Summer and Winter
The boundary AB between sunlight and shadow--between day and night--is always perpendicular to the Earth-Sun line, as it was in the example shown at the beginning.
But because of the tilted axis, as each point on Earth is carried on its daily trip around the rotating Earth, the part of the trip spent in daylight (unshaded part of the drawing) and in the shadow (shaded) are usually not equal. North of the equator, day is longer than night, and when we get close enough to the north pole, there is no night at all. The Sun is then always above the horizon and it just makes a 360-degree circuit around it. That part of Earth
A mirror-image situation exists south of the equator. Nights are longer than days, and the further one gets from the equator, the larger is the imbalance--until one gets so close to the pole that the sun never rises. That is the famous polar night, with 24 hours of darkness each day. In that half of the Earth, it is winter time.
Half a year later, the Earth is on the other side of the Sun, that is, the Sun's position in the above drawing should be on the right, and the shaded part of the Earth should now be on the left (light and dark portions in the drawing switch places). The Earth's axis however has not moved, it is still pointed to the same patch of sky, near the star Polaris. Now the south pole is bathed in constant sunshine and the north one is dark. Summer and winter have switched hemispheres.
A big difference between summer and winter is thus the length of the days: note that on the equator that length does not change, and hence Spring, Summer, Fall, and Winter do not exist there (depending on weather patterns, however, there may exist a "wet season" and a "dry season"). In addition (as the drawing makes clear), the Sun's rays hit the summer hemisphere more vertically than the winter one. That, too, helps heat the ground, as explained further in section #4, "The Angle of the Sun's Rays."
At equinox, the situation is as in the first drawing, and night and day are equal (that is where the word "equinox" comes from)
Some interesting facts
If June 21 is the day when we receive the most sunshine, why is it regarded as the beginning of summer and not its peak? And similarly, why is December 21, the day of least sunshine, the beginning of winter and not mid-winter day?
Blame the oceans, which heat up and cool down only slowly. By June 21 they are still cool from the winter time, and that delays the peak heat. Similarly, in December the water still holds warmth from the summer, and the coldest days are still ahead.
And what about our distance from the Sun? It, too, varies, because the Earth's orbit around the Sun isn't an exact circle. We are closest to the Sun--would you believe it? --when it's cold winter for most people, around January 3-5. This may have an interesting implication for the origin of ice ages, as will be explained in section (7).
Questions from Users:
Where on Earth is the boundary between summer and winter?
What if the Earth's axis were tilted 90° to the ecliptic?
Does Earth rotate clockwise or counterclockwise?
Does atmospheric refraction give polar winters extra daylight time?
Does any location on Earth get the same number
of sunshine hours each year?
The Sun and Seasons
"Zenial Days" on Hawaii
Asian tradition on the start of winter
One year of continuous sunlight?
And for something completely different:
When does Jewish Sabbath begin far north?
Earth tilt and climate (1)
Earth tilt and climate (2)