就在10月21日！来看“水火之约”

Core Points - On October 21, a rare astronomical event will occur where Mercury and Mars will appear together in the southwestern sky after sunset, providing a unique opportunity for the public to observe this celestial phenomenon [1][10] Group 1: Definition and Frequency of Conjunctions - The term "conjunction" in astronomy refers to two celestial bodies having the same geocentric right ascension or geocentric ecliptic longitude [3] - Conjunctions of planets are relatively common, but the frequency varies among different planets. Mercury, being the closest planet to the Sun, has a maximum angular distance of only 28 degrees from the Sun, while Mars has an average conjunction cycle of about 780 days [5][6] Group 2: Observational Conditions - Although Mars and Mercury may have multiple conjunctions within a 780-day cycle, typically only two are observable under favorable conditions [8] - On October 21, the angular distance of both planets from the Sun will be approximately 22 degrees, making the observational conditions particularly favorable for this event [8][10] - The best viewing conditions will be in southern China compared to northern regions, with a theoretical observation window of about half an hour after sunset [10]