When we look up to the night sky where there is no cloud and light pollution, we can distinctly see many bright objects suspend on the sky. Some are very bright and large in size, and some are very small and difficult to determine what they are. Moreover, some of them are blinking, but some of them are not. Why?
Often, we point to the sky at any points and say “What a bright star!” However, not all of them are stars. Some of them are stars, but some of them are not. So, what are they? Light? Airplane?
Usually, stars are blinking when we see them. And, the not blinking objects are normally we call the planets. Planets? Are they the planets in our solar system? Yes, it is absolutely correct. But, why do they appear like a star? Why isn’t there any differences between star and planets?
Actually, there are differences. Before the telescope was invented, Galileo Galilei was the first person to see and differentiate the planets and stars. He can see until the sixth planet from the Earth counting from the center of Solar System. Mercury, Venus, Mars, Jupiter and Saturn. How did he determine these?
It’s not that difficult as what we thought. Firstly, planets are usually not blinking. It is because it is so close to our Earth, and the light emitted to the Earth is what the planet reflects the sunlight. So, the reflected light is always the same brightness. But, the other stars are blinking because they are too far where only can be counted by using Astronomy Unit(A.U.) or Light year. In addition, they produce light themselves and so, just like our Sun, are sometimes cooler and sometimes hotter. The temperatures are not constant. So, the brightness are also flexible.
Secondly, the motion of planets. We are standing on the Earth and look into the space to the stars and planets. So, we can use few days or months to view at the point what we are going to determine. In these periods, we can figure out the changes of location or motion of the planets or stars. The location of stars are remain, but the planets are changing. Planets are orbiting the Sun, as what the Earth does. This is another way to differentiate the planets and stars.
However, it is not very difficult to figure out whether an object is planet or star. It is because, nowadays, we have the telescope which can be used to view objects at few light years away!