The sun subtends a half a degree from were we stand. If the lower limb of the setting sun touches the horizon at 6:30 PM when does it disappear completely below the horizon (that is how how does it take to set.) ?

6:31 PM

Explanation:

The sun subtends half a degree angle from where we stand. The lower limb of the sun touches the horizon at 6:30 PM.

Now time it takes to disappear completely below the horizon can be calculated as

The Sun diameter is diameter is 32 arc minutes. And its lower limb is 16 arc minutes from the center.

Now, 31 arc minutes = 0.52°

⇒0.52° = 31 arc minutes

therefore, if it crosses its angular diameter sun will set completely

Angular diameter = 0.52° = 31 arc minutes

therefore Sun will set at 6:31 PM