Sentence:  Since it gets light before the sun rises and remains light after the sun sets, the actual period of darkness at the <WSD>equinox</WSD> will be substantially less than twelve hours, the exact amount depending on latitude and how one defines the boundary between twilight and night. 
Meaning of the word marked with <WSD> tags:  One of the two days on which this intersection occurs each year: (for the Northern hemisphere) March 20 or 21 in the spring and September 22 or 23 in the autumn.