Final answer:
In September and March, Earth's axis is no longer pointing at the North Star, and neither end of Earth's axis is tilted toward the sun. The sun's rays strike Earth with the same intensity everywhere.
Step-by-step explanation:
In September and March, Earth leans "sideways"-neither into the Sun nor away from it-so the two hemispheres are equally favored with sunshine. This means that near the end of both March and September, Earth's axis is no longer pointing at the North Star, and neither end of Earth's axis is tilted toward the sun. The sun's rays strike Earth with the same intensity everywhere, resulting in the equinoxes.