The sun rotates once every 25.4 days (average period; The equator rotates faster than high latitudes) and revolves around the center of the Milky Way every 200 million years. The sun is slightly flattened due to its rotation, which is 0.00 1% different from the perfect sphere, which is equivalent to a difference of 6km between the equatorial radius and the polar radius (the earth is 2 1km, the moon is 9km, Jupiter is 9000km, and Saturn is 5500km). Although the difference is small, it is important to measure this flatness, because any slightly larger flatness (even 0.005%) will change the influence of solar gravity on Mercury's orbit, making the test of general relativity based on Mercury's perihelion precession unreliable.

Solar radiation energy reaching the upper limit of the earth's atmosphere is called astronomical solar radiation. When the earth is located at the average distance between the sun and the earth, the total energy of the whole spectrum of solar radiation received by the unit area where the upper boundary of the earth's atmosphere is perpendicular to the solar rays in unit time is called the solar constant. The common unit of solar constant is watt/square meter. Due to different observation methods and techniques, the obtained solar constant values are also different.