What defines sunset?

• A. When the center of the sun is 6º below horizon
• B. When the upper limb of the setting sun crosses the visible horizon
• C. When the lower limb of the setting sun crosses the horizon
• D. When the sun disappears below horizon completely

Answer: (B)

Sunset is defined by the Sun’s upper limb touching the horizon and beginning to disappear. As the Sun sinks, the top edge reaches the horizon first, and once that upper edge crosses below the horizon, the Sun is no longer visible. The center being 6° below horizon would occur well after sunset, and the lower limb crossing horizon happens at sunrise, not sunset. The Sun disappearing completely below the horizon is the moment after sunset.