What are the directions with your back to the Big Dipper at night?

There are two easily recognizable constellations in the northern sky, one is Ursa major and the other is Ursa minor (Figure 1). ?

The constellation Ursa major is arranged like a water ladle in the northern sky, so it is also called the Big Dipper. Beidou refers to the Big Dipper, not a star. ?

The arrangement of Ursa minor is similar to that of the Big Dipper, but the brightness is not as good as that of the Big Dipper. Therefore, it is the Big Dipper that attracts people's attention at all times and all over the world. ?

As early as in ancient Greece, navigators wanted to go to the North Pole, which was guided by the Big Dipper. When they arrived at the North Pole, their first perceptual knowledge was that this was the world of bears. Therefore, they call the Arctic region "Great Bear Country", and the seven bright stars above "Great Bear Country" are called "Ursa Major". It can be seen that the name "Ursa Major" has been used for more than 2000 years. ?

The last star on the barrel handle of Ursa minor is called Polaris. Because it is located near the north pole of the celestial sphere (the zenith of the earth's north pole), it is closely related to some human activities. For example, a lost traveler can find the approximate true north direction according to the Polaris. ② According to the horizon height of Polaris, we can know the geographical latitude of the observer. For example; If the height of Polaris observed in Beijing is 40 degrees, then the geographical latitude of Beijing is 40 degrees north latitude (Figure 2). ?

As can be seen from the above, Polaris is of great significance to astronomy, navigation and aviation. ?

So, how to find Polaris? This can be found with the help of the big dipper. As shown in figure 1, it extends from the pointing star in the Big Dipper (that is, the collective name of ① and ② stars) to the direction of Doukou, and the star at a distance of 5 times between the pointing stars is the Polaris. ?

However, during the day and night, the Big Dipper always rotates counterclockwise around the North Star, so its position is constantly changing. When Polaris runs below the horizon, it must be found according to the Hou Xian constellation. ?

The position of the Big Dipper also changes with the seasons, as shown in Figure 3. At dusk in spring, the bucket handle of the Big Dipper points to the east. Summer guide (i.e. upward); Autumn refers to the west; Winter refers to the north (that is, face down, below the horizon). These phenomena are very helpful for the ancients to infer the seasons.