leonids

How often does the meteor shower appear? Is it a constellation? Which one is the most frequent?

1 1 once a year Among all kinds of meteor phenomena, the most beautiful and spectacular is the meteor shower. When it appears, millions of meteors are like shining ribbons. Meteor shower is a special phenomenon, in which groups of meteors seem to come from a point in the sky and fall with them. This point or a small sky area is called the radiation point of meteor shower. The radiation point is a perspective effect. The fundamental reason for the formation of meteor shower is the fragmentation of comets. Comets are mainly composed of ice and dust. When the comet approaches the sun, the ice will evaporate, so that dust particles will be ejected from the mother body and enter the orbit of the comet like water in a fountain. However, large particles still remain around the mother comet, forming a dust comet head; Small particles are blown away by the radiation pressure of the sun, forming tails. The rest of the material stays near the comet's orbit. However, even a small eruption speed will cause a great difference in particle period of revolution. Therefore, in the next comet return, small particles will lag behind the mother, while large particles will lead the mother. When the earth passes through the dust tail orbit, we have a chance to see the meteor shower. Meteor shower activity cycle: Five dust particles located in the orbit of comets are called "meteor swarms". When meteoroid particles are just ejected from comets, their distribution is relatively planned. Due to the gravity of the planet, these particles gradually spread all over the orbit of the comet. At present, this process is not very clear. When the earth passes through the meteoroid group, there may be various forms of meteor showers. Every year, the earth passes through the orbits of many comets. If there are meteoroid particles in orbit, periodic meteor showers will appear at the edge. A meteor shower that only happens when the mother comet runs to the perihelion is called a near-comet meteor shower. This shows that the meteoroid group is still near the comet. Most meteor showers formed by comets with a period of less than several hundred years are of this type. Due to the gravitational perturbation of the planet, the meteoroid group of the long-period comet may be far away from the parent comet. Meteor showers may also occur when the parent comet is not at perihelion, which is the far comet meteor shower. In order to distinguish meteor showers in different directions, meteor showers are usually named after the constellations in the sky where the meteors radiate. For example, every year165438+1October 17 or so, the radiant point of the meteor shower is in Leo, so it is named Leonid meteor shower. Other meteor showers include Aquarius meteor shower, Orion meteor shower and Perseid meteor shower. Some meteors are single, the direction and time are random, and there are no radiation points at all. This kind of meteor is called accidental meteor. Meteor shower and accidental meteor are essentially different. Sometimes only a few meteors appear in an hour, but they all seem to "flow out" from the same radiation point, so they also belong to the category of meteor showers; Sometimes, in a very short time, thousands of meteors can be seen at the same radiation point, just like people setting off fireworks in festivals. When the number of meteors per hour exceeds 1000, we call it "star cluster".

There are many records about meteor showers in China ancient books. The record of "In the fifteenth year of Guangxi, the stars died like rain at night" in the annals of bamboo books is a meteor shower in the Shang Dynasty in BC16th century, and it is also the earliest meteor shower record in the world. "Spring and Autumn Annals" recorded a meteor shower in the seventh year of Duke Zhuang of Lu (687 BC): "In summer and April, Xin Mao, at night, the stars disappear, and the stars are like rain." This is the earliest record of Lyra meteor shower. The Book of Song recorded a meteor shower at the end of February (April 9, 433 AD) in Yuanjia, Liu Song in the Northern and Southern Dynasties: "A meteor as big as a peach left Tianjin Gate and entered the Purple Palace. In a short time, there were thin meteors or five or three meteors in a row, and another big meteor flew out of the purple palace, which made people feel like Beidou Kui. After a while, another big meteor came out, crossed the cable, crossed the sky and the city wall, and the meteor went north until dawn. " This is an observation record about the η meteor shower in Aquarius.

The appearance time and orientation of many celestial phenomena can be predicted in advance according to calculation, but the prediction level of the size and accurate time of meteor shower by modern science is not satisfactory. Leonid meteor shower has appeared on the earth 1799, 1833 and 1866. According to the law of the return of Comet Temple-Tuttle after about 33 years (orbiting the sun 1 week), it is predicted that the meteor rainstorm will come to the earth again in 1899, and will exceed 65,438. The result is disappointing: there is only sporadic light rain, only 40 times per hour. For the Leonid meteor shower of 1998, the maximum center time is predicted to be 0 1 18+0 1 Beijing time, when the radiation point is very low in the west and slightly higher in the east of China. It coincides with the end of the lunar calendar and there is no moonlight interference. In the second half of the night, you can observe the second half of the meteor shower, which is a rare observation opportunity. However, when people look up at the sky as scheduled, they can only see meteors without rain. The meteor shower occurs around 17+0 1. It is believed that the dense area of comet debris may deviate due to the gravitational force of the planet, which changes the trajectory of meteor group. In order to predict meteor showers more accurately, astronomers need to make greater efforts in exploring the mechanical and physical mechanisms of meteor swarms.

Meteor shower is a godsend beauty that can be feasted on. Meteor shower will not directly harm people on the ground, because the solid particles in the debris-intensive area formed in Xiangxing orbit are very small, and when they break into the earth at a high speed of tens of kilometers per second, they will be rubbed and burned in the atmosphere of tens of kilometers on the ground. However, meteor storms pose a threat to the safety of spacecraft in the earth's atmosphere. Before each meteor shower comes, it is a big headache for the people concerned.