The Milky way is a huge galaxy with a radius of about 50000 light-years. The Milky Way near the center of the Milky way is a star dense area, while the number of stars near the edge of the Milky way is less. The sun is about 26000 light-years away from the center of the Milky Way galaxy, slightly inclined to the edge of the Milky Way galaxy.
The earth revolves around the sun day and night. In summer, the earth is just between the sun and the center of the Milky Way galaxy, and the night sky faces the center of the Milky Way galaxy. We can see the center of the Milky Way galaxy with the most dense stars, which is naturally twinkling with stars. In winter, when the sun turns to the edge of the Milky way, and the night sky faces the outside of the Milky way, we see mostly the edge of the Milky way with sparse stars, with few and sparse stars.
Of course, the position of the earth is excellent only for stargazing. If the earth is in the center, the night sky may be many times brighter than it is now, and the lights may not be on at night, but we will no longer see stars like diamonds, because most of the centers of the Milky Way galaxy are old stars and red. If the earth is unfortunately on the edge of the Milky Way galaxy, there may be few stars in the night sky, or even empty.
Meteors are a kind of interplanetary matter that breaks into the atmosphere and frictions with air in the atmosphere to emit light. In the space near the earth, in addition to other planets, there are all kinds of interplanetary matter. This interplanetary material is called meteoroids. The small one is like dust and the large one is like a mountain. It moves in space according to its own speed and orbit. They do not emit light by themselves. When they "collide" with the earth, the speed of meteoroids relative to the earth's atmosphere is very high, up to 10 ~ 80 kilometers per second, dozens of times faster than the fastest plane. When the meteoroid penetrates into the earth's atmosphere at such a high speed, it rubs violently with the atmosphere and burns, heating the air to thousands of degrees Celsius or even tens of thousands of degrees Celsius. Under the action of such high-temperature airflow, the meteoroid itself also vaporizes and emits light. The burning of meteoroids in the atmosphere is not finished at once, but gradually burns with the movement of meteoroids, thus forming arc light. There are two kinds of meteors. One is a single meteor, and the other is a meteor shower that appears in batches like raindrops.
To facilitate research and observation, naming stars is the first step. It has a specific nomenclature. The international common method of naming stars is to arrange all stars in each constellation from light to dark, and then use Greek letters α、β、γ And so on, and add the name of the constellation before the Greek letter. For example, Ursa Major α、 andromeda β Wait. What happens when you run out of 24 Greek letters? Then put it down in Arabic numerals, such as "61 Scorpio". In this way, the stars in the sky can be given different names.
Before the 11th century, the starry sky was divided into many "stars". The meaning of stars and constellations is similar. Sanyuan 28su "fixed" the stars in the sky and named them, such as weaving actress, old man star, Sirius and so on. These names have always been used in astronomy.
In 1610, Italian astronomer Galileo discovered IO, Europa, Ganymede and Callisto with his homemade telescope. Later, these four satellites were called Galileo satellites. The mass of these four Galileo satellites is larger than Pluto, and Ganymede is the largest satellite in the solar system, with a diameter larger than mercury. Inside the orbit of the four Galileo satellites, there are four Jupiter satellites, namely Ganymede 16, Ganymede 15, Ganymede 5 and Ganymede 14, of which Ganymede 16 is the nearest satellite to Jupiter. From Galileo's satellite, the distances are Ganymede 13, Ganymede, Ganymede and Ganymede. They rotate clockwise around Jupiter with a period of about 260 days. Then outward, there are Ganymede 12, Ganymede 11, Ganymede 8 and Ganymede 9, which rotate in reverse, with a period of about 700 days. With so many satellites orbiting Jupiter, it's no wonder Jupiter has the reputation of "small solar system".
When the moon does not rotate (left), it rotates in the same cycle as the revolution (right), and faces the earth differently.
The moon revolves around the earth on one side and rotates on the other side. The time of its rotation is exactly the same as that of its revolution around the earth, which is 27.3 days. Therefore, when the moon turns an angle around the earth, it also rotates the same angle. If the moon turns 360 degrees around the earth, it also rotates a circle, always facing the earth on one side and carrying the earth on the other side. More accurate observation can find that the moon moves around the earth along an elliptical orbit, and its revolution speed is not as uniform as its rotation speed, and its rotation axis is not perpendicular to the orbit surface of revolution. Therefore, sometimes we can still see a small part of the back of the moon. Compared with the front, the terrain on the back of the moon is more uneven and uneven; The plain occupies less area, while the crater is more.