The Sun appears to be about 0.5° in diameter. On the equator, approximately how long does it take for the Sun to set, from first contact with the horizon to complete disappearance below the horizon?

It takes about 2 minutes for the sun to completely disappear below the horizon.

Explanation:

Earth's rotation in a day = 360 degrees

Sun's Diameter = 0.5 degrees

360/24hr = Earth's rotation is 15 degrees/hour

If the sun's diameter is 0.5 degrees it takes:

15/0.5=30

60/30 = 2 minutes (since there are 60 minutes in an hour)