How much does the direction of perihelion change each orbit ?
近日点方向使每个轨道改变了多少？
According to marsden 's calculations , this comet reaches its closest point to the sun ( perihelion ) on march 18th .
根据marsden的计算，这颗彗星会在3月18日过近日点。
Although predicting the brightness of comets with any degree of accuracy is very difficult , hale-bopp met or exceeded most predictions when it passed perihelion on april 1 , 1997 . The comet was dubbed the great comet of 1997 .
