the battle of brightness
Have you ever noticed that your computer screen seems much brighter in a dark room? Or that on sunny days, it’s sometimes hard to read the screen on your phone? This is because different sources of light compete with each other when you look at them. To really understand what’s happening, you need to learn about how your eyes work and what makes up light. Those things are worth learning about, but they are a bit off-topic here. For now, all you need to know is that two main things affect how bright something looks: the strength of a light source and how far away it is. If there are competing light sources, the one that’s strongest and closest to your eyes will win the battle of brightness.
When you try to use your phone on a sunny day, there is another light source--the Sun--that is much more powerful than your phone and can make your phone’s screen hard to see. Your phone is much closer to you than the Sun is, but the Sun is much, much more powerful than your phone. On the other hand, if you look at your phone in a dark room, your screen will be easy to see--maybe even too bright. That’s because, in this case, your phone is the strongest light source and the closest light source.
Light pollution
Like the Sun, the stars are actually shining all the time. We can only see them at night, though, when the Sun’s light isn’t in the sky to compete with them. Other sources of light can make the stars tougher to see. One of the largest is the moon, especially when the moon is full. Because it’s so bright (and so close) the moon is the easiest thing to see in the night sky.
Artificial lights can hide the stars as well. Artificial lights are things like flashlights, spotlights, street lights, electric lamps, neon signs, even bug zappers. Any light source built by humans is considered an artificial light--and cities have a lot of them. Even small cities have hundreds of lights that shine all through the night. That’s why it is difficult to see stars when you are in a city.
Artificial lights can hide the stars as well. Artificial lights are things like flashlights, spotlights, street lights, electric lamps, neon signs, even bug zappers. Any light source built by humans is considered an artificial light--and cities have a lot of them. Even small cities have hundreds of lights that shine all through the night. That’s why it is difficult to see stars when you are in a city.
Even if you live near a city, its light sources may be strong enough to hide many of the stars in the sky. You may only see a few of the brightest stars. The images below show recent pictures of North America and Europe from space. These were pictures taken by NASA, the United States government's agency that explores and studies space.
Notice how bright both continents look? Especially the eastern half of the North America? That's because of the light from all of the large, brightly lit cities in those areas.
Astronomers call this effect light pollution because light is added to the sky with a bad consequence--starlight becomes harder to see, if you can see it at all. The best places to view the night sky are away from cities.
Astronomers call this effect light pollution because light is added to the sky with a bad consequence--starlight becomes harder to see, if you can see it at all. The best places to view the night sky are away from cities.