主题：科学英语:每日星相(8.24)

每日星相

Daily Astronomy


规则：参与的朋友可以每人翻译一句，也可全部翻译，依照自己情况而定。
注意：抄袭灌水者，无论是谁，一律删贴处理！
说明：每天悬赏50分。

Mercury, the solar system’s innermost planet, never strays far from the sun. As seen from Earth, Mercury swings farthest out from the sun 6 to 7 times a year. When Mercury swings farthest west of the sun, this world appears as a morning “star” in the east before sunrise. When it’s farthest east, it appears as an evening “star” in the western twilight.

Today, Mercury reaches its greatest eastern elongation from the sun. If the elongation is particularly favorable, Mercury can stay out for 2 hours or more after sundown. However, Mercury sets less than one hour after sunset at mid-northern latitudes. But at middle latitudes in the southern hemisphere, Mercury stays out for nearly 2.5 hours after the sun.

Why the big difference between the two hemispheres? It’s because August is a late winter month in the southern hemisphere. On late winter and early spring evenings, the ecliptic – the pathway of the planets – is most inclined for the year and nearly perpendicular to the horizon. That’s why Mercury sets so long after sunset in the southern hemisphere.

However, it’s now late summer in the northern hemisphere. On late summer and early autumn evenings, the ecliptic hits the horizon at its shallowest angle. Therefore, Mercury sets soon after the sun in our northern hemisphere sky.

Written by Bruce McClure , 24 August 09