Hi, there! :)
The law that allows us to calculate the semimajor axis of the orbit of distant objects is known as Kepler's Third Law of Planetary Motion, also known as the harmonic law. Therefore, the answer is e) Kepler.
Johannes Kepler was a German mathematician, astronomer, and astrologer who lived in the 16th and 17th centuries. He discovered his laws of planetary motion, including the third law, which states that the square of the period of an orbit is proportional to the cube of the semimajor axis of the orbit. Kepler's laws of planetary motion were a major breakthrough in the development of modern astronomy and laid the groundwork for Isaac Newton's theory of gravitation.
Hope that helps! Good luck! ^_^