Motion of the Earth and their effects

Galactic Movement

This is the movement of the Earth with the sun and the rest of the solar system in an orbit around the centre of the Milky Way Galaxy.  This movement has little effect upon the changing environment of the Earth.

The earth has two main motions (i) Rotation and (ii) Revolution.

Rotation

• Earth rotates along its axis from west to east. (Axis is the imaginary line connecting the two poles running through the centre of the earth)
• Earth rotates on a tilted axis. Earth’s rotational axis makes an angle of 23.5° with the normal i.e. it makes an angle of 66.5° with the orbital plane.

• It takes approximately 24 hours to complete one rotation. (23 hours 56 minutes 4 seconds precisely)
• Days and nights occur due to rotation of the earth. When a place on Earth faces the sun, it has daytime. When it faces away from the sun, it has night.
• The circle that divides the day from night on the globe is called the circle of illumination.

What would happen if the earth did not rotate ?

• The portion of the earth facing the sun would always experience day, thus bringing continuous warmth to the region.
• The other half would remain in darkness and be freezing cold all the time.
• Life would not have been possible in such extreme conditions.

Effect of the rotation of the Earth

• Formation of days and nights
• Change in the direction of winds, ocean currents, etc. (concept of Coriolis force)
• Occurrence of sunrise, noon and sun set.
• Flatness at the poles and bulge at the equator
• Concept of time and dates
• Apparent movement of the sun, the moon and the stars
• Magnetic field of the earth

Revolution

• The second motion of the earth around the sun in its orbit is called revolution. It takes 365¼ days (one year) to revolve around the sun.
• Six hours saved every year are added to make one day (24 hours) 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 in its orbit around the sun moves in a constant plane.  This plane is called the PLANE OF THE ECLIPTIC.   The plane of the Earth’s equator makes an angle of 23½° with the plane of the ecliptic.  Thus the imaginary Earth axis[1], being perpendicular to the equator, has a constant ANGLE OF INCLINATION, as it is called, of 66½° with the plane of the ecliptic.

In addition to a constant angle of inclination, the Earth’s axis maintains another characteristic called PARALLELISM.  As the Earth revolves around the Sun, the Earth’s axis remains parallel to its former position.  That is, at every position in the Earth’s orbit the axis remains pointed towards the same spot is close to the star we call the NORTH STAR or POLARIS.  Thus, the Earth’s axis is fixed with respect to the stars system, but not with respect the Sun.

The path on which it describes its motion is called orbit. The shape of the orbit is an ellipse and not a circle. The mean distance of the Earth from the Sun is 150 million km but because of the elliptical shape of the orbit the distance varies from time to time. On or about January 3, the Earth is closest to the Sun. At this time the Earth is said to be in Perihelion (nearest) and its distance from the Sun is 147 million km. Similarly, the Earth is at its farthest point from the Sun on or about July 4. In this position the Earth is said to be aphelion and is at a distance of 152 million km. It should appear that there is summer in perihelion because the Earth is the closest to the sun. But reverse is the case. It is summer in aphelion and winter in perihelion.

 Effect of the Revolution of the Earth

• Change of seasons
• Length of days and nights
• Shifting of wind belts

The Seasons

It has been made clear that the Earth revolves around the Sun with two characteristics:

(i)         Its axis of rotation is inclined to the orbital plane at an angle of 66½ degree.

(ii)        The northern end of the axis of rotation points towards the pole star wherever the Earth is in the orbital path.

There is one important effect of this type of revolution.  The northern and southern hemispheres in turn are tilted towards the Sun while at two places both the hemispheres are equally inclined to the Sun.

Duration of Seasons

From the point of view of the Earth’s inclination, there are four positions of SOLSTICES and EQUINOXES.  Hence, there are the following four seasons according to the positions of the Earth in one complete revolution of the Earth around the Sun.

Solstice

• 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. So, 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 the 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. 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 tilts towards it. As the sun’s rays fall vertically at the Tropic of Capricorn (23½° S), a larger portion of the southern hemisphere gets light.
• Therefore, it is summer in 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.

Equinox

• 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.

• On 23rd September, it is autumn season (season after summer and before the beginning of winter) in the northern hemisphere and spring season (season after winter and before the beginning of summer) 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 (Rotation= Days and Nights; Revolution = Seasons)

Varying Length of Day and Night-

• The axis of the earth is inclined to the plane of the ecliptic (the plane in which the earth orbits round the sun) at an angle of 66½°, giving rise to different seasons and varying lengths of day and night.
• The revolution of the Earth and its inclination to the plane of the ecliptic cause the variation in the length of day and night at different times of the year.

Why regions beyond the Arctic Circle receive sunlight all day long in summer?

• Reason- Tilt of the earth
• Earth’s axis at the North Pole is tilted towards the sun in summer.
• So the whole of Arctic region falls within the ‘zone of illumination’ all day long in summer.

Why Arctic Circle is popularly referred as Land of the mid night Sun’?

• In the northern hemisphere in winter (December) as we go northwards, the hours of darkness steadily increase.
• At the Arctic Circle, the sun never rises and there is darkness for the whole day in mid-winter on 22 December.
• Beyond the Arctic Circle the number of days with complete darkness increases, until we reach the North Pole (90° N) when half the year will have darkness. In summer (June) conditions are exactly reversed. Daylight increases as we go polewards.
• At the Arctic Circle, the sun never sets at the midsummer (21 June) and there is a complete 24 hour period of continuous daylight. In summer the region north of the Arctic Circle is popularly referred to as ‘Land of the mid night Sun’.

