People like to look at the starry sky. Whatever the circumstances around you, no matter how life forces you to “dig into” the ground from danger, sooner or later someone will still look up. Maybe that’s what makes us human.
Sides of the starry sky
When we go out into the open space on a clear night, a panorama of the starry sky opens above us. Good eye can see hundreds of bright dots in the sky. Some of them are stars, others are planets or artificial objects launched by man into space. This incredible picture is impressive, and for many people it becomes an impetus to learn more about the Universe.
But the way to science begins with understanding the basic things. We perceive the starry sky as a hemisphere. Orientation in it begins with the definition of the main directions: North, South, East, West, zenith and nadir. Zenith is the easiest to find: this is a point directly above the observer’s head. Nadir is in the opposite direction to it, under our feet.
Orientation to the cardinal directions should begin with determining the direction to the North. On a starry night in the territory of Ukraine, it is easiest to do this with the help of Dippers. The brightest stars of the Ursa Major and Ursa Minor form something similar to dippers in the sky. No wonder the Chumaks, who went to the Crimea for salt centuries ago, gave them the names of Big and Small Dippers
How to find the North
The Ursa Major is easier to find because it is much bigger. We are interested in the two front stars of the “dipper”, which are called Dubhe and Merak. It is necessary to draw a line through them in the direction from where the load would be placed in the dipper. If we measure approximately 5.5 distances between these two stars on this straight line, then we can find the “tail” of Ursa Minor. At its end is the Polaris. However, in the Southern Hemisphere this method does not work: There the Polaris does not appear above the horizon (at least in our era).
Polaris practically does not change its position in the sky, while other objects participate in the daily rotation of the celestial sphere. To be more precise, this star makes a small circle with a radius of less than a degree around a conditional point, which is called the Celestial pole. If we draw a line from the zenith to the horizon through this point, we get a direction to the North.
In the evening and in the morning, you can also orient yourself by the place of sunrise and sunset. This method is not so correct, because during the year the points of its sunrise and sunset shift. At sunrise, it is necessary to stand with your right hand on the bright side of the sky. After sunset, the point where it hid behind the horizon should be left on the left. In both cases, the North is in front of you, and the Polaris can be found just by looking up. Its height above the horizon is equal to the latitude of the observation site.
How the starry sky rotates
After that, it’s enough to spread your hands to the sides and use your imagination. Ahead is the North, behind — the South, left — the West and to the right — the East. If the area is still unknown to you, you should link these directions to some noticeable objects: trees, buildings, bright lights on the horizon. After all, then you will have to deal with the rotation of the celestial sphere.
Actually, there is no rotating celestial sphere. The Earth orbits around the Sun and rotates around its own axis, that’s why the surface point on which we are located turns in the direction of certain stars that are not attached to any sphere and are located at different distances from us.
However, it is easier for us to imagine the sky as a sphere rotating with the stars around an axis passing through the North Celestial Pole. Polaris is located near it (there is no such bright star near the South Pole now). This imaginary line is called the celestial axis, and during the day all the stars rotate around it. At this time, they move in the same direction as the Sun, i.e. clockwise. They rise above the horizon, up to the highest position, and then set down.
Сelestial equator, meridian and non-setting stars
If the celestial sphere, as the Earth, has an axis of rotation, then it should also have an equator. It is called the “Celestial equator”. It is a circle on this sphere, equidistant from both poles. Another important large circle is called the “Celestial meridian”. It passes through the celestial pole and the Zenith. Despite the fact that all the stars rise above the horizon at different heights, during their daily movement they cross the celestial meridian. During its intersection they rise above the horizon, i. e they pass the culmination. It should be noted that when people say “High sun”, they most often mean its culmination, which marks the local noon. For observers in Ukraine, the High sun never rises.
There are also stars on the territory of Ukraine that do not hide under the horizon either during the daily or during the annual rotation. These are objects that are located close enough to the North Celestial Pole. These include all the stars of Ursa Major and Ursa Minor, Capella, Vega and the constellation Cassiopeia, similar to the letter M (or W).
The Ecliptic and the Milky Way
Everything mentioned above concerns only the movement of the stars. The Sun and planets rotate according to their own rules and constantly move relative to the “fixed” stars. Because of this, the planets got their name, which means “travelers”. But they also have something in common.
All the planets and the Moon move across the sky not far from the line along which the Sun moves among the stars. This line is called the ecliptic. In fact, this is a projection of the plane of the Earth’s orbit on the celestial sphere. The planes of other planetary orbits deviate from it by a maximum of a few degrees.
Another important detail of the starry sky is the Milky Way, a light band “encircling” the entire celestial sphere. This is the main plane of our Galaxy. And in the constellations of Archer and Scorpio, where this line bifurcates, there is actually a galactic nucleus hidden by dark clouds of dust. Furthermore, this plane does not coincide with either the celestial equator and the ecliptic.
Local coordinate system
How can we clearly indicate the position of a particular object in the sky so that other observers can find it? The easiest way is to “start from” the horizon and a fixed direction to one of the cardinal directions. This is the so-called horizontal system of coordinates. It can also be called local, because it is tied to the point where the observer stands.
In this system, a celestial body has two coordinates: azimuth “Az” and height above the horizon “h”. Instead of the latter, the zenith distance (90°-h) can also be used. It determines how high you need to raise your head or telescope tube to see an object.
Azimuth (from the Arabic “as sumut” — “direction”) is defined as the angle between the point of the north and the point of intersection with the horizon of a large circle drawn through the zenith and the celestial body. It is counted from left to right. The azimuth of 0° corresponds to the North in this coordinate system, 90° to the East, 180° to the South, 270° to the West.
Since our planet, as already mentioned, rotates around its axis, all astronomical objects constantly change their position in the reference system, “attached” to the horizon. This greatly complicated the lives of astronomers and forced them to invent another system of celestial coordinates. It will be discussed in the next article.