Since we can’t get to all the planets to study them, we learn what we are able to from the places we can reach and then compare them with any facts we can gather about other places. This helps us make good guesses about what they are like.
We have been to the moon. It’s the closest celestial body to earth. Mercury is the closest planet to the sun. The moon orbits earth and Mercury orbits the sun. They are both gray and covered in craters.
We only ever see one side of the moon. The moon orbits the earth once every 27 days. It also spins around once every 27 days.
Mercury turns very slowly, but even more slowly than the moon. It takes two years for Mercury to turn around once. Can you imagine morning and night each being a year long? That’s what it’s like on Mercury.
Mercury is larger than the moon and is made of heavy iron instead of the lightweight rock of the moon.