... In low Earth orbit there is not enough atmosphere to diffuse and scatter sunlight, so shadows are black and the sky is dark - even when the Sun shines. ... While modern astronomers still refer to M57 as a planetary nebula, at one light-year across M57 is not a planet but the gaseous shroud of a dying sun-like star. ... But while Wednesday's leisurely transit of Mercury across the Sun entertained observers all over the dayside of planet Earth, the audience for this lunar transit was more restricted. ...