Orbital Plane

·   The Earth has two types of motions, namely rotation and revolution. Rotation is the movement of the earth on its axis.

·   The movement of the earth around the sun in a fixed path or orbit is called revolution.

·   The axis of the Earth which is an imaginary line makes an angle of 661/2 with its orbital plane. The plane formed by the orbit is known as the orbital plane. The earth receives light from the sun. Due to the spherical shape of the earth, only half of its gets light from the sun at a time. The portion facing the sun experiences day while the other half away from the sun experiences day while the other half away from the sun experiences night.

Circle of Illumination

The circle that divides the day from night on the globe is called the circle of illumination. This circle does not coincide with the axis. The earth takes about 24 hours to complete one rotation around its axis. The period of rotation is known as the earth day. This is the daily motion of the earth.

Rotation and Impact of Rotation

•    Rotation: -The earth spins on its axis from west to east. This spinning of the Earth around its own axis is known as rotation. The Earth around takes about 24 hours to complete one rotation. One rotation makes an earth day. It is also called solar day.Effects of the Earth’s rotation: The rotation of the Earth causes:•    Day and night•    Bulging of the earth at the equator, and flattening at the poles.•    The deflection (pushing away from the straight path) of winds and ocean currents.•    Tides (the alternate rise and fall of the oceans due to the gravity of the Moon and the Sun, and the rotation of the earth.•    The apparent movement of all the heavenly bodies from east to the west.What do you think will happen if the Earth fails to rotate?It will be disastrous if the earth fail to rotate.•    While the side of the earth facing the Sun will always be lighted. The other side will always remain dark.•    The lighted side of the Earth will become extremely hot and the dark side will be freezing cold.•    Life forms would not be able to exist in such extreme conditions.

Revolution and Impact of Revolution of Earth

Revolution:-The second motion of the Earth around the sun in its orbit is called revolution. It takes 365 1/4 days (one year) to revolve around the sun.Effects of the Earth’s revolutionThe revolution of the Earth, along with the tilt in the Earth’s axis, causes:•    Varying lengths of day and night•    changing seasonsIf the Earth’s axis were straight and not titled, there would be no seasons, since every point on the Earth would receive the same amount of light each day of the year. Nor would there have been any variation in the lengths of day and night-- they would last 12 hours each.

Day and Night

Varying lengths of day and night: The lengths of days and nights can be very different, even at the same time of year. This variation is due to the inclination of the earth’s axis. For half the year, the Northern Hemisphere faces the Sun, while for the next six months; the Southern Hemisphere faces the Sun. A larger part of the hemisphere that faces the Sun gets longer days and shorter nights. At the equator, the days and nights are of equal length.

Elliptical Orbit

An orbit is the path an object takes around another object, like the path the Earth takes around the Sun. An elliptical orbit is a path that has an oval-like shape.

How Do Objects Stay in Orbit?

An object in motion will stay in motion unless something pushes or pulls on it. This statement is called Newton's first law of motion. Without gravity, an Earth-orbiting satellite would go off into space along a straight line. With gravity, it is pulled back toward Earth.

An elliptical orbit can be useful to a communications satellite because it allows the satellite to travel over a specific region for a long portion of its orbit.

What is a Leap Year?

It takes approximately 365.25 days for our planet Earth to orbit the Sun — that is a solar year. We usually round the days in a calendar year to 365, that is 365 days in a year. To make up for the missing partial day, we add one day to our calendar approximately every four years and that is known as a leap year.

In an ordinary year, if you were to count all the days in a calendar from January to December, you’d count it as 365 days. But approximately every four years,  the month of February has 29 days instead of 28. So, there are 366 days in the year. This is called a leap year.

·   Six hours saved every year are added to make one day (24 years) over a span of four years. This surplus day is added to the month of February. Thus every fourth year, February is of 29 days instead of 28 days. Such a year with 366 days is called a leap year. The earth goes around the sun in an elliptical orbit throughout its orbit, the earth remains inclined in the same direction.

The Changing Season on Earth

The changing Seasons: A year is usually divided into summer, winter, spring and autumn seasons. Seasons change due to the change in the position of the earth around the sun.

·   On 21st June, the northern hemisphere is tilted towards the sun. The rays of the sun fall directly on the tropic of cancer. As a result, these areas receive more heat. The areas near the poles receive less heat as the rays of the sun are slanting. The North Pole is inclined towards the sun and the places beyond the Arctic Circle experience continuous daylight for about six months.

Since a large portion of the Northern Hemisphere is getting light from the sun, it is summer in the regions north of the equator. The longest day and shortest night at these places occur on 21st June. At this time in the southern Hemisphere all these conditions are reversed. It is winter season there.

Equinox

The Equinox (Vernal & Autumnal)

There are only two times of the year when the Earth's axis is tilted neither toward nor away from the sun, resulting in a "nearly" equal amount of daylight and darkness at all latitudes. These events are referred to as Equinoxes. At the equator, the sun is directly overhead at noon on these two equinoxes.  The "nearly" equal hours of day and night are due to refraction of sunlight or a bending of the light's rays.

In the Northern Hemisphere, the March equinox is called the vernal or spring equinox while the September equinox is called the autumnal or fall equinox.

What is an Equinox?

As the Earth revolves around the Sun, it experiences seasonal changes. These include changes in weather and the length of days. This is due to the tilt of the Earth's axis causing different amounts of sunlight at different times of year. Four points along the Earth's path have been defined: two equinoxes and two solstices.

The solstices, which take place in June and December, occur when the Earth's tilt either towards or away from the Sun is most significant.

The equinoxes, which are observed in March and September, occur when the Earth's axis is tilted neither towards nor away from the Sun. These are the days when the Sun is exactly above the Equator, which makes day and night of equal length. So, on 21st march and September 23rd, direct rays of the sun fall on the equator. At this position, neither of the poles is tilted towards the sun; so, the whole earth experiences equal days and equal nights. This is called an equinox.

So, in the Northern Hemisphere you have:

Summer and Winter Solstice

·   The nights are longer than the days. This position of the Earth is called the summer solstice. On 22nd December, the Tropic of Capricorn receives direct rays of the sun as the South Pole tills towards it. As the sun’s rays fall vertically at the Tropic of Capricorn, a larger portion of the southern Hemisphere with longer days and shorter nights. The reverse happens in the Northern Hemisphere. This position of the earth is called the winter solstice.

  On 23rd September, it is autumn season in the Northern Hemisphere and spring season in the Southern /hemisphere. The opposite is the case on 21st march. When it is spring in the Northern Hemisphere and autumn in the Southern Hemisphere.

Thus, you find that there are days and nights and changes in the seasons because of the rotation and revolution of the earth respectively.