Earth Tilt lesson:
Study this picture very carefully. Notice how the Earth is tilted and look at the names of the seasons under the Earth at the top right. This tells you which seasons are happening in the Northern and Southern hemispheres at the same time.
Many people believe the Earth is closer to the sun in the summer and farther away in the winter, because it is hotter in the summer. This answer seems to make perfect sense. If you are closer to a fire, you are warmer. But in relation to the Earth and the sun, this explanation is not correct, especially since the Earth is farther away from the sun in July than it is in January.
Why isn't the Earth's distance important in explaining the causes of the changing seasons? The most important reason is that the Earth's orbit or path around the sun is almost circular. The distance of the Earth from the sun does change, but it is so slight that it makes very little difference in the heat energy felt on the Earth. For example, if you were sitting 30 feet from the fireplace and moved 30 inches closer, you would not feel that much warmer than what you felt before. Therefore, it is not the Earth's distance from the sun that causes the change in our seasons.
What causes the changes in the seasons on Earth? The two things that make an impact on the seasons are the angle of the Sun above the horizon and how long the sun stays in the sky (the amount of daylight). When the sun is most nearly overhead, the ground gets the hottest. At , the sun is at an angle that is most nearly vertical. At sunrise or sunset, the angle from the horizon is smaller so the ground is cooler.
When daylight lasts longer, there is more time for the sun to warm the ground each day.
The angle above the horizon at noontime is important because it determines how much concentrated light and energy the Earth receives at a specific point. An area that receives more light more directly overhead, gets the hottest. Also, the longer the sun is above the horizon on a given day, the longer it will radiate light and energy.
What causes the Earth to be at a different angle from the sun at different times of the year? The tilt of the Earth's axis is what makes this difference. The angle of the Earth's axis is 23.5°. The seasons are caused because the Earth is tilted in its orbit around the sun. The Earth’s orbit is not quite circular, it is elliptical. An ellipse is an oblong circle, sort of egg shaped. The reason for the seasons is the angle at which the sun's rays strike the earth, caused the by the tilt of the Earth on its axis. The tilt of the Earth also causes the change in the number of daylight hours experienced throughout the year.
Because the direction of the Earth's tilt changes in relation to the sun, the northern and southern halves of our planet get differing amounts of sunlight during the year. When the Northern Hemisphere of the Earth is leaning toward the sun, it receives direct rays of sunlight and is warmer, while the Southern Hemisphere receives more indirect rays.
When the northern part of the Earth is leaning away from the sun, the opposite happens—the Northern Hemisphere gets cooler, more indirect sunlight while the southern half receives direct rays. Because of this, the seasons in the Northern and Southern Hemispheres are reversed, about six months apart from each other.
The changing position of the Earth's tilt is the reason for the differences in temperature and length of daylight that we get during the seasons. When the Northern Hemisphere is leaning toward the sun, the direct rays cause spring and then summer in that part of the globe. When the Northern Hemisphere is leaning away from the sun, the sunlight strikes the Earth at an angle and causes autumn and winter.
The start of spring, summer, autumn, and winter is marked by special days that correspond to different points in the Earth's orbit:
The summer solstice is the first day of summer and the longest day of the year, occurring in the Northern Hemisphere when the North Pole is leaning more directly toward the sun than it does on any other day. During the period marked by the summer solstice, the Northern Hemisphere is warmed by more direct sunlight and days are long and hot.
The winter solstice, by contrast, is the first day of winter and the shortest day of the year. As you might have guessed, the winter solstice in the Northern Hemisphere occurs when the North Pole is leaning away from the sun. When the North Pole is pointing away from the sun, the Northern Hemisphere receives only indirect sunlight; that is why winter is so much colder than summer. Brrrr!
Equinoxes, on the other hand, occur during transition periods when the North Pole is pointing neither directly toward nor directly away from the sun; these days are have equal periods of light and darkness. The autumnal equinox is the first day of autumn and occurs when the North Pole begins to lean away from the sun;
the vernal equinox is the first day of spring and occurs when the North Pole begins to lean toward the sun again.