavs simply called a bucket.
But the most common are the names of the constellations, given in honor of the characters of the ancient Greek epic. It is possible, albeit with a slight stretch, to say that the names of the constellations and stars in the sky are their first primitive coordinates.
Pearls of the sky
Astronomers and the most beautiful bright stars did not pass their attention. They also received names in honor of the Hellenic gods and heroes. So the alpha and beta constellations of the Gemini are respectively named Castor and Pollux by the names of the sons of Zeus, the Thunderer born after his next love adventure.
Of particular note is the star Algol, the alpha of the constellation Perseus. According to legend, this hero, having defeated the evil creature of the dark Tartar - the gorgon Medusa, who turns all life into stone with a look, took his head with him as a kind of weapon (the eyes of even the severed head continued to “work”). So, the star Algol in the constellation is 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 earth observer).
Naturally, the "winking" star also became the eye of a fabulous monster. The coordinates of the star Algol in the sky: right ascension - 3 h 8 min, declination + 40 °.
Sky calendar
But we should not forget that the Earth rotates not only on its axis. Every 6 months, the planet is on the other side of the sun. The picture of the night sky naturally changes. It has long been used by astrologers to accurately determine the seasons. For example, in ancient Rome, students were impatiently waiting for Sirius to appear in the morning sky (the name of the Romans sounded the Vacation), because these days they were allowed to go home to rest. As you can see, the star name of these student vacations has survived to this day.
In addition to school holidays, the position of objects in the sky determined the beginning and end of sea and river navigation, gave a start 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 stars. On all continents, where the remains of ancient civilizations are located, whole stone complexes are discovered, built for astronomical observations and measurements.
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. The value is angular, measured in degrees. The maximum value is + 90 ° (zenith). Zero coordinate values are luminaries located on the horizon. And finally, objects located at the nadir point or at the observer "underfoot" have a minimum height of -90 ° - the zenith is the other way around.
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 due to the binding of coordinates to a specific 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
But how is such a system used? If you move around the planet long enough, the star picture will certainly change. This was noticed by ancient sailors. The observer, standing at the very North Pole, the North Star will be at its zenith, directly above his head. 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 or sunset of certain celestial objects will also change.
This is what sailors learned to use to determine their position in the oceans. By measuring the elevation angle above the horizon of the North Star, the navigator of the ship received the latitude value. Using an accurate chronometer, sailors compared the local noon time with the reference (Greenwich) and got the longitude. It was obvious that both earth coordinates could not be obtained without calculating the coordinates of stars and other celestial bodies.
For all its complexity and approximation, the described system for determining the location in space faithfully served travelers for more than two centuries.
Equatorial first stellar coordinate system
In it, the celestial coordinates are attached 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 is measured (a plane perpendicular to the axis of the world - the direction line to the North Star). Thus, for stationary objects of the sky, such as stars, this coordinate always remains unchanged.
The second coordinate in the system will be the angle between the direction of the star and the celestial meridian (the plane in which the axis of the world and the plumb line intersect). Thus, the second coordinate depends on the position of the observer on the planet, as well as the point 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 with a dome of heaven. This is done to increase the exposure time when photographing areas of the sky.
Equatorial No. 2 Star
And how on the celestial sphere determine the coordinates of the stars? There is a second equatorial system for this. Its axes are motionless relative to distant space objects.
The first coordinate, like that of the first equatorial system, is the angle between the star 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 intersection with the axis of the world. The first line is drawn to the point of the vernal equinox, the second - to the point of projection of the star on the celestial equator.
The right ascension angle is plotted along the arc of the celestial equator in a clockwise direction. It can be measured both in degrees from 0 ° to 360 °, and in the "hours: minutes" system. Every hour is 15 degrees.
How to measure the right ascension of the star, the diagram shows.
What else are the coordinates of the stars?
None of the systems listed above is suitable for determining our place among other stars. Scientists fix the position of the nearest luminaries in the ecliptic coordinate system. It differs from the second equatorial in that the base plane is the ecliptic plane (the plane in which the earth's orbit lies around the sun).
And finally, to determine the location of even more distant objects, such as galaxies, nebulae, a galactic coordinate system is used. It is easy to guess that the plane of the Milky Way galaxy is taken as its basis (as our native spiral galaxy is called).
Is everything perfect?
Not really. The coordinates of the polar star, namely the declination, are 89 degrees 15 minutes. This means that it is almost a degree away from the pole. For orientation on the ground, if a lost person is looking for a way, such an arrangement is ideal, but for planning the course of a ship that has to go thousands of miles, you had to make an amendment.
And the stillness of stars is an apparent phenomenon. A thousand years ago (quite a bit by cosmic standards) the constellations had very different outlines.
So scientists could not determine for a long time why the inclined tunnel to the surface of one of the faces leaves the pyramid of Cheops from the burial chamber. Rescued astronomy. The coordinates of the brightest stars at different time periods were thoroughly calculated, and astronomers suggested that during the construction of the pyramid exactly on the line where this tunnel "looks" was the star Sirius - a symbol of the god Osiris, a sign of eternal life.