Lagerstroemia Yangzhou

Although this question is relatively simple, it is not easy to make it clear in a few words.

The reason why the sun moves back and forth in the tropic of cancer is because of the revolution of the earth and the inclination (23.45) of its axis relative to the ecliptic plane.

With the revolution of the earth, the latitude of the earth facing the sun will change. We look at the sun on the earth as if the sun itself were moving back and forth from north to south. Every time it turns, the sun goes back and forth between the Tropic of Cancer. Just try it with balls and candles. The angle between the Tropic of Cancer and the equator is equal to the angle between the equatorial plane and the ecliptic plane. This forms the seasonal changes on the earth.

There are four points in the earth's orbit, namely the vernal equinox, the summer solstice, the autumnal equinox and the winter solstice.

In winter solstice, the sun is directly above the tropic of Capricorn, and the daytime in the northern hemisphere reaches the minimum, then the sun gradually moves northward, and the daytime in the northern hemisphere gradually becomes longer. When moving directly above the equator, the whole world is as long as day and night, which is the vernal equinox. Then continue to move north, and the days in the northern hemisphere begin to get longer and the nights get shorter. When moving to the tropic of cancer, it is the summer solstice, and the number of days in the northern hemisphere reaches the maximum.

Then the sun moved south, and the days in the northern hemisphere began to get shorter, but the days were still long and the nights were short. When it runs above the equator, the global day and night are equal in length, that is, the autumn equinox. Moving further south, the nights in the northern hemisphere are long and the days are short, until it moves to the tropic of Capricorn, and the daytime in the northern hemisphere reaches the minimum.

So far, the sun has made one revolution, that is, the earth has made one revolution around the sun, with a period of one year, and so on.

Because Yangzhou is located in the northern hemisphere (about 32 north latitude, north of the Tropic of Cancer), the sun's trajectory in a day does not start from a straight line, but an arc protruding to the south (if it is in the North Pole, this arc will close into a circle, that is, the sun will circle along a certain height above the horizon), so the sun can't reach directly above Yangzhou for a period of time around noon, but it can appear in the east or west direction before noon.

If the sun sets due west of Yangzhou, the day is getting longer and longer, which means that the sun is in the northern hemisphere (latitude can be calculated) and moving northward. In other words, the earth should be between the vernal equinox and the summer solstice (moving to the summer solstice).