The September Equinox falls on Monday 23 September this year (2018), when the day and
night will be of equal length in both of the Earth's hemispheres. The
Earth spins on its axis at an angle to the plain of the Solar System.
This means that the poles of the Earth do not remain at 90° to the Sun,
but rather the northern pole is tilted towards the Sun for six months of
the year (the northern summer), and the southern pole for the other six
months (the southern summer). This means that twice a year neither pole
is inclined towards the Sun, on days known as the equinoxes
Simplified diagram showing the tilt of the Earth throughout the year. Not to scale. National Weather Service.
The equinoxes fall each year in March and September, with the March
Equinox being the Spring (or Vernal) Equinox in the Northern Hemisphere
and the Autumn Equinox in the Southern Hemisphere, while the September
Equinox is the Autumn Equinox in the Northern Hemisphere and the Spring
Equinox in the Southern Hemisphere. On these two days the day and night
are both exactly twelve hours long at every point on the planet, the
only days on which this happens.
See also...
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