I received an interesting inquiry recently from a woman who wanted to know the meaning of the terms "morning stars" and "evening stars."
"I mean, they're not stars … they're planets, right?" she asked. "So why do we call them stars? And is a morning star only visible in the morning, like at the crack of dawn, or at other times as well? Hope you can clear this up for me because I'm totally confused!"
Here is an explanation for what qualifies as a "morning star" and an "evening star." [Night Sky: Visible Planets, Moon Phases & Events, February 2016]
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It began with Venus
Originally, the terms "morning star" and "evening star" applied only to the brightest planet of all, Venus. Far more dazzling than any of the actual stars in the sky, Venus does not appear to twinkle, but instead glows with a steady, silvery light. The fact that Venus was a "wandering star" soon became obvious to ancient skywatchers, who noticed its motion relative to the background stars, going from the eastern sky in the morning to the western sky in the early evening. Nicolas Camille Flammarion, a noted French astronomer in the late 19th and early 20th century, referred to Venus as "The Shepherd's Star." I myself like to refer to Venus as the "night light of the sky." So, it's easier to understand the origin of the terms "evening star" and "morning star" if we only considered Venus.
Of course, Venus is not the only wandering "star" in the sky; there are four others that are also visible to the unaided eye (five, if you include Uranus, which is barely perceptible without any optical aid on dark, clear nights). The difference is that, with the possible exception of Jupiter and, on rare occasions, Mars, none of the others stands out in the same manner as Venus. Nonetheless, somewhere in the distant past, "morning star" and "evening star" became plural in order to account for the four other planets.
When an evening star is branded as a morning star
This upcoming week, there are officially no evening stars, only morning stars.
But the distinction between these terms is not very precise. As an example, let me call attention to the fact that rising in the east this week, between 7:30 p.m. and 8 p.m., is the planet Jupiter. It will attain its highest point in the southern sky between 1:30 a.m. and 2 a.m. local time, and will still be visible, sinking in the western sky at dawn. The giant planet is thus ideally situated for observations of its changing cloud bands and four big satellites for much of the night.