Planets of the Solar System: Orbits & Visibility

Planets of the Solar System: Orbits & Visibility
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  • 0:02 Are All Twinkling…
  • 0:38 5 Naked-Eye Planets
  • 1:18 Planets in the Night Sky
  • 2:33 How the Planets Move…
  • 5:18 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
This lesson will tell you which planets in our solar system you can see with the naked eye and which ones you cannot. You'll also learn how they appear to move in our sky and why we can see them in the first place.

Are All Twinkling Lights Stars?

The next time you are outside on a clear night, with as little night pollution as possible, you'll be lucky to see a sky filled with twinkling lights. What are those twinkling lights? Do you know? They are, for the most part, distant stars shining brightly. I know you know that. But not every light that's twinkling is a star. Some of those lights are actually planets, but they don't shine brightly on their own like distant stars. You'll learn which those planets are, why we can see them if they don't produce any light of their own, and how they move in our night sky.

The Five Naked-Eye Planets

The planets in the night sky that we can see with the unaided eye, the ones that look like shining stars, are known as the five naked-eye planets. They are Mercury, Venus, Mars, Jupiter, and Saturn.

If you've got some good eyesight and the conditions are just right, you might, just might, be able to see Uranus with your unaided eyes. Urban light pollution has made it near impossible for modern humans to see Uranus without binoculars or a telescope. But Neptune, the farthest planet from the Sun, is invisible to us without a telescope, regardless of anything else.

Why We Can See Planets in the Night Sky

None of the planets you can see with your eyes or with a telescope produce any visible light. If you go to the bathroom, where you probably have a mirror, shut the door and turn off the lights, what will you see? Nothing. Why not? Because nothing in the room is producing any light. So how is it then that we see the planets shining like stars in the night sky if they produce no light?

It's because they reflect light. Reflection is simply the process where a wave bounces off a surface it has struck. Light travels as a wave. So when it hits a reflective surface, it bounces off and travels in another direction.

In the dark bathroom, you can use a flashlight and shine it at the mirror. You'll notice that much more than the mirror will light up in the bathroom because light will reflect off of the mirror and travel to other points in the washroom. Like that flashlight, our Sun's light waves bounce off of the planets, and make it seem like the planets are producing visible light when they're actually simply reflecting it back in a way similar to, but not exactly like, a mirror.

How the Planets Move on Our Sky

All of the planets in the solar system move in orbits that are almost circular. If you were able to look at how the planets orbit the Sun in space from above, that is to say, from the north celestial pole, then you'd see they all orbit in a counterclockwise direction.

And like runners who are farthest away from the leader out in front, the farther the planet is away from the Sun, the slower it moves compared to a planet nearer the Sun. But how do our solar system's planets appear to move to us in the night sky?

The four outer planets of the solar system - Jupiter, Saturn, Uranus, and Neptune as well as Mars - generally appear to move slowly eastward in our sky along the ecliptic. However, retrograde motion sometimes makes it appear these planets move backwards, or westward in the sky. It's like when a faster runner passes a slower runner on the racetrack. It appears the slower runner is moving backwards in comparison, but you know it's only an illusion.

The ecliptic I mentioned a second ago is the circle around our sky the Sun traces out over a year. It represents the plane in outer space where the Earth's orbit around the Sun is located. The orbits of these outer planets are tilted only slightly to this plane, and so, their paths in the sky are thus basically close to the ecliptic. Just in case you were wondering, the ecliptic is called the ecliptic because an eclipse can occur only when the Moon crosses it.

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