WBBSE Class 7 Geography Chapter 1 Earths Revolution Introduction
Any two bodies in the universe attract each other with a force. This force is known as the gravitational force. The imaginary line that joins the North and the South Poles and passes through the center of the Earth is known as the Earth’s axis. The Earth takes nearly 24 hours to rotate once around its axis.
The extreme northern and southern ends of the Earth’s axis are known as the Poles. If an object is thrown upwards at a velocity of 11.2.km/sec, it will go past the gravitational force of the Earth and will start revolving around the Earth. This velocity is known as the escape velocity of the object in the case of the Earth.
The elliptical path along which the Earth. revolves around the Sun is called its orbit. The imaginary plane on which the orbit of the Earth is situated is known as the orbital plane of the Earth. Aphelion is the position when the Earth is farthest from the Sun. The Earth reaches its Aphelion position on 4 July when it is 15 crore 20 lakh km away from the Sun.
Perihelion is the position when the Earth is nearest to the Sun. It occurs on 3 January every year when the distance between the Sun and the Earth is 14 crore 70 lakh km. The Earth’s satellite, Moon takes 27 1/3 days to revolve around the Earth. This time is called a lunar month.
The Earth orbits the Sun once in about 365 days. This period is known as a solar year. Leap year is a special kind of calendar year that contains 366 days, having 29 days in February. The actual time taken by the Earth to complete one revolution is 365 days 5 hours 48 minutes and 46 seconds.
Read And Learn Also WBBSE Class 7 Geography Notes
This is rounded off as 365 days for ease of calculation. But to balance this extra 6 hours is added to February (6×4-24 hours or 1 day) after every four years. Due to the Earth’s rotation, it seems that the Sun rises in the east and sets in the west. The Sun’s daily movement across the sky from the east to the west is known as the apparent diurnal movement of the Sun.
The sun rays are incident vertically on the Tropics and Equator during different times of the year. Thus, the Sun seems to march between the Tropic of Cancer (23½°N) and the Tropic of Capricorn (23½°S). This apparent movement is known as the apparent annual movement of the Sun or the Solar Ecliptic.
For six months from 22 December to 21 June, the Sun moves towards the northern direction. This movement of the Sun is known as the apparent movement of the Sun towards the north. For six months from 21 June to 22 December, the Sun gradually moves in a southerly direction. This movement of the Sun is known as the apparent movement of the Sun towards the south.
On 21 June, the sun rays are incident on the Tropic of Cancer at 90°. This day is known as Summer Solstice. This is the longest day and shortest night in the northern hemisphere. Conversely, this day is the shortest day and longest night in the southern hemisphere. On 22 December, the Sun comes overhead on the Tropic of Capricorn. This day is known as the Winter Solstice. This event is characterized by the shortest day and longest night is the northern hemisphere and the longest day and shortest night is the southern hemisphere.
On 21 March each year, the Sun remains overhead on the Equator. This day is known as the Vernal Equinox. Since this day marks the beginning of spring in the northern hemisphere is also known as the Spring Equinox. The Sun again remains overhead on the Equator on the 23 September every year. This day is called the Autumnal Equinox since it marks the beginning of autumn in the northern hemisphere. There is 12 hours of the day and 12 hours of night everywhere on Earth on the equinoxes.
In regions north of the Arctic Circle (places of countries like Denmark, Canada, Norway, Sweden, Iceland, etc) the Sun is visible at the horizon even at night according to local time from the months of March-July. This is known as the midnight Sun. The Sun can be clearly seen at night according to local time from May to July in the port of Hammerfest to the north of Norway. That is why Norway is called the Land of the Midnight Sun.