Why do the poles experience about six months day and six months night?

• Reason- The tilt of the Earth on its axis.
• Because of this tilt, each Pole is tilted towards and away from the Sun for about six months each.
• When the North Pole is tilted towards the Sun, it experiences continuous daylight for six months. It is night for the same time period at the South Pole.
• These conditions are reversed when the South Pole is tilted towards the Sun.

Why does the Southern Hemisphere experience winter and Summer Solstice in different times than that of the Northern Hemisphere?

• When the North Pole is tilted towards the Sun, the northern Hemisphere experiences Summer Solstice. At this time, since the South Pole is tilted away from the Sun, the Southern Hemisphere experiences Winter Solstice.
• When the North Pole is tilted away from the Sun, the Northern Hemisphere experiences Winter Solstice. At this time, since the South Pole is tilted towards the Sun, the Southern Hemisphere experiences Summer Solstice.
• Why summer is usually associated with much heat and brightness and winter with cold and darkness?
• In summer, the sun is higher in the sky than in winter. When the sun is overhead its rays fall almost vertically on the earth, concentrating its heat on a small area; temperature therefore rises and summers are always warm.
• In winter the oblique rays of the sun, come through the atmosphere less directly and have much of their heat absorbed by atmospheric impurities and water vapour. The sun’s rays fall faintly and spread over a great area. There is thus little heat and temperatures remain low.
• The days are longer than nights in summer and more heat is received over the longer daylight duration. Nights are shorter and less heat is lost. There is a net gain in total heat received and temperature rises in summer.

Temperature distribution on earth

• Sun is the ultimate source of heat. And the differential heat received from sun by different regions on earth is the ultimate reason behind all climatic features.
• The patterns of distribution of temperature in different seasons will help in understanding various climatic features like wind systems, pressure systems, precipitation etc.

Insolation

Earth intercepts only one in two billion parts of solar radiation. This intercepted radiation is called Insolation.

• Definition- Insolation is the proportion of solar energy received or intercepted by earth.
• Some heat within the core and mantle is transferred to the surface and ocean bottoms through volcanoes, springs and geysers. But this heat received at the surface form interiors of the earth is negligible compared to that received from sun.
• Earth receives Sun’s radiation (heat) in the form of short waves which are of electromagnetic nature. The earth absorbs short wave radiation during daytime and reflects back the heat received into space as long-wave radiation i.e. infrared radiation during night.

Ways of Transfer of Heat Energy

The heat energy from the solar radiation is received by the earth through three mechanisms—

1. Radiation- Heat transfer from one body to another without actual contact or movement. It is possible in relatively emptier space, eg. from the sun to the earth through space.
2. Conduction- Heat transfer through matter by molecular activity. Heat transfer in iron and other metals is by conduction. Generally, denser materials like water are good conductors and a lighter medium like air is a bad conductor of heat.
3. Convection- Transfer of heat energy by actual transfer of matter or substance from one place to another.

Factors Affecting Temperature Distribution

• The Angle of Incidence or the Inclination of the Sun’s Rays
• Duration of Sunshine -
• The amount of heat received depends on day or night; clear sky or overcast, summer or winter etc.
• Transparency of Atmosphere
• Aerosols, dust, water vapor, clouds etc. effect transparency
• If the wavelength of the radiation is more than the radius of the obstructing particle (such as a gas), then scattering of radiation.
• If the wavelength is less than the obstructing particle (such as a dust particle), then total reflection.
• Absorption of solar radiation takes place if the obstructing particles happen to be water vapour, ozone molecules, carbon dioxide molecules or clouds.
• Most of the light received by earth is scattered light.

Land-Sea Differential

• Albedo of land is much greater than albedo of oceans and water bodies. (For example, snow covered areas reflect up to 70%-90% of insolation) (Albedo is the measure of reflectivity of a surface)
• Average penetration of sunlight is more in water than in land. Therefore, land cools or becomes hot more rapidly compared to oceans.
• In oceans, continuous convection cycle helps in heat exchange between layers keeping diurnal and annual temperature ranges low.
• The specific heat of water is 2.5 times higher than landmass; therefore water takes longer to get heated up and to cool down.

Prevailing Winds

• Winds transfer heat from one latitude to another. They also help in exchange of heat between land and water bodies.
• The oceanic winds have the capacity to take the moderating influence of the sea to coastal areas – reflected in cool summers and mild winters. This effect is pronounced only on the windward side (the side facing the ocean).
• The leeward side or the interiors do not get the moderating effect of the sea, and therefore experience extremes of temperature.

Aspects of Slope

• The direction of the slope and its angle control the amount of solar radiation received locally. Slopes more exposed to the sun receive more solar radiation than those away from the sun’s direct rays.
• Slopes that receive direct Sun’s rays are dry due to loss of moisture through excess evaporation.
• Slopes that are devoid of direct sunlight are usually well forested.

Ocean Currents

Ocean currents influence the temperature of adjacent land areas considerably.

Altitude

• With increase in height, pressure falls, the effect of greenhouse gases decreases and hence temperature decreases (applicable only to troposphere).
• The normal lapse rate is roughly 1° C for every 165 metres of ascent.

Earth’s Distance from Sun

• During its revolution around the sun, the earth is farthest from the sun (4th July).
• On 3rd January, when the earth is the nearest to the sun the annual insolation received by the earth is slightly more than the amount received on 4th July.
• The effect of this variation in the solar output is marked by other factors like the distribution of land and sea and the atmospheric circulation.
• Hence, this variation in the solar output does not have great effect on daily weather changes on the surface of the earth.