How do scientists measure the size of the earth?
Trigonometry is one of the answers.
The first estimate of the size of the Earth was done by Erastotenes 2200 years ago. All the following were done by improvement of the method.
He determined that the distance, between Aswan and Alexandria was, by the present unit measures, about 880 km. In Aswan the Sun was completely in zenith, (above our heads) in the day of Summer solstice (about the 21st June), but in Alexandria in the same day an angle of about 7 degree was determined, between the zenith and the position of the Sun (using the shadow of a vertical pole). He realized that the 880 km corresponded to 7º degrees of the circumference of Earth. Just by the rule of three he reasoned;