[Jimbee (OP)]

Why does sunrise and sunset always begin when the center of the Sun is at the horizon? I think they both begin when the tip of the Sun is at the horizon. Because otherwise, it is usually daylight a few moments before and after it reaches the center.

[alancalverd]

"Daylight" is separately defined by the  Civil Aviation Authority as 0.5 hour before sunrise to 0.5 hour after sunset, these times being the appearance and disappearance of the upper perimeter of the sun above the horizon. 

[paul cotter]

Thanks, Alan, I had often wondered what the actual definition was.

[Halc]

I had often wondered what the actual definition was.

Alan gave the aviation definition of daylight, and unfortunately I think his fractions didn't work (they haven't since the server was moved).

I did a web search and sunrise/sunset is first rays (just an edge) to last rays, not when the center of the sun breaks the horizon. Maybe google is showing me an American definition since I'm west of the pond and the EU folks define it as center to center, but I could find no reference to the claim made in the OP.

Then there is dawn/dusk which seems to be bounded by the sun (what part of it?) being 6^o below the horizon, which could be a difference of hours/never if you're close enough to the poles.

Twilight is more loosely defined as when the sun produces a glow on the horizon, a distinction that got harder to delimit since cities produce a similar glow all night long.

[alancalverd]

I have corrected the awful server character set. 0.5 hours each way.

No idea where JB's definition came from: sunrise and sunset seem to be internationally defined by the top edge of the disc, but legal day and night can vary according to national aviation authority definitions: just one more thing to worry about if you don't have a Night Rating (which is internationally recognised) on your licence - so get night trained (10 hours minimum)!

[vhfpmr]

"Daylight" is separately defined by the  Civil Aviation Authority as 0.5 hour before sunrise to 0.5 hour after sunset, these times being the appearance and disappearance of the upper perimeter of the sun above the horizon. 

The Road Vehicle Lighting Regulations are the same, too.
https://www.legislation.gov.uk/uksi/1989/1796/made