Yesterday was the equinox and I noticed that the day was longer than 12 hours according to the (secular) sunrise/sunset times. Curious, I did some investigation. One reason for the difference is how we measure -- the equinox is based on when the center of the sun is visible while sunrise/sunset times are based on when the top edge is visible. That only accounts for a couple minutes; though; the bigger factor is atmospheric refracion (discussed in simpler terms here), which, in a nutshell, means that the sun is visible to us in the sky even when it is below the horizon, for several minutes.
This made me wonder about the effect on halachic times. This article says that the halacha is that Shabbat begins at sunset, though we add time for safety. I know that, l'hatchila, we add (usually) 18 minutes before the start of Shabbat or Yom Tov to provide a cushion. That's more than enough to account for any false effects due to refraction, but I've also been told that, b'dieved, if one is running late and has to cut into those 18 minutes, it's still ok so long as you actually began Shabbat/Yom Tov before the end of those 18 minutes, at sunset. And therein lies my question.
Regardless of how we halachically define sunset, we might see the sun above the horizon and conclude that it is not yet sunset when the sun has actually already set. Does this mean that the day has in fact started -- we go by where the sun is, not where it appears to be -- and if you light at the 15th of the 18 minutes you're kindling fire on Shabbat? Or, halachically, is sunset based on what we see, not what we now know happens scientifically?
I know there's discussion (somewhere) of Shabbat times if you're in the mountains or have an obstructed view of the horizon, and maybe that's relevant here. That's a more localized situation, while atmospheric refraction happens everywhere all the time, so I don't know if that makes it a different case.
Does Shabbat really start sometime during the 18 minutes?