Is it a Star or a Planet?
Have you ever looked up into the night sky, seen a bright light, and wondered to yourself: “Is that a star or a planet? And if it’s a planet, which one is it?” If so, wonder no further! Ask Science has simple rules to help make you become an expert on the night sky.
Watching the starry sky is one of my favorite pastimes – especially when I’m in the countryside and there’s not a ton of pollution blocking the view. Some of the most exciting things to watch in the night sky are the other planets in our solar system. But how do you know exactly what you’re looking at? Is that shiny orb a planet? A star? a UFO?
I’m glad you asked.
There are several ways to tell whether that point of light you’re seeing is a planet or a star. The most common method is to see if the light appears to twinkle, that is change in color or brightness.
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Is it a Planet or a Star?
Even though stars are enormous, they are so far away from the earth that they appear very, very small. (With the notable exception of our sun). The light from the star gets refracted, or bent, ever so slightly as it passes through the different layers of the Earth’s atmosphere. This bending causes the star to appear to twinkle.
The planets in our solar system are much closer to the Earth. The light we see from the planets is sunlight being reflected off of the planet. Since the planets are so much closer to the Earth than the stars, they appear larger to us. When their light gets bent by the Earth’s atmosphere, the amount the light bends is small compared to how big the planet looks, so there’s no twinkle.
Like all great rules however, this one has an exception. Sometimes the Earth’s atmosphere is moving around so much, or has such a large amount of pollution, that even the light from planets can appear to twinkle.
Caught in My Orbit
The way ancient astronomers were able to first distinguish between planets and stars is that while the stars do appear to change position in the night sky, they do so all together. Planets on the other hand move in specific orbits independent of the motion of the stars.
The planets also follow the general path of the ecliptic, the imaginary path that the sun follows across the sky from east to west.
So if you see a bright, non-twinkling light along that path, there’s a good chance that it’s a planet.
And the Planet Is…
So now that you can tell a star from a planet, how do you know which planet you’re looking at? Some of them are pretty easy to recognize. Here’s how…
If you see a very bright light near the horizon just after sunset or just before sunrise, it is probably Venus. If you see a bright red light that isn’t twinkling, there’s a good chance that it is Mars.
Jupiter is a very bright, yellow light. Unfortunately Saturn, while not as bright as Jupiter, is also a fairly bright yellow, making it difficult to tell the two of them apart. Fortunately you can find pleny of information about which planets to expect to see during different times of they year from places like stardate.org, or in references books like my personal favorite, The Peterson Field Guide to Stars and Planets.
Mercury is only rarely visible since it is so close to the sun, and the rest of the outer planets are typically too far away to see with the naked eye.
Conclusion
So now you have a better idea of how to tell a star from a planet, and a general idea of how to tell which planet is which. As for UFOs…that’s another story.
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Solar System image by Kabsik Park at Flickr CC BY 2.0.
About the Author
Dr. Lee Falin earned a B.S. in Computer Science from the University of Illinois, then went on to obtain a Ph.D. in Genetics, Bioinformatics, and Computational Biology from Virginia Tech.
