The summer solstice is the day when the Earth's North Pole is closest to the sun. It is also the day when the sun reaches its zenith in the sky. The winter solstice, or the shortest day of the year, occurs when the Earth's North Pole is tilted away from the Sun the most. At this time, the Sun is located on its southernmost point in the sky and begins its descent toward the south pole.
At mid-summer, the days are long but they get shorter as we move into fall and spring. However, at mid-winter the days are getting longer but they don't seem to get any longer as spring approaches.
This is because the amount of daylight changes throughout the year. At mid-summer, there is almost no nightfall in central Europe because the sun rises so early in the morning. At mid-winter, it is beginning to get light earlier every day because the sun is going down later in the evening.
These are just average values. In some regions, such as the Arctic, midsummer and midwinter have very short days. There may be only a few hours of sunlight each day, if anything at all.
In general, the length of day increases as we move north. At the South Pole, where mid-summer is defined as the date when the Sun is highest in the sky, the days are forever dark.
The tilt of the North Pole fluctuates as the Earth travels in its orbit (see diagram). It is winter in the northern hemisphere when it is turned away from the sun. At this time there are few hours left in December.
At spring equinox, when the north pole is aligned with the center of the earth and therefore the axis of rotation, the days start getting longer and longer until summer. At fall equinox, when the south pole is aligned with the center of the earth, the nights start getting longer and longer until winter again. So the seasons change because the Earth goes through these two events each year: one where it gets light all the time at mid-summer and another where it gets dark all the time at mid-winter.
These phenomena have important implications for astronomy since we can use them to determine information about the location of the North Pole over time. They have also been used by ancient civilizations to predict future seasons and agricultural yields!
The first scientific observation of the solstices was made by Thales of Miletus around 600 B.C. He predicted that there would be more sunlight during the summer months than during the other times of the year, which was correct. He also predicted that it would be colder during the winter months than it is today, which was also correct.
When the Earth's axis is pointing straight toward the Sun, the solstices occur. During Earth's orbit, this occurs twice a year. On June 21, the north pole is inclined 23.5 degrees toward the sun, and the northern hemisphere celebrates the summer solstice, the longest day of the year. On December 21, the south pole is inclined 23.5 degrees away from the sun, and the southern hemisphere celebrates the winter solstice, the shortest day of the year.
The axis of the Earth becomes its greatest distance from the Sun at the summer solstice, when it is called the aphelion. At the winter solstice, it is closest to the Sun, when it is called perihelion. The terms "aphelion" and "perihelion" are Greek words meaning "far removal" and "near removal."
The angle between the equator and the axis of the Earth is called the tilt of the earth. The tilt changes over time because the Earth's rotation is not a perfect circle. As a result, the distance between the North Pole and the South Pole changes. This is why you will never see a person standing under the Arctic Ocean with their feet in Canada and their head in Russia.
For example, right now the tilt is nearly 23.5 degrees. But about 5 million years ago, it was much more flat.