Rising and Setting Times of the Sun and Moon
The rising and setting times given for the Sun and Moon refer to the moment when the topmost point of the solar or lunar disc can be seen on the horizon. This is the established astronomical method of calculation. The celestial body does not have to be exactly conjunct the Ascendant or Descendant at that time.
When the Sun is exactly conjunct the Ascendant, it means that the center of the solar disc, not its top margin, is right on the geometrical horizon. Due to the atmospheric refraction of the Sun's rays, however , the whole Sun can be seen at that point in time.
For the Moon, the factual situation is even more complex, because it is not usually found directly on the plane of the zodiac (ecliptical latitude).