Why is today so long?

Today is, of course, the day of the equinox when the day and night are supposedly of equal length. But today in Montreal will last 12 hours and 8 minutes (6:58 to 7:06). About three minutes of this excess is caused by the definitions of sunrise (when the first ray of sun appears) and sunset (when the sun disappears). It ought to be the time between the horizon bisecting the sun. This ought to be 2 minutes at the equator (the sun subtends an arc of half a degree and moves 15 degrees an hour—for this purpose geocentric coordinates are better) but this has to be divided by the sine of our latitude, about .7. I understand there is some atmospheric refraction. But does that really extend the day by five minutes?

At the horizon refraction is about half a degree. Multiply by two, that’s about enough to account for the difference.