When calculating the length of daylight hours by from latitude and longitude, does one add a few minutes to winter days or summer ones...
Yes. The simple formula
assumes the Sun is a point and that sunrise/sunset occur when the Sun's centre is exactly on the horizon.
In reality, two effects lengthen the day slightly:
Atmospheric refraction bends sunlight upward, allowing you to see the Sun even when it is geometrically below the horizon.
The Sun has an apparent diameter of about 0.5°, and sunrise/sunset are conventionally defined by the upper edge of the Sun crossing the horizon.
Together these effects make the Sun visible when its centre is about 0.83° below the horizon.
The result is:
Every day is a little longer than the simple geometric calculation predicts.
The correction is largest near the solstices, especially at higher latitudes.
At your latitude (about 45° N, near Saint-Jean-sur-Richelieu), the correction is typically several minutes.
A useful rule of thumb is:
Winter days gain more minutes than summer days.
Why? Near the winter solstice, the Sun's path crosses the horizon at a steeper angle, so that extra 0.83° translates into a larger increase in daylight time. Near the summer solstice, the Sun skims the horizon more shallowly and the effect on total day length is somewhat smaller.
For example at 45° N:
Geometric winter-solstice day length: roughly 8 h 35 min
Actual observed sunrise-to-sunset: roughly 8 h 50 min
So the correction can be on the order of 10–15 minutes in winter.
This is one reason that if you calculate day length using only the above formula, your answer will typically be a few minutes shorter than the sunrise and sunset times published in almanacs or weather reports.
Interestingly, this same correction is also what delays the first 24-hour daylight period in the Arctic and the first 24-hour night in the Antarctic by several days compared with a purely geometric model.
I have read the Terminator barrier moves at over 1,000 miles per hour. So that means nightfall must be really fast in human terms?
It seems counterintuitive, doesn't it?
The terminator (the day-night boundary) does indeed sweep across Earth's surface at roughly 1,000 mph (1,600 km/h) at the equator because Earth rotates once every 24 hours. But that does not mean darkness falls like a curtain racing across the ground.
The reason is geometric.
Imagine standing in Saint-Jean-sur-Richelieu watching a sunset. The Sun doesn't move straight down. Instead, it follows an arc across the sky. Near sunset, the Sun's apparent motion is only about:
Since the Sun's disk is about 0.5° wide, it takes roughly 2 minutes for the entire Sun to disappear below a level horizon.
So from a human perspective:
The Sun noticeably approaches the horizon over perhaps 20-30 minutes.
The last sliver of Sun disappears over about 2 minutes.
Twilight then continues for another 30-90 minutes depending on season and latitude.
At your latitude (~45°N), the Sun usually meets the horizon at a fairly oblique angle, making sunset feel gradual.
What's interesting is that the terminator's speed depends strongly on latitude:
Near the equator, it moves fastest across the ground, over 1,000 mph.
Near the poles, it slows dramatically.
Around the Arctic Circle in summer, the terminator can creep so slowly that twilight lasts for hours.
A nice way to think about it is that the terminator is an enormous line wrapping around half the Earth at once. Even though that line is moving very fast across the surface, you only experience the tiny angular motion of the Sun in your local sky, which is why sunset feels leisurely rather than sudden.
In fact, if you were standing on the Moon, where a day lasts about 29.5 Earth days, sunset would take hours, not minutes. By comparison, Earth's sunset is already quite "fast".
* * *
The terminator is an arc on a 3-D Earth:
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