The sun's beams can only reach half of the earth's surface at once. Only twice a year, on September 23 and March 21, do the sun's rays reach the North and South Poles in equal measure. The sun's vertical rays fall directly on the equator on these two days. On all other days, the sun's vertical rays are refracted by the atmosphere, resulting in an angle between them and the horizon.
Sunlight travels through space until it reaches something solid like the earth. When it enters the atmosphere, some of its energy is absorbed by molecules in the air, while the rest passes over or around the planet. The part that passes over us is called direct sunlight. The part that is reflected back towards the sun is called indirect sunlight.
During a day with no clouds or pollution layers in the atmosphere, about half of all incident solar radiation is direct sunlight, and half is indirect. This means that on average, sunrays fall on the earth vertically, i.e., at a 90-degree angle.
However, because the rotation of the earth causes local changes in the direction of this average, the amount of direct and indirect sunlight at any given location varies throughout the day. At times of high pressure over land, more of the direct sunlight makes it to the ground than at times of low pressure. The wind also affects how much direct sunlight reaches various parts of the globe.
Along the equator The equator receives direct sunlight on March 21st and September 23rd. Because neither pole is slanted towards the sun in this configuration, the entire world has equal days and equal nights. This is known as an equinox.
At the poles The polar regions never see direct sunlight. On January 1st at the North Pole and July 1st at the South Pole, the sun is directly over the center of Earth. It takes light traveling through water or ice to reach the ground there, so those areas are called dark. But even though the North and South Poles are dark, the sky is not completely black - it's just that no stars can be seen from below 65 degrees north or south latitude because the planet blocks their glow.
In between The parts of the globe that are not at either the equator or the poles experience both day and night throughout the year. During certain times of the year, however, these areas will experience conditions where the sun is high in the sky during the daytime, but low in the sky at night.
On a clear day you could see all the way to the horizon from any location on Earth because there are no clouds or other obstacles between you and infinity. But on a cloudy day you would only see far away objects if the clouds were transparent, which they are not. Clouds consist of tiny particles that scatter light waves.
The equator receives direct sunlight on March 21st and September 23rd. Because neither pole is oriented towards the sun in this configuration, the entire planet experiences equal days and nights. The equator also receives indirect sunlight during the other months of the year.
The equator is located at the center of our planet. It is a line extending out from the center of the Earth to infinity. It can be thought of as the inside surface of a sphere with radius equal to that of the Earth. All points on the equator experiencethe same amount of daylight and darkness because they are always facing the Sun (or Moon).
The angle between the equator and the Sun varies by season and location. At the spring equinox, when we are experiencing direct sunlight at the equator, the Sun is directly over the poles at its lowest point in the sky. At the fall equinox, when we are experiencing direct sunlight at the equator, the Sun is directly over the equator at its highest point in the sky.
During the rest of the year, the Sun never gets completely below the horizon at the equator. There is always some sunlight reaching all parts of the globe, but it is mostly diffuse light coming from above and beyond the clouds.
The Sun's perpendicular rays wander from 23.5o N latitude to the equator from June to September 21. (0o latitude). The Sun's perpendicular rays wander from the equator to 23.5o S latitude from September to December 21. (Tropic of Capricorn). From December to March 22, they wander from the tropics back to the poles.
The horizontal rays of the Sun are stationary at the equator during the entire year, but its vertical rays wander within these limits: from the north pole to 23.5o N during the summer months; and from the south pole to 23.5o S in the winter.
It is this apparent movement of the Sun that causes us to have seasons: when the Sun is near the horizon it causes darkness; when it is high in the sky it causes lightness.
This is why plants need to change their behavior depending on whether it is day or night. During the day, plants use the sunlight to make food using the process of photosynthesis. At night, they store the nutrients they have produced during the day in order for them to be available when it is not light anymore. This is why plants have roots that go down into the ground and leaves that face upward toward the sky.
Animals also follow a daily cycle of sleeping and eating. They need sleep in order to grow and reproduce.
During the June solstice, the sun's vertical rays touch the Tropic of Cancer, 23.5 degrees north of the Equator. During the December solstice, the subsolar point begins its descent south, and vertical rays touch the Tropic of Capricorn, 23.5 degrees south of the Equator. The sun is directly over the equatorial line on both sides of the planet at all times during the year.
The word "solstice" comes from the Latin words for "sun" and "stick". It refers to the fact that at one point in time during the year, the center of the sun is exactly between the Earth and the moon. At other points in time, the center of the moon is between the Earth and the sun.
At Northern Hemisphere spring and autumn equinoxes, the Sun is directly over the equatorial line of the Earth at noon, while at mid-summer and mid-winter it is not. The term "solsticial" means "pertaining to the sun", and describes the relationship between the sun and the seasons. At any given location on the surface of the earth, there are two events each year when the sun is precisely over the equatorial line: spring equinox and fall equinox. These are called solar eclipses because the Moon is completely covered by the Earth's shadow.
Eclipses were important to certain cultures due to their connection with deity.