Venus transit is a celestial phenomenon in which the apparent disk of Venus appears as a small black spot projected onto the surface of the Sun. It occurs when Venus moves between the Sun and Earth and the three celestial bodies are aligned in a straight line. Venus transits are rare and beautiful, making them a rare celestial spectacle worth observing. Venus transits occur in groups of eight years, either around June 8 or December 10. After each eight-year cycle, they occur at intervals of 105.5 years and 121.5 years.
The regular occurrence of Venus transits is due to the orbital motion of Earth and Venus around the Sun. Venus and Earth have orbital periods of 224.6960 days and 365.2564 days respectively, with Venus orbiting faster than Earth. When Venus reaches a position between Earth and the Sun, it is called a "conjunction". If the orbits of Venus and Earth were in the same plane, every conjunction would result in a transit; however, the two orbital planes have a 3.39° angle, so from Earth's perspective, the maximum angular distance between Venus and the Sun during a conjunction can reach 8.86°. Only when Venus reaches a conjunction near the intersection of the two orbital planes can it align with the Sun to form a transit.
The intersection of the two orbital planes has two ascending and descending points. The dates when Earth passes through these two points are fixed annually, around December 10 and June 8. The interval between two Venus conjunctions is called the "conjunction period". Let T and E be the orbital periods of Venus and Earth respectively, and S be the conjunction period, all measured in days. Starting from the conjunction, Venus completes 1/T orbit in one day, and Earth completes 1/E orbit. Venus overtakes Earth by (1/T - 1/E) orbits. When the cumulative overtaking reaches one full orbit, Venus returns to its conjunction position with Earth. The conjunction equation for Venus and Earth is given by: S = T + E - 2T This equation can also be used to calculate transits of Mercury and Venus.
It can be calculated that the conjunction period S of Venus is 583.920 days. The orbital period of Earth is E = 365.2564 days. The two periods are different and cannot be reduced to a common denominator, so it is almost impossible to accurately coincide with the conjunction position and simultaneously reach the intersection of the two orbital planes, ensuring that the black spot of Venus passes exactly through the center of the Sun's disk. Fortunately, the Sun has a large surface area with an angular diameter of about 0.5°. When Venus is in conjunction, if the angular distance between Venus and the center of the Sun's disk is less than 15', a transit can be observed. This condition is still quite stringent. There are three time periods that meet this stringent condition: eight years, 105.5 years, and 121.5 years: 8E - 5S = 8 × 365.2564 - 5 × 583.920 = +2.45 days 105.5E - 66S = 105.5 × 365.2564 - 66 × 583.920 = -4.17 days 121.5E - 76S = 121.5 × 365.2564 - 76 × 583.920 = +0.73 days After these three time periods, the dates of Earth's arrival at the intersection and Venus's conjunction coincide closely, satisfying the conditions for a transit. The cyclic combination of these three time periods forms the seemingly mysterious regularity of Venus transits.
The motion of celestial bodies is purely an objective natural law, without any supernatural mysterious forces. However, various natural laws interact with each other, leading to many complex situations. The motion of Venus and Earth is not only mainly influenced by the gravitational force of the Sun, but also by the Moon and other planets. Therefore, the positions of the orbital intersection, perihelion, and vernal equinox, as well as various time periods, are not absolutely fixed. The regular occurrence of Venus transits will also show inconsistencies over a longer period of time. For example, there were no transits in 1396 and 3089 AD every eight years.This is a phenomenon of group behavior, and it only occurs once.