The beam is providing 20 LUX of illumination, outshining in the ambient light on the night half by a factor of two. (From the book “What if”)
I am confused by the expression by a factor of two.
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Context makes all the difference. What the sentence means is:
The beam is providing 20 LUX off illumination, which is double the natural illumination of the dark side of the moon.
In the context of illuminating the dark side of the moon when what is normally visible on earth is half the moon,”night half” meaning “normally invisible” is decipherable. “Half by a factor of two” is an unfortunate use of language, but the author is viewing “night half” as a compound noun.
This is why people hire editors. In the author’s mind, what is intended is so clear that it is impossible to see that the words are obscure or even absurd without already knowing the intended meaning.