Why sometimes there are stars at night and sometimes there aren't?

It's normal to see it, but it's not good, or the light pollution there is too serious.

Most stars are dark and can only be seen when the background of the night sky is darker than it. During the day, the sky background is brighter than the brightest star, so you can't see the stars during the day, but occasionally you can see the moon; At night, there will be more stars as long as the background is dark enough; If you can't see the stars at night, it's nothing more than clouds or smog or your background light in the city center is too harmful. If you want to see the Milky Way in China, go to a place without light pollution, such as suburbs, countryside or wild forests in the mountains. If you happen to be in a big city, basically don't consider seeing the Milky Way.

Light pollution prevents us from seeing the stars, which is only a small part of its harm. In fact, people's love for light is much more harmful to animals and plants than we thought. The earth's ecosystem has completely adapted to and relied on the cycle of alternating day and night in the past billions of years. The disappearance of night caused by the invention of artificial light source has seriously affected the habits of some creatures.

If the moonlight is dark, corals are likely to miss the correct breeding time, resulting in a decrease in coral population. Light pollution is not good for some terrestrial creatures. Birds that migrate at night often bump into brightly lit buildings, or fly around the light source until they are exhausted and fall to the ground. Amphibians such as lizards feed at night. They will be misled by the bright night sky and come out for food later, so they will find less food. More seriously, frogs, their males only make courtship calls when they feel dark enough. Too bright sky will hinder their reproduction.