Motions of the Earth
The Earth has two types of motions:
- Rotational Motion
- Revolutional or Orbital Motion
- Rotation : The Earth rotates on its axis, from west to east like a top. This motion is called Rotation of the Earth.
- Revolution : While rotating on its axis, the earth also goes around the sun in an elliptical path and completes one round in 365 days and 6 hours. The elliptical path traced by the earth is called its orbit. This motion of the earth is called revolution.
- Perihelion : When the earth is at the minimum distance from the sun, while in orbit, this position is known as perihelion. The earth is at this position on 4th July.
Change in Seasons
The earth moves not only on its axis but also in its orbit around the sun. Therefore, the earth changes its position continuously with respect to the sun. There are four major positions of the earth, while it revolves around the sun.
- Position on 21st June : In this position the sun shines perpendicularly over the Tropic of Cancer. This position in called the Summer Solstice. It is the time of summer season in the northern hemisphere, while in the southern hemisphere, it is winter season. The day in the northern hemisphere is longest on 21st June.
- Position on 22nd December : In this position the Sun shines perpendicularly over the Tropic of Capricorn. This position is called Winter Solstice. During this period, days are long and nights are short in the southern hemisphere. This positions marks the summers in the southern hemisphere and winters in the northern hemisphere.
- Positions on 21st March and 23rd September : In these two positions the Sun shines directly overhead on the Equator. Therefore, half part of all latitudes receives the sun-light at these times. Hence, everywhere, the duration of day and night is equal.. Seasons are also similar in both the hemispheres. These two positions are referred to as Equinoxes. 21st March is called Spring or Vernal Equinox whereas 23rd September is called Autumn Equinox.
- Both the Earth and the Moon get hght form the Sun. Only one part of the Moon can be seen from the earth because of equal rotational speed of both the earth and the Moon. Illuminated face of the Moon is seen on the Earth once a month on the Full Moon. On the New Moon the Moon is not seen at all because on this night the completely dark Moon faces the Earth.
- When the Earth comes in between Sun and the Moon, the hght of the Sun is not able to reach the Moon, rather, the shadow of the Earth falls on the moon. This is called the Lunar Eclipse. Lunar Eclipse always occurs on a Full Moon. When the Moon comes in between the Sun and the Earth, the light of the Sun is interrupted and shadow of the Moon falls on the Earth. This is called the Solar Eclipse, which occurs on New Moon day.