Planets' sizes are measured from their angular width as seen from the Earth. The basic scale of distances is calibrated by measurements of Venus from different points on Earth at the time of a transit of Venus.
Astronomers use a telescope to view planets directly.
Astronomers measure it by predicting with the stars and using scientific tools
Parallax is the method that astronomers use to measure the distance from the sun to the earth.
Astronomers cannot locate extrasolar planets with telescopes because these planets are outside of our solar system and too far away for telescopes to directly observe. Instead, astronomers use indirect methods such as radial velocity and transit techniques to detect these distant planets.
Teams of astronomers have studies all possible planets.
Their eyes, ground-based telescopes, space telescopes.
Average density of the planet
Many astronomers, historians, regular people, and scientisis know about the planets in our Solar system.
They use complicated trigonometry and mathematics.
the sun
astronomers
How do they measure what?