Heliocentric view of the seasons

Heliocentric view of the seasons

The Earth's axis is tilted at an angle of approximately 23.44° from the orbital plane; this tilt is called the As a consequence, for half of the year (i.e. from around March 20 to around September 22), the northern hemisphere tips toward the Sun, with the maximum around June 21, while for the other half of the year, the southern hemisphere has this honor, with the maximum around December 21. The two instants when the Sun is directly overhead at the Equator are the equinoxes. Also at that moment, both the North and South Poles of the Earth are just on the and day and night are divided equally between the hemispheres.

The table gives the dates and times of equinoxes and several years. A few remarks can be made about the equinoxes:

• Because the Sun is a spherical (rather than a single-point) source of light, the actual crossing of the Sun over the Equator takes approximately 33 hours.
• At the equinoxes, the rate of change for the length of daylight and night-time is the greatest. At the Poles, the equinox marks the start of the transition from 24 hours of nighttime to 24 hours of daylight (or vice versa). High in the Arctic Circle, Norway has an additional 15 minutes more daylight every day around the time of the Spring equinox, whereas in (which is virtually on the Equator), the amount of daylight each day varies by just seconds.
• It is 94 days from the June solstice to the September equinox, but only 89 days from the December Solstice to the March equinox. The seasons are not of equal length, because of the variable speed of the Earth in its orbit around the Sun.
• Currently, the most common equinox and solstice dates are March 20, June 21, September 22 and December 21; the four-year average will slowly shift to earlier times in coming years. This shift is a full day in about 70 years (compensated mainly by the century "leap year" rules of the Gregorian calendar). This also means that in many years of the twentieth century, the dates of March 21, June 22, September 23 and December 22 were much more common, so older books teach (and older people may still remember) these dates