Equinoxes and Solstices
- Every day the Sun lies in front of a slightly different location on the
celestial sphere due to the Earth's orbital motion.
- In the course of a year the changing position of the Sun
among the stars traces out a complete circle around the celestial sphere.
- This circular path is called the ecliptic and represents
the apparent path followed by the Sun around the
celestial sphere as the Earth orbits the Sun.
- The 12 constellations located along the Ecliptic are called the
Zodiac. The Sun passes through about one constellation per month.
- On your birthday, the Sun is in line with your "birthsign"
constellation, making it impossible to see!
- Since the Earth's rotation axis is tilted 23.5 degrees relative
to its orbital plane the ecliptic is a circle around the sky tipped
23.5 degrees relative to the celestial equator.
- Since the celestial equator is a fixed line on the sky
for any particular observer on Earth, the tilt of the
ecliptic causes the Sun's altitude at Noon to vary throughout the year.
- There are two points on the Ecliptic where the Sun lies
as far as possible from the Celestial Equator (23.5 degrees away).
- At the Summer Solstice the Sun is 23.5 degrees North of
the Celestial Equator -- high in the sky at noon. It shines almost directly
down on North America and heats the ground effectively. The image at right
shows the Earth as seen from the Sun on the Summer Solstice.
- At the Winter Solstice the Sun is 23.5 degrees South of
the Celestial Equator -- low in the sky at noon. For North America it shines obliquely
across the ground and does a poor job of warming the ground -- movies.
- Note that the seasons are reversed in the Southern Hemisphere. When it is winter in North America it is summer in South America.
- Note also that, although the Earth's orbit is slightly elliptical,the Earth is closest to the Sun in January and farthest in July. The Earth's slightly varying distance from the Sun has little to do with the Seasons.
- The ecliptic crosses the celestial equator at two points
called the Vernal and Autumnal Equinoxes.
- At these points the Sun lies on the Celestial Equator
and, as a result, the length of day and night are nearly
equal -- thus the name Equinox.
- Why so? Any object, in this case the Sun, located on the celestial equator will be above the horizon exactly 1/2 of the day.
- This is because exactly 1/2 of the circle of the celestial equator lies above the horizon (no matter where you are on Earth with the exception of the poles where it lies on the horizon).
- An object "below" the celestial equator (e.g. the Sun in Winter) is "up" for less than 1/2 a day.
- An object "above" the celestial equator (e.g. the Sun in Summer) is "up" for more than 1/2 a day.
- Days are short in the Winter and Long in the Summer.
- When the Sun lies on these crossing points it marks the first
day of either Spring or Autumn.
Updated February 3, 2010