The Five Planets That Are Visible To The Naked Eye
Because the naked-eye planets can be seen without lenses, they have been known to astronomers for centuries. However, they are not all visible at the same time, and only rarely are observers able to see them all in the same night. Mercury in particular can be hard to spot. It's best seen when it's farthest from the sun in its orbit, and it appears briefly during morning or evening twilight.
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from Crash Course
Key Facts In This Video
In astronomy, "naked eye" observations are those that don't require binoculars or telescopes. (0:25)
There are 88 officially recognized constellations with delineated boundaries. (3:51)
Polaris, the North Star, appears fixed in place because it is close to the north celestial pole. (8:53)