In recent years, the word “Shuini” is quite popular. In Western astrology, there is a saying that Mercury is retrograde and bad luck. Many people think that when things go wrong, they make fun of whether it’s “water adversity” or not. There are also popular Internet sayings such as “young people don’t work hard, old people blame water adversity” and “as long as they look ugly, water adversity happens every day.”. In fact, from an astronomical point of view, to see Mercury is really not an easy thing. How many people have actually seen mercury? Not to mention retrograde Mercury. The reason why mercury is not easy to see is mainly because it is too close to the sun and is often submerged in bright sunlight. The maximum angular distance between it and the sun is no more than 28.3 ° and appears only as a morning or evening star. People can only see it at dawn or dusk, and the longest observation time is only a few minutes. < / P > < p > when will it be easier to observe mercury? It depends on whether mercury is high enough when it’s dark. The higher the horizon height of mercury is, the better the observation is. At this time, the observation time is longer, and the influence of atmospheric extinction and scene occlusion is smaller. There are only a few days of the year when Mercury reaches a relatively high altitude, which is determined by the following two factors. The first is the angle between the ecliptic and the horizon. The ecliptic is the sun’s orbit in the background of the stars. The zodiac, as we know it, is distributed on the ecliptic, and the planets basically all move on the ecliptic. The ecliptic is a large circle, which has two intersections with our horizon, and forms an inclination angle. Since the earth is rotating, the position of the intersection and the size of the dip angle change all the time. For mercury observation, we are concerned about whether the inclination is high enough at sunrise or sunset. Mercury is basically located in the ecliptic, and not far from the sun, so when the inclination is large, it may appear in the higher sky. On the contrary, if the inclination is very small, Mercury will “lie” with the ecliptic near the horizon, and the height will not be optimistic. The higher the latitude, the smaller the inclination, and the more difficult it is to see mercury. The famous astronomer Copernicus lived in Poland, about 52 degrees north latitude. He had to worry about observing mercury. < / P > < p > then is Mercury the highest on the day with the largest tilt angle? The answer is no, and there is a second factor at work, that is, the angular distance of the star from the sun. If the distance is too close, the height is naturally limited. When Mercury and the sun are seen from the earth, the maximum angular distance is called “large distance”, and the angular distance between the two is called the “angle of departure”. Because Mercury’s orbit is relatively flat, the angle of departure varies from 18 degrees to 28 degrees at large distances. The largest talus angle occurred in late March and late August. If you use an upright stick to compare the height of mercury, and the angle is equal to the length of the stick, it actually limits the maximum height of mercury on a given date. The tilt angle is equivalent to the tilt angle of the stick. It is not only related to the date, but also varies with latitude, which ultimately determines the maximum height of mercury at a given date and place. < / P > < p > if the inclination angle is the largest, then mercury has a chance to reach the maximum height. Unfortunately, in the northern hemisphere, the large angle always meets the small angle, and in the southern hemisphere, the large angle superimposed with the large inclination angle appears. Therefore, generally speaking, mercury observation conditions in the southern hemisphere are generally better. In the mid latitude region of the southern hemisphere, the maximum height of mercury is about 27 ° at sunset. In the mid latitude region of the northern hemisphere, the maximum height is about 21 degrees. And the farther north, the lower the altitude of mercury, the more difficult it is to see. < / P > < p > in most areas of China, there are usually only two or three best mercury observations a year. Mercury is a faint star. From the end of March to the beginning of June, especially in the middle and late May, mercury has the opportunity to reach a higher altitude, which can be found in the western sky in the evening. Mercury is the morning star. From the beginning of September to the beginning of December, especially in the middle and last ten days of October, mercury has the opportunity to reach a relatively high altitude, and can look to the east at dawn. It is worth noting that it is not to say that Mercury in these two periods will be relatively high. Only when Mercury has a large distance during this period, the height will be relatively large. Otherwise, we will have to wait for the coming year. The year 2021 will be a good day to observe mercury, and two large distances are very close to the best observation date. They are the evening star on May 17 and the morning star on October 25. < / P > < p > the eight major planets in the solar system all revolve around the sun from west to East, and the closer the planet is to the sun, the faster the speed of rotation is. On earth, all other planets are retrograde, and mercury is no exception. The so-called retrograde means that when a planet moves in a starry background, the direction of its motion is from east to west, which is exactly opposite to that in peacetime. For an earth planet like mercury, retrograde occurs when it is on the same side of the sun as earth. Because Mercury goes faster, it will overtake the earth like a overtake. In our opinion, during this period of time, it is moving from east to West in the background of stars. When Mercury is retrograde, the angular distance between mercury and the sun is the smallest, plus the influence of the inclination angle of the ecliptic, leading to the famous water inversion, which can not be seen by the naked eye. In addition, Mercury retrogrades only 20 days at a time, the shortest of all planets. Venus and Mars retrograde for about 40 days and 60 days each time, and the slow-moving Saturn retrograde for nearly five months. However, Mercury moves fast and retrogrades every three or four months, much more frequently than other planets. < / P > < p > this kind of characteristic is just suitable for giving people all kinds of unexplained misfortune. Mars retrograde happens only once every two years. Naturally, as a shield, Venus is not much better than Mars. Jupiter and Saturn are retrograde for four or five months a year, and the time is too long, so the water retrograde becomes the most appropriate “outlet”. As a matter of fact, both anterograde and retrograde movements are the natural result of the planets’ movement according to the physical laws, which cannot be changed or avoided. Even if you change the position of earth and mercury, or observe on Pluto, you will still see water inversion and fire inversion. < / P > < p > the universe will still work as usual, and the planets won’t bring any bad luck. If there is no water reverse, then it is really a bad thing. (author’s unit: Beijing Planetarium) < A= target=_ blank>Global Tech

By ibmwl