In astronomy, what term is used for the moment when the Sun is directly above the Earth's equator, resulting in roughly equal day and night lengths?

• A. Solstice
• B. Zenith
• C. Equinox
• D. Aurora

Answer: (C)

Equinox is the moment when the Sun’s subsolar point lies on the Earth’s equator, so the Sun appears to cross the celestial equator. At that time the Sun illuminates roughly half the globe, and day and night are about equal in length. Technically the lengths are nearly equal (not always perfect) because of atmospheric refraction and the Sun’s finite size, but the key idea is that the Sun is positioned over the equator, producing nearly equal daylight across most places on Earth. This happens twice a year, around late March and late September.

Solstice refers to the Sun’s greatest distance north or south of the equator, giving the longest or shortest day. Zenith is the point directly overhead an observer, not this global alignment. Aurora is a visual atmospheric phenomenon, not related to day length.