In ancient Greece, it was believed that Sun was a God and traveled in a chariot. Every morning, he would come out of his palace and light up the world with his aura. At night, he would go back to his palace, and the Earth would turn dark.
Day and night are the result of the Earth's rotation on its own axis and also its relative position to the Sun in our solar system. To understand the complete process that leads to morning and nighttime, we have to understand the basics of our solar system, which have been touched upon in the second half of the write-up.
Causes of Day and Night
Speed of Rotation
The Earth rotates on its own axis. This axis is an imaginary line running across the North and South Pole, through the center of the Earth. One rotation is completed in approximately 24 hours. The speed, however, varies from one place to another on the Earth's surface. For example, on the equator, the imaginary line divides the Earth into the northern and southern hemisphere. The Earth completes a rotation at the speed of 1669.8 km/h. On the contrary, its rotational speed at the poles is almost zero. This effect is caused due to the varying diameter of the Earth. The speed of the rotation is calculated by dividing the circumference at the equator by the total number of hours in a day.
Direction of Rotation
The Earth rotates in the counterclockwise direction (with North pole as point of observation) -- that is from west to east on its axis. As the Earth is a sphere, while rotating, only half of the Earth faces the Sun and is exposed to sunlight, causing day in that area. The other half is concealed from the Sun and experiences nighttime. This is the reason why countries placed exactly opposite to each other experience day and night at different times. For example, if it is midnight in Washington D.C., it is exactly noon in Singapore.
Different Lengths of Day and Night
Places that are located on the side of the Earth exposed to the Sun have varied lengths of day. This is caused due to the axial tilt, or the tilt of the Earth's axis. The axis on which the Earth rotates is tilted by 23.5 degrees such that the North Pole always points towards Polaris -- another star in the Milky Way.
This tilt has a great impact on how much surface is exposed to sunlight and thereby, the lengths of daytime and nighttime. It is this tilt that makes the days interesting in the North and South Pole. During the Earth's revolution around the Sun throughout the year, sunrays reach the North Pole only from the month of March to September. Thus, in the North Pole, a day is as long as six months. Similarly, the Sun rises in the month of September in the South Pole and sets in March. So, the South and North poles have six months each of day and night.
Solar System
Our Solar system has one star -- the Sun and eight planets that revolve around the Sun in their own paths known as orbit. The sequence of planets according to their proximity from the Sun is Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto's status as a planet is controversial. Mercury is the smallest planet, while the Earth is the fifth largest planet. Every planet has its own atmosphere, although only the Earth's atmosphere is fit for survival of living beings.
The Sun
The Sun is the center of our solar system. However, it is one of the billion stars of our galaxy -- the Milky Way. It is a spherical mass of hot plasma and is about 1,300,000 times bigger than the Earth. Its diameter is 1,392,684 km, which is 109 times that of the Earth. Its mass is 1.9891×10
30 kg and the surface temperature is 5,778 °K. The Sun makes up for 99.86% of the total mass of our Solar system. Our solar system derives all its energy from the Sun.
The Earth
The Earth is the third planet in our solar system and probably the only planet in the whole universe with life on it. It revolves around the Sun, in its orbit, and takes approximately 365.25 days to complete one revolution at the speed of 108,000 km/h. The Earth's mass is approximately around 5.98 x 10
24 kg, and the average density is about 5520 kg/m
3. The diameter is 12,756 km. It has one natural satellite -- the Moon. The Moon is around 1/4th of the Earth's diameter.
Eratosthenes, a Greek scholar, was the first person to determine the circumference of the Earth. He calculated the circumference by comparing the shadow of the Sun (at noon) in the deep wells of Syene and Alexandria. In his calculation, he assumed that the rays of the Sun are parallel. He stated the circumference of the Earth to be around 250,000 stadia (an ancient Greek measure - one stadia is equal to a modern furlong or 660 feet). He also calculated the distance of the Earth from the Sun and the Moon, and the tilt of the Earth's axis.
So, to conclude, day and night are not caused due to the Sun (God), but it's because our planet rotates about its axis, giving us the wonderful experience of warm days and cool nights.
