The vernal equinox (or spring equinox), when the Sun reaches
its ascending node (i.e., crosses
the celestial equator moving northward), falls on or around March 21 and
marks the start of spring; the autumnal equinox, when the
Sun reaches its descending node,
occurs on or around September 22 and marks the start of autumn. These are
the two days of the year on which, everywhere on Earth, day and night are
of equal duration: hence the name.
An equinox is either of the two points on the celestial
sphere where the celestial
equator intersects the ecliptic.
|The location of the vernal and autumnal equinoxes
on the celestial sphere
- An equinox is also either of the times at which the center of the Sun's disk passes through
Movement of the equinoxes
The equinoxes drift very slightly across the sky because of precession.
For example, in the time of Hipparchus,
about 2,100 years ago, the vernal equinox lay in Aries and it is still referred
to as the First Point in Aries, even though it has now
moved into Pisces. Likewise, the autumnal equinox, also called the First
Point in Pisces, now lies in Virgo.
|The region of the sky near the First Point in Aries,
or vernal equinox, the point of intersection of the celestial equator
and the ecliptic. Because of precession of the Earth's poles, the
First Point in Aries moves backward along the ecliptic at a rate of
about 0.014° annually. As can be see, the point lies no longer
in Aries (one star of which, β Arietis, can be seen) but is moving
through Pisces toward Aquarius.