Are Day and Night Each Exactly 12 Hours Long on the Spring Equinox?

It is a common misconception that the day and night are each exactly 12 hours long on the spring equinox — the day when the center of the sun crosses over the Earth's equator, which is typically about 20 March. On the spring equinox, also known as the vernal equinox, there is slightly more daylight than darkness. It is actually two to three days before the equinox when the days are nearly equal parts day and night. For example, three days before the equinox, there is about one more minute of darkness, but on the day of the event itself, there might be about eight more minutes of daylight.

More about the spring equinox:

• The spring equinox and the fall equinox, or autumnal equinox, are the only days of the year when the sun rises from due east and sets at due west.
• Before 1582, the design of the commonly used Julian calendar caused the spring equinox to fall one day earlier every 128 years — which eventually would have made it happen during the winter. Pope Gregory XIII altered the calendar slightly in 1582 to prevent this occurrence, which is why the commonly used calendar is now known as the Gregorian calendar.
• The vernal equinox begins six months of uninterrupted daylight for the North Pole and six months of darkness for the South Pole.

Frequently Asked Questions

Is the length of day and night exactly equal on the spring equinox?

No, contrary to popular belief, day and night are not exactly 12 hours each on the spring equinox. The date when day and night are closest to being equal is known as the equilux, which occurs a few days before the spring equinox due to atmospheric refraction and the way sunrise and sunset are defined.

What causes the discrepancy in day and night length on the equinox?

The discrepancy is caused by atmospheric refraction, which bends sunlight above the horizon before the sun has fully risen, and the definition of sunrise and sunset, which are considered to occur when the top edge of the sun's disk is on the horizon, not the center. This makes the day slightly longer than the night at the equinox.

How much longer is the day compared to the night on the spring equinox?

The exact difference in length between day and night on the spring equinox varies depending on your location. However, typically, the day can be up to several minutes longer than the night. For example, in Washington, D.C., the day is about 12 hours and 8 minutes long on the equinox.

Does the equinox occur at the same moment everywhere on Earth?

Yes, the equinox occurs at the same moment worldwide. It is the instant when the Sun crosses the celestial equator, making day and night approximately equal in length everywhere. However, local times for the equinox differ due to time zones.

What is the equilux, and how is it different from the equinox?

The equilux is the day when the length of day and night are closest to being exactly equal, and it occurs a few days before the spring equinox. The equinox, on the other hand, is a specific astronomical event marking the point when the sun crosses the celestial equator, not necessarily when day and night are equal in length.

Do the equinoxes signify the start of spring and autumn seasons everywhere in the world?

No, the equinoxes do not mark the beginning of spring and autumn for everyone globally. In the Northern Hemisphere, the spring (vernal) equinox marks the start of spring, while the autumnal equinox signifies the beginning of autumn. Conversely, in the Southern Hemisphere, the seasons are opposite, with the spring equinox indicating the start of autumn and the autumnal equinox the beginning of spring.