Earthed

How did scientists work out the size of planets?

I've been looking for an answer but all I can find is the dimensions of planets. Does anyone know how scientists figured out the size in the first place?

Public Comments

1. i believe they use complex geometric equations to find the size of planets.
2. dont know of a mathematical way. perhaps, they started with knowing the size of the earth and its relationship to the moon and sunlight /shadow, etc.
3. The angle subtended by the particular planet is measured in the telescope. Typically it will be on the order of 5 to 20 arcseconds. Since we know the distance to the planets (bounce radar off them if nothing else), it is simple geometry to figure what size goes with the angle and distance.
4. Nothing mentioned above!
   
   The measure the revolution time of a planet!
   
   They take a point on the equator of a planet and they measure the time when that point occurs in the same position. Using the day duration they calculate the radius and the rest measurements of a planet.
5. For planets in our solar system, the angular diameter can be measured visually through a telescope. Then, knowing how far away the planet is, you can calculate its diameter. The distances to the other planets were calculated first using parallax and Kepler's laws, and later with radar and other techniqes to a high degree of accuracy. Of course, we have also sent probes to orbit several of the planets, so we've had a good close look at those.
   
   
   
   For extrasolar planets, often all they have is a mass estimate, from the amount of wobble the planet causes in the star, and some assumptions about density. They've been able to make a few size estimates on extrasolar planets by seeing how much of its star's light was blocked when the planet transited in front of the star.