Which term describes a day when day and night are equal in length?

• A. Solstice
• B. Perihelion
• C. Equinox
• D. Aphelion

Answer: (C)

Day and night are equal in length during an equinox. This happens when the Sun is directly above the Earth’s equator, so the dividing line between day and night (the terminator) passes through the middle of the globe. As a result, roughly twelve hours of daylight and twelve hours of darkness occur, and this occurs twice a year—in March and September—around when the Sun crosses the celestial equator. The other terms describe different ideas: a solstice marks the extreme lengths of day or night (longest day or longest night), while perihelion and aphelion refer to the Earth’s closest and farthest points from the Sun, which don’t determine the equality of day and night.