EARTH AND ITS MOVEMENTS

                             

THE SOLAR SYSTEM

• Sun has most of the mass of the solar system.

• The solar system started forming around 5.6 billion years ago and was fully formed 4.6 billion years ago.

EARTH

• As of our knowledge till now, life exists only on the Earth.

• Pre-conditions for life like conducive temperature(0-100 C) which allows H2O as the liquid makes life possible on the Earth.

• Any planet found in the Goldilock zone of a star has the possibility of supporting life-Earth is in the Goldilock zone of the Earth.

• Hence, whenever we are searching for life anywhere in the universe we look for the Goldilock zone of different stars.

 

Shape of the Earth:

• We found that the Earth's shape is much closer to Oblate Ellipsoid (non-rotating earth) and Oblate Spheroid(rotating earth) which are slightly flattened at their tops and bottoms. These shapes when used for mathematical calculations for the earth gives us more accurate results for different conditions.

• Geoid is the shape of the earth when gravity effects and the relative tidal bulges in the ocean due to the sun and moon are not considered.

 

Movements of the Earth:

(I)Major movements:

1.Along with the Galaxy:

• This movement along with the galaxy is due to the expansion of the universe where because of the expanding universe all the galaxies are moving away from each other and because the earth is also a part of the galaxy/Milky Way which is moving at a speed of 20-21 lakh km/hr-the earth is also moving with that speed along the Milky Way. This movement of the galaxy or the earth does not affect us in any tangible way.

2.Along with the sun around the Milky Way galaxy:

• The sun moves around the Milky way in an orbit.

• The earth also revolves around the Milky way along with all the objects of the Solar system.

• It takes around 250 million years for one revolution around the Milky Way.

• This movement of the sun and solar system and hence the earth is at a speed of around 8.25 lakh km/hr. This movement does not affect us in any tangible way.

 

3.Revolution of the Earth around the Sun at a speed of 1, 10,000 km/hr:

• The earth revolves around the sun in an elliptical orbit-and the sun is located at one of the focuses of the ellipse.

• Because of this, when the earth revolves around the sun in its orbit, at one of the places it is closest to the sun and at another location, it gets farthest from the sun.

• The closest location is the Perihelion(147 million Km) and the farthest location is the Aphelion(152 million Km).

• The earth is farthest from the sun on 4th July and the closest to the Sun around on 3rd January. (Note: The average distance between the sun and the earth is 150 million km=1 Astronomical Unit-which is the measurement unit for astronomical distances).

 

• The earth revolves around the sun in such a manner that it tilts around 23.5 degrees along the direction-always pointing towards the North Star(Polaris star). The earth's tilt with respect to the orbit is fixed in angle and direction both.

• Because the earth is tilted towards the Pole star always, as the Pole star is too far from the solar system, hence the earth is always tilted with the respect to its orbit at a constant angle of 23.5 degrees vertical to the orbit and also in a constant direction with respect to the revolution orbit of the earth. Hence, it is simply said earth's tilt angle and direction are constant with respect to the orbit of the earth.

• The earth takes approximately 365 and 1/4 days for one complete revolution.

 

 4.Rotation of the Earth around the Sun:

• The earth rotates on its own axis in a West-East direction with its axis always tilted at a constant angle 23.5 degrees vertical to the orbit of revolution.

• The speed of rotation of different locations of the earth is different when seen in km/hr-maximum at the Equator and minimum at the Poles.

 

• The time taken for the Earth's rotation is 23 hrs 56 minutes 4 seconds for 360-degree rotation with respect to itself. But it takes 24 hours for one complete rotation with respect to the sun.

(II)Minor movements: As the earth moves around the sun, it behaves like a spin top, and hence in addition to the rotation and revolution it does, it also does some minor movements like:

1.Axial tilt/Obliquity:

• The earth's axis keeps changing its tilt in a cyclic manner between 22.1 degrees and 23.5 degrees with respect to the vertical to the orbit of revolution.

• This process continues as a cycle over a period of 41,000 years. Due to this change in axial tilt, the earth's tilt with respect to the sun also changes so that for a few thousand years, the earth might be tilted more with respect to the Sun (24.5 degrees ) and later on for few thousand years tilted less with respect to the Sun(22.1 degrees).

• Hence, axial tilt also affects the earth's tilt with respect to the sun over thousands of years.

