The path of the total solar eclipse represents a relatively narrow strip of the continental United States, but the entire country will be treated to at least some fraction of this cosmic event. That means all of America is getting at least a little darker, with those in the path of totality experiencing a midday twilight.
Admittedly, the further outside the path of totality you are, the less likely you are to notice much of a dimming effect. The moon obscures a decent amount of the sun even more than a thousand miles out — Maine, for instance, is in the worst spot in the continental United States for the eclipse, yet the sun will still be about 50 percent covered. But that won’t have as much impact as one might think on the overall brightness.
It’s only at about 90 percent coverage that a darkening effect becomes noticeable. The upper limit for that is about a couple hundred miles. According to NASA, it’s only at 99 percent coverage that the darkness is comparable to that of twilight. Cities like Portland, Oregon, or Missouri’s St. Louis and Kansas City aren’t in the path of totality, but they are close enough that they should experience noticeable darkening.
Only those in the path of totality will get the full experience of midday darkness, and even then it’s not even going to last a full three minutes. The Total Eclipse 2017 website has a poetic description of the experience of the onrushing darkness for those in totality:
The sky surrounding the Sun will grow very dark very quickly. In real time, you will be able to see the deep blue turn to twilight blue, and then to bluish-black. Stars and planets will pop out of nowhere. Roosters will crow and insects will chirp as though night is falling. If you look to the west, you’ll see a beautiful black curtain rising up out of the Earth, with hints of sunset-orange north and south of it, while off to the east, the sky at the horizon is still rather light. On the ground, your shadow will become impossibly clear and thin, and then will vanish completely as the Sun’s light fades to about the intensity of the full Moon. In the last few seconds before totality, that dull blackness you saw off to the west will suddenly spring up out of the Earth, and take over the whole sky like a gigantic curtain being pulled over you.