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In the clear night sky, there are countless twinkling stars, forming various patterns in groups of three or five, some bright and some dim, and the colors are also different. The magnificent starry sky arouses people's infinite reverie and inspires people to explore the mysteries of cosmic celestial bodies. Knowing the starry sky, observing celestial bodies and further understanding the physical properties of stars is one of the entry ways to learn astronomy. Observing the starry sky can cultivate scientific observation ability; Understand the constellation, master the law of starry sky operation, and also determine the time, season and direction. Recognizing stars can not only cultivate scientific interest, but also be a beneficial scientific and cultural activity. Changes in the starry sky Almost everyone will notice that the starry sky is not static. The huge sky rotates silently from east to west. The stars in the west gradually sink below the horizon, and new constellations are constantly rising on the horizon in the east, while the stars in the south gradually tilt to the west. As we all know, the apparent motion of stars on Sunday is a reflection of the earth's rotation from west to east. Every 23 hours and 56 minutes, the star orbits in parallel at the speed of 15 degrees per hour on Sunday. Therefore, we can see all the local visible stars from dusk to the whole night before dawn the next day. Generally speaking, people can see six stars with the naked eye, and the total number of stars is about 6000. Obviously, this number will vary greatly due to different eyesight. In addition, the number of stars seen in a place varies with geographical latitude. People in equatorial regions can see all the stars overnight, while observers in polar regions can only see half of the stars in the sky even at night for half a year. Observers at other latitudes (such as geographic latitude φ) will never see the stars in the constant hidden circle (that is, stars with declination δ less than or equal to -(90 degrees-φ)). The higher the geographical latitude, the fewer stars you see. Astronomers usually divide the luminous power of stars into 25 magnitudes, and the difference between the strongest luminous power and the worst luminous power is about 65.438+000 billion times. The brightness of a star is usually expressed in magnitude. The brighter the star, the smaller the magnitude. The magnitude measured on the earth is called apparent magnitude; The magnitude reduced from the earth to 10 parsec is called absolute magnitude. The magnitude of the same star measured by detection elements sensitive to different bands is generally unequal. At present, one of the most commonly used magnitude systems is the three-color system of U (ultraviolet), B (blue) and V (yellow) (see photometry system). Photometric system); B and v are close to photographic magnitude and visual magnitude, respectively. The difference between them lies in the commonly used color index. The sun's V=-26.74, absolute apparent magnitude M=+4.83, color index B-V=0.63, U-B=0. 12. The color temperature can be determined by the color index. Stars close to people have strong luminous ability, so people will light up when they see them. However, even if a star with a fairly strong luminous ability is far away from people, its brightness may not be as good as that of a star with a luminous ability of tens of thousands of times. Because the earth rotates and revolves at the same time, it forms the seasonal variation of the starry sky: when you look at the stars at the same time in different seasons, the constellations in the starry sky are different. Stars appear and cross the border four minutes in advance every day. That is to say, the time of the appearance and transit of stars is different from that of the sun day by day. Suppose the Altair rose from the eastern horizon at 7 o'clock after dusk yesterday, then look at today's Altair, which rose at 6: 56 after dusk; It will rise at 6: 52 after dusk tomorrow; After half a month, it was 1 hour early and rose at 6 pm; Get up at 5 pm a month; A quarter of an hour later, the star appeared and disappeared six hours ago. In other words, the starry sky at 7 o'clock after dusk at the beginning of the month is equivalent to the starry sky at 6 o'clock after noon in the middle of the month, and it is also equivalent to the starry sky at 5 o'clock at the end of the month (early next month); It is equivalent to the starry sky at 3 pm two months later (the beginning of the third month), and by analogy, it is also equivalent to the starry sky at 7 am at the beginning of the month six months later. In other words, the starry sky at dusk in spring is the starry sky at dawn in autumn, and the starry sky at dawn in winter is the starry sky at midnight in autumn or dusk in summer. In other words, if we see Altair near the eastern horizon at 19 today, and arcturus, which is about 90 degrees west of Altair, is due south, then one month later, we will find that arcturus is 30 degrees west of due south at 19. Three months later, when observing the starry sky at 19, Altair is now very high to the south, while arcturus sinks to the northwest, close to the horizon. The starry sky images at the same time in different months are different, and the constellations seem to have changed, so people often divide the sky into four seasons of starry sky according to the four seasons of spring, summer, autumn and winter. The so-called starry sky in four seasons refers to the starry sky at dusk in each season. After understanding the law that the stars rise and fall once a day and rise and fall four minutes in advance every day, it is not difficult to understand why the same Zhang Chun night star map represents 23 o'clock in March, 265,438+0 in April and 65,438+09 in May. It can also represent the starry sky in mid-February 1 and at 3 o'clock in mid-February. If observed at the beginning of the month, the same star map is marked with the starry sky at 24 o'clock in early March, 22 o'clock in early April and 20 o'clock in early May, and also marked with 65438+ night sky images at 2 o'clock in early February and 4 o'clock in early June. If this star map is not used in the current month, the star map is equivalent to the starry sky at 22: 00 on March, 20: 00 on April, 24: 00 on February and 65438+2 on1October. On the other hand, if the observation is not made at the above time, such as 3: 00 in mid-April, it is not difficult to infer that the summer star map (June 23: 00, July 265, 438+0, August 65, 438+09) represents the starry sky image at that time. In short, using four star maps of four seasons can basically meet the needs of observing the starry sky at any date and at any time. It should also be pointed out that observers at different latitudes see different starry sky. The farther south they go, the more stars they see in the southern sky. Sanya, Hainan Island, China, has a geographical latitude close to 18 degrees, where you can see all the stars north of -72 degrees declination. In the capital Beijing (φ=40 degrees), only stars with declination greater than -50 degrees can be seen.