How do they know how far is a planet?

Answer 1

Within our solar system, it's really easy to tell each planet's distance from the sun.

From Newton's law of gravitation, we realized that the time it takes a planet to revolve around the sun only depends on the planet's distance from the sun, and if we know the distance of one of them, we can figure out all the others.

If we watch a planet move through the sky, we can see how long it takes for that one to revolve once around the sun. Then we need to know both the distance and time of one planet, and from that one we can calculate all the others.

The one for which we know both the distance from, and time to orbit, the sun, is ... our Earth ! With this information, plus watching another planet to see how long it takes to orbit the sun once, we can calculate how far it is from the sun.

Answer 2

Scientists can determine the distance of a planet from Earth using various methods, such as parallax measurements, radar ranging, or spectroscopic measurements. Parallax involves measuring the apparent shift in a star's position when viewed from different locations on Earth. Radar ranging uses radar signals bounced off a planet to calculate its distance. Spectroscopic measurements analyze the light emitted by a planet to estimate its distance based on factors like brightness and spectral characteristics.