Astronomical spring officially begins with the spring or vernal equinox, a milestone that announces the transition from winter to spring in the Northern Hemisphere. The equinox, occurring around March 20 each year, marks the moment when the sun appears directly overhead along the equator.
The word “equinox” comes from the Latin aequus (“equal”) and nox (“night”), reflecting the idea that day and night are nearly the same length on this date. Observers around much of the globe see just over 12 hours of daylight and darkness because of the balance of Earth’s tilt and atmospheric effects on sunrise and sunset.
The equinox also marks the point at which the Northern Hemisphere begins to tilt more directly toward the sun, and there are more daylight hours than darkness. Increasing daylight eventually leads to warmer conditions. After the equinox, the accumulation of solar energy accelerates, ushering in longer days and the progression of the seasons toward summer.
Why the date varies
Even though the equinox usually falls on March 20, it can sometimes occur on March 19 or as late as March 21. That happens because Earth takes slightly more than 365 days to orbit the sun, and our calendar adjusts with leap years. These slight shifts ensure that our civil (or Gregorian) calendar stays aligned with Earth’s position in space.
Astronomical vs. meteorological spring
There are different ways to define the start and end dates of spring:
- Astronomical spring begins at the equinox, the precise moment when Earth’s tilt is perpendicular to the sun’s rays and the sun appears directly overhead along the equator.
- Meteorological spring uses calendar months, March 1 through May 31, for seasonal statistics and climate records.
Both viewpoints are useful: astronomers define seasons by Earth’s orbit, while meteorologists define spring as the transition months between the warmest and coldest calendar quarters of the year (summer spanning June-August, winter from December-February and spring and fall occurring in between).
What you’ll notice after the equinox
In the weeks following the spring equinox:
- Days keep getting longer, with earlier sunrises and later sunsets.
- The sun rises due east and sets due west, then gradually moves toward the northern horizon.
- Temperatures generally rise, although cold snaps can linger, especially in early spring.
- Nature responds: flower buds burst, bird migrations accelerate, and signs of spring become more visible.
Climatologically, March is often a transition month – when lingering winter cold and warm air masses clash. Even though the equinox signals spring on the astronomical calendar, it may take weeks for temperatures respond to increasing sunlight. Eventually, warmer conditions win out.
