In astronomy, what term describes the biannual event when the Sun is exactly above the Earth's Equator, yielding nearly equal day and night?

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

Answer: (B)

The key idea is that an equinox is when the Sun crosses the celestial equator, causing day and night to be nearly the same length. This happens twice a year because Earth’s axis is tilted; as it orbits the Sun, the Sun’s apparent path (the ecliptic) crosses the celestial equator in two places. On those days, the Sun’s declination is 0 degrees, so the amount of daylight is nearly equal to darkness. There are two equinoxes each year: a vernal (spring) one and an autumnal (fall) one.

Solstice, by contrast, marks the longest and shortest days of the year, when the Sun is farthest north or south of the celestial equator. Aurora and longitude describe unrelated concepts—an atmospheric light display and a geographic coordinate, respectively.