Chapter 50 Section 3 Planetary Position Calculation

The positions of planets in the background of the starry sky were generally calculated in the early days based on the average daily orbital degrees.Planetary orbits are almost circular, so the average position is close to the real position, and the approximate calculation has little effect.Comparing the positions of planets in the background of stars, Chinese astronomers are more concerned with the distances of planets from the sun.In the previous observations, we already know how long it takes for the planets to come to special positions such as conjunction and stay, but from the stages of conjunction to stay, prograde and retrograde, the speed of planetary rendezvous changes. The distance must be added or subtracted from the average value.After Zhang Zixin of the Northern Qi Dynasty discovered the inhomogeneity of the solar movement, this correction value also includes the variation data of the solar movement.After correction, the difference between the true celestial longitude of the planet and the true celestial longitude of the sun is what you want.

The above corrections made by ancient astronomers are correct in thinking, but the change in the speed of planetary conjunction is partly due to the relative position changes between the earth and the planets, which cannot fully reflect the real situation of planetary motion, so the apparent motion The perihelion and aphelion obtained from the speed change are far away from the real position.The correction value table is compiled according to the perihelion, which affects the correction value and makes the actual calculated planetary position error larger.