Which of the following dates marks the equinox, the two times of year when day and night are equal?

The correct answer is 21 March and 23 September.

Key Points

• The equinoxes occur twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, resulting in a nearly equal duration of day and night.
• The vernal (spring) equinox typically occurs on March 21, marking the start of spring in the Northern Hemisphere.
• The autumnal (fall) equinox typically occurs on September 23, marking the start of autumn in the Northern Hemisphere.
• During an equinox, the Sun crosses the celestial equator, an imaginary line projected into space above the Earth's equator.
• Both equinoxes are significant astronomical events used to define the changing of the seasons.

Additional Information

• Celestial Equator: An imaginary line that is the projection of the Earth's equator into the celestial sphere, dividing it into the northern and southern celestial hemispheres.
• Solstices: The points in the year when the Sun is at its greatest distance from the celestial equator, occurring around June 21 (summer solstice) and December 21 (winter solstice).
• Equilux: A day when the duration of light and dark is exactly equal; it occurs a few days before or after the equinox due to atmospheric refraction.
• Earth's Tilt: The 23.5-degree tilt of the Earth's rotational axis relative to its orbital plane around the Sun, responsible for the changing seasons.
• Day and Night Cycle: The rotation of Earth on its axis, which leads to the alternation of day and night. During equinoxes, this cycle results in nearly 12 hours of daylight and 12 hours of night.