Answer:

a equator

Explanation:

The subsolar point appears at the Equator twice a year (during the equinoxes), and migrates north and south across the tropics during the rest of the year. The solstices mark when the subsolar point reaches its northernmost and southernmost latitudes.

The sun’s vertical rays strike the Tropic of Cancer, 23.5° north of the Equator, during the June solstice. The subsolar point then begins its migration south, and vertical rays strike the Tropic of Capricorn, 23.5° south of the Equator, during the December solstice. The subsolar point will cross every latitude between these extremes twice every year.

Polar Regions

The subsolar point never reaches Arctic and Antarctic regions. At the North Pole and South Pole, the solstices mark the time when the sun is highest or lowest in the sky. In this way, solstices are the extreme examples of “midnight sun” and “polar night.”

“Midnight sun” describes the phenomenon surrounding the summer solstice, when the sun remains visible at midnight in the weeks leading up to and following the event. The “polar night” surrounds the winter solstice, when the sun remains below the horizon during the weeks leading up to and following the event.

Extraterrestrial Solstices

Every planet in our solar system experiences solstices. The timing and extent of solstices are largely determined by the planet’s axial tilt, orbital eccentricity, and distance from the sun.

Venus, the planet closest to Earth, has a very small axial tilt, just 3°. Venus experiences very little seasonal variation, and its solstices are separated by about three months.

Mars, our other close neighbor, has an axial tilt similar to Earth (24°). However, Mars has a significantly greater orbital eccentricity, meaning it orbits the sun in a more elliptical shape than Earth. As a result of Mars’ larger orbital eccentricity and axial tilt, the Red Planet experiences extreme seasonal variations and its solstices are about 11 months apart.