The "true" sunrise happens when you can already see the sun. At "true" sunset, you can still see the sun. (assuming other conditions to be helpful). Under "normal" circumstances, the lower limb of the sun will be seen about 2/3 of a sun's diameter above the horizon at the times of astronomical sunrise or sunset.
On board ship, we called this the "Dachshund Rule". You could check a compass with the azimuth of the sun when a small dog could just pass under it. That would be the time of astronomical sunset/rise which you could look up in the Nautical Almanac. Rudolf
