This article is from the Calendars FAQ, by Claus Tondering email@example.com with numerous contributions by others.
Equinoxes and solstices are frequently used as anchor points for
calendars. For people in the northern hemisphere:
- Winter solstice is the time in December when the sun reaches its
southernmost latitude. At this time we have the shortest day. The
date is near 21 December.
- Summer solstice is the time in June when the sun reaches its
northernmost latitude. At this time we have the longest day. The
date is near 21 June.
- Vernal equinox is the time in March when the sun passes the equator
moving from the southern to the northern hemisphere. Day and night
have approximately the same length. The date is near 20 March.
- Autumnal equinox is the time in September when the sun passes the
equator moving from the northern to the southern hemisphere. Day and
night have approximately the same length. The date is near
For people in the southern hemisphere these events are shifted half a
The astronomical "tropical year" is frequently defined as the time
between, say, two vernal equinoxes, but this is not actually true.
Currently the time between two vernal equinoxes is slightly greater
than the tropical year. The reason is that the earth's position in its
orbit at the time of solstices and equinoxes shifts slightly each year
(taking approximately 21,000 years to move all the way around the
orbit). This, combined with the fact that the earth's orbit is not
completely circular, causes the equinoxes and solstices to shift with
respect to each other.
The astronomer's mean tropical year is really a somewhat artificial
average of the period between the time when the sun is in any given
position in the sky with respect to the equinoxes and the next time
the sun is in the same position.