以前写过金星西大距。天文学上的大距，指的是在地球上观测地球与太阳之间的行星，相对于太阳位置关系。东大距与西大距，都是水星与太阳相对位置关系变化过程中的一个特定时刻。这一时刻，太阳与水星的视角距离最远。大距前后的一段时间，更适合水星观测。 大距这一概念 ...

Factory equipment and machinery can fail if used for a long time.However, repairing equipment every time it breaks down leads ...

When a large number of people head toward the same exit from an event venue or building, people can become congested near the ...