Video Transcript
Which planet in the solar system
moves slowest along its orbit around the Sun?
In our solar system, we have the
Sun at the center and eight full-sized planets that orbit around the Sun. Starting closest to the Sun, these
eight planets are Mercury, then Venus, then Earth, then Mars, then Jupiter, then
Saturn, then Uranus, and then finally Neptune. We need to work out which of these
planets moves the fastest around the Sun.
Normally, when we think about the
speed of a planet, we would use the formula 𝑠 equals two 𝜋𝑟 divided by 𝑇, where
𝑠 is the orbital speed, 𝑟 is the orbital radius, and 𝑇 is the orbital period. However, we haven’t been given any
information whatsoever about each planet’s orbital period. That means we can’t use this
equation to help us with this problem. Instead, we should recall that when
a planet orbits a star, the planet’s speed is determined by its distance away from
the star and the mass of the star.
Here, because all the planets are
orbiting the same star, the Sun, we’ll only need to consider their distances from
the Sun in order to compare their speeds. We can say that these two
quantities, the planet’s speed and the planet’s distance from the Sun, vary
inversely because as one quantity increases, the other quantity must decrease and
vice versa. Because of this, a planet that’s
closer to the Sun moves faster around the star. And if a planet is further away
from the Sun, it moves slower around the star.
If we look back at our drawing, we
can see that Neptune is the furthest planet away from the Sun. This means that Neptune will be the
slowest planet to orbit around the Sun. And this is our final answer.
