Which word describes the phenomenon of both day and night being equal?

The phenomenon where both day and night are of equal length is referred to as an equinox. An equinox occurs twice a year, in March and September, when the tilt of the Earth's axis is such that the sun is directly over the equator. This results in approximately equal amounts of daylight and darkness across the entire planet. On these days, the sun rises exactly in the east and sets exactly in the west.

In contrast, a solstice refers to the times of the year when the sun reaches its highest or lowest point in the sky at noon, leading to the longest day (summer solstice) or the longest night (winter solstice). The term lunar phase relates to the changing appearance of the moon as it orbits Earth, which does not involve the balance of day and night. Lastly, season change is a broader concept referring to the transition between different seasons as affected by Earth's tilt and orbit, but it does not specifically address the equal division of day and night.