The spring equinox in the Northern Hemisphere is almost upon us.
This event marks the moment when the sun crosses the celestial equator—an imaginary line in the sky above the Earth's equator—from south to north.
In 2021, it occurs on March 20 at 5:37 a.m. ET, or 2:37 a.m. in the Pacific time zone.
The word "equinox" comes from the Latin aequus and nox, which mean "equal" and "night" respectively.
On the March equinox, the length of the day and night is almost equal wherever you are in the world, with the sun shining directly on the equator. This is also the case for the other equinox in September.
The March and September equinoxes occur in between the summer and winter solstices, marking the two points in the year when the Earth's axis is tilted neither towards or away from the sun.
The Earth's axis of rotation is tilted 23.4 degrees to its orbital motion around the sun—a phenomenon that gives rise to the seasons.
The March equinox marks the beginning of astronomical spring in the Northern Hemisphere and the start of astronomical fall in the Southern Hemisphere. The opposite is true for the September equinox.
There is, however, another way to define the start dates of seasons—the meteorological definitions. These are based on average temperatures rather than astronomical events such as the equinoxes and solstices.
For millennia, these events have played an important role for cultures around the world. There are many ancient archeological sites that mark them, with the designs of temples and other structures taking into account the sun's movements on these days.
There are also several traditions linked to the March equinox that persist to this day.
For example, there is a Chinese custom where people try to balance eggs—a symbol of fertility—on the day of the March equinox in the hopes of bringing good luck and prosperity.
This has given rise to the urban myth that the March equinox is the only day of the year when eggs can be balanced on their ends. However, this is not true.
Iranian New Year—known as Nowruz—occurs on the March equinox, according to the Persian calendar. It has been celebrated for more than 3,000 years.
In Japan, Buddhists celebrate a holiday known as Higan or Ohigan for three days before and after the spring and fall equinoxes. It is a chance for people to return to their hometowns and remember their ancestors.
