Final answer:
An equinox is the event during which the northern and southern hemispheres receive equal amounts of sunlight, occurring around March 21 and September 21 each year. During this time, day and night are about equal in length due to the Sun crossing the celestial equator.
Step-by-step explanation:
When an equinox occurs, the northern hemisphere and the southern hemisphere receive the same amount of sunlight. The term equinox refers to the two times in the year, around March 21 and September 21, when the day and night are approximately equal in length. It is during an equinox that the Sun crosses the celestial equator, and both the northern and southern hemispheres are equally illuminated, unlike during the solstices when one hemisphere is more directly exposed to the Sun's rays than the other.
Specifically, the vernal equinox (or spring equinox) occurs around March 21, signaling the start of spring in the northern hemisphere and autumn in the southern hemisphere. Conversely, the autumnal equinox (or fall equinox) occurs around September 21 and marks the beginning of autumn in the northern hemisphere and spring in the southern hemisphere. At these points in Earth's orbit, due to the planet's 23.5° tilt, there is a balance in sunlight received, contributing to approximately 12 hours of daylight and 12 hours of night regardless of location on Earth.