Final answer:
July's longest period of darkness occurs at the South Pole, in the Southern Hemisphere, where there is continuous 24-hour darkness due to the polar night phenomenon.
Step-by-step explanation:
The longest period of darkness in July for any location on Earth occurs at the South Pole. In the Southern Hemisphere, the month of July falls within the polar night, a period during winter when the Sun does not rise above the horizon.
This polar night phenomenon happens because the Earth's axis is tilted in relation to its orbit around the Sun. During this time, regions inside the Antarctic Circle experience continuous darkness. The South Pole itself is shrouded in darkness for a full 24 hours each day throughout the winter months, including July.
By contrast, in the Northern Hemisphere, where July is a summer month, the longest nights are comparatively short, with the longest days and shortest periods of darkness typically occurring around the summer solstice, which falls on or around June 21st.