Final answer:
On December 21st, you would be on the Tropic of Capricorn if the Sun is directly overhead, marking the winter solstice in the Northern Hemisphere and the longest day in the Southern Hemisphere.
Step-by-step explanation:
If the Sun passes directly overhead on December 21st, you would be on the Tropic of Capricorn, which is the latitude approximately 23° South of the Equator. This geographical line marks the southernmost point on Earth where the Sun can be directly overhead at noon.
This event occurs during the winter solstice in the Northern Hemisphere, which is the shortest day of the year for that hemisphere and the longest day of the year for the Southern Hemisphere. On this day, the Earth's axis is tilted away from the Sun,
leading to the decrease in altitude of the noontime Sun the further north one goes from the Tropic of Capricorn. For example, at the Tropic of Cancer (23° N latitude), the Sun's altitude at noon on December 21 would be approximately 44° above the horizon.