Earth, Moon, and the Sun

Class 07 Science

The Earth spins (or rotates) on its own axis in space. The Earth’s axis of rotation passes through its geographic North Pole and the South Pole. The Earth completes one rotation in about 24 hours.

Rotation of the Earth

When viewed from the top of the North Pole, the Earth is rotating in the anti-clockwise direction, that is, from West to East. The Earth’s rotation from West to East causes the day-night cycle. The side facing the Sun experiences daytime, while the other side is dark and experiences night.

Due to the rotation of the Earth, the Sun appears to rise in the eastward direction, move across the sky from the East to the West and set in the westward direction. Then the night begins and the stars become visible in the sky.

The Earth’s axis of rotation points very close to the Pole Star in the Northern Hemisphere. Therefore, the Pole Star appears nearly stationary in the sky from the Earth. All the stars appear to move around it. Just like the Sun, the Moon also appears to rise in the eastward direction and set in the westward direction because the Earth rotates from West to East.

Revolution of the Earth

While rotating on its own axis, the Earth also revolves around the Sun. This movement is different from rotation. Revolution is the motion of an object around another object.

The path an object takes while revolving around another object is called its orbit. If viewed from the top, the orbit of the Earth around the Sun is nearly circular. The Earth completes one revolution around the Sun in about 365 days and 6 hours.

Every evening the Sun sets in the westward direction and the night sky becomes visible. This occurs due to the Earth’s rotation. As the Earth also revolves around the Sun continuously, the stars seen in the night sky after sunset gradually change over a year.

Seasons on the Earth

The Earth’s axis of rotation is not upright with respect to the orbit, but is tilted. The Earth maintains this tilt as it orbits around the Sun. The tilt of the Earth’s axis and the spherical shape of the Earth gives rise to seasons.

In June, the Northern Hemisphere is tilted towards the Sun while the Southern Hemisphere is tilted away from the Sun. A given amount of sun rays are spread in a smaller area in the Northern Hemisphere as compared to the Southern Hemisphere due to the spherical shape of the Earth’s surface. So that area is heated more.

Further, the Northern Hemisphere receives sunlight for more than 12 hours in June. So, the Northern Hemisphere experiences more intense sunlight, which lasts for a longer time, causing the summer season. In December, the situation is opposite in the Northern Hemisphere and it experiences winter season with sunlight for shorter time.

The seasons and length of daytime are reversed in the Southern Hemisphere as compared to the Northern Hemisphere. There, it is winter in June and summer in December.

On the equator, there is always 12 hours of sunlight and 12 hours of darkness. There is little difference in the intensity of the sun rays falling on the equator in different months. Thus, for the southern states of India that lie close to the equator, the effect of seasons is not very prominent. Other effects, such as local geographical features and proximity to oceans or seas, can also influence these broader patterns seen in the two hemispheres.

Eclipses

The Moon is a natural satellite of the Earth and it revolves around the Earth as the Earth revolves around the Sun.

Solar Eclipse

At certain times, the Moon can come in between the Sun and the Earth in a way that obstructs the light from the Sun from reaching us. This is known as a solar eclipse.

Though the Moon is much smaller in physical size than the Sun, the Moon is much closer to us compared to the Sun. Therefore, the Moon can appear to cover the entire Sun as viewed from the Earth.

Lunar Eclipse

As the Moon revolves around the Earth, sometimes the Earth can block the sunlight from reaching the Moon. This is known as a lunar eclipse. On such days, we see the Earth’s shadow falling on the full disc of the Moon.

When the Moon is completely in the Earth’s shadow, it is called a total lunar eclipse. The bright disc of the Moon starts to appear dark red in colour and stays that way until the Moon moves out of the shadow of the Earth. When part of the Moon is in the Earth’s shadow and the rest of the Moon is visible, it is called a partial lunar eclipse. Unlike the Sun, we can safely watch the eclipsed full Moon with our naked eye.