All About the Autumnal Equinox

Autumnal Equinox Definition

The autumnal equinox is the transitional point between summer and winter. It is the day where night and day are nearly equal in length. Before the autumnal equinox days are longer than nights, after the autumnal equinox days are shorter than nights. We can see this understanding from the roots of the word equinox which comes from Latin. Æquus meaning equal and nox meaning night.

To put it in more scientific terms, an equinox is when the solar ecliptic (the path the Sun appears to take through the sky throughout the year) intersects with the celestial equator (a theoretical extension of the earth's equator out into space). This happens twice a year, once at the vernal or spring equinox and the other at the autumnal equinox.

Generally the autumnal equinox comes on about the 22^nd or 23^rd of September. In the United States and many other countries it marks the first day of autumn (or fall). It is also known as the fall equinox for this reason.

What Causes the Autmnal Equinox?

Equinoxes are caused by the tilting of the earth as it revolves around the Sun. This is also the cause of the seasons. In summer the northern hemisphere is tilted towards the Sun and the southern hemisphere is tilted away. In winter the northern hemisphere is tilted away from the Sun while the southern hemisphere is tilted towards it.

As the tilt inverts itself each year it comes to a point where the earth's equator is directly in line with the Sun. There is effectively no tilt and both the southern and northern hemispheres are at equal distance from the Sun. This occurs twice a year and are our two equinoxes. Because the earth is directly in line with the Sun there is no difference between the lengths of day and night.