2.PRECESSION:

• Precession is the circular movement of the North pole of the Earth taking around 26,000 years to complete one cycle. During this cycle, the Earth changes its pole star between Vega and Polaris. And due to precession the Earth's tilt with respect to the Sun also changes for different parts of the year for different years. 
• This means, for eg. presently, the Earth's tilt with respect to the Sun during different parts of the year is such that A, B, C, D. A( around June) when Northern Hemisphere is tilted towards the Sun and Southern Hemisphere is tilted away from the Sun. B ( during December), the Northern hemisphere is tilted away from the Sun and the Southern hemisphere is tilted towards the sun.
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• C and D Earth is so tilted on its orbit that neither the Northern Hemisphere nor the Southern Hemisphere is tilted towards the Sun. Hence, at these March and September positions, we say that Earth is not tilted w.r.t. the Sun.
• This tilt due to precession might change its months, for eg. when 1/4th of the precession cycle is completed, Earth's tilt during June and December w.r.t. Sun might not be tilted. And during March and September, it might be tilted w.r.t. Sun. 
• The day in June when we get maximum tilt w.r.t. Sun we call it June Solstice, and this tilt angle is 23.5 degrees. June solstice happens somewhere between 21st-23rd June. 
• Similarly, the December date on which we have maximum tilt w.r.t. The Sun (angle 23.5 degrees) is called the December Solstice( will be discussed later). 
• Whereas, the dates of March and September when Earth is not tilted w.r.t. The Sun is called Equinoxes, for September we call September Equinox and for March, March Equinox.
• Precession also changes the climate on Earth on a long-term basis i.e. over thousand of years.

3.ECCENTRICITY:

• Eccentricity is the phenomenon, where Earth's elliptical orbit of revolution keeps changing the position of its orbit by slightly shifting it every year and this way completing its eccentricity cycle over a period of around 1 lakh years. 
• Eccentricity is mainly due to the gravitational pull of Jupiter and Saturn on the Earth.

 

OVERALL EFFECT OF THE THREE MINOR MOVEMENTS

• The three minor movements over their cycle of thousands of years go on to change the Earth's tilt or distance w.r.t. the Sun. We know that the seasons or climate on the Earth are affected by Earth's distance and tilt w.r.t. the Sun for eg. part of the earth tilted towards the Sun experiences hotter summer seasons and part of the Earth away from the Sun experiences winter season. 

• Simply said, the minor movements affect the long-term climate of the Earth, sometimes making the Earth hotter for a few thousand years making the climate hotter, and sometimes making the Earth cooler for a few thousand years hence a colder climate. So, they bring natural climate change on Earth.

• All the minor movements together are also known as MILANKOVITCH MOVEMENTS or Cycles.

MAJOR MOVEMENTS OF EARTH AND THEIR EFFECTS

1)REVOLUTION OF THE EARTH:

• CASE 1: If the revolution of Earth took place alone, without rotation and without tilt. Then outcomes of every one complete revolution will be:

1. The 1-year cycle will be the same that is (365 days and 1/4th days)

2. Half the year there will be a day and half the year there will be a night. The day and night cycle will also be (365x24 hours).

3. No Seasons on Earth: Since Earth is not tilted w.r.t. Sun, there are no seasons.

• CASE 2: If the Revolution without tilt of 23.5 degrees w.r.t. Elliptical orbit, but, with Rotation. Then the outcome will be

1. The 1-year cycle will be the same that is (365 days and 1/4th days)

2. No seasons on Earth.

3. 24 hour day and night cycle will be there i.e. 12 hour day and 12 hour night for every place on Earth. It means no long or short day/night concept.

2)CIRCLE OF ILLUMINATION

 

• This is a circle which separates the dayside from the night side. The circle of illumination is always vertical ( zero tilt) because of the sunlight coming from the Sun.

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• CASE 1: When the Earth is not tilted w.r.t. Sun: 

• The circle of illumination overlaps with the Earth's axis of rotation at the two poles as shown. On these days(when Earth is not tilted) every place on the Earth remains for 12 hours on the dayside of the Earth and the remaining 12 hours on the night side of the Earth.

• CASE 2: When Earth is tilted (w.r.t. Sun): 

• The places in the hemisphere tilted towards the Sun., for eg., Northern Hemisphere in June, will experience longer daylight and shorter night. While places in the Southern Hemisphere will experience longer nights and shorter days.

• CASE 3: Revolution with 23.5 tilt (found in reality) with normal rotation:

• The outcome will be that the Hemisphere which receives more amount of sunlight, more duration of sunlight, and closer to vertical sunlight would be overall heated much more and hence would experience hotter season or summer season.

• Whereas the Hemisphere that is tilted away from the sun, the Southern Hemisphere receives a lesser amount of sunlight, lesser duration of the sunlight, and more inclined sun rays falling on the surface. Because of this, this Hemisphere experiences cooler or winter climates.

• Hence, when we see the Earth's revolution diagram we find that during the June Solstice the Northern Hemisphere experiences Summer, where the Southern Hemisphere experiences Winter.

 

• In the month of December solstice, when Northern Hemisphere is tilted maximum away from the sun and Southern Hemisphere is tilted maximum towards the sun, the Northern Hemisphere experiences winter and Southern Hemisphere experiences summer, this is the summer solstice.

• During the September and March equinoxes, the Earth tilts w.r.t. The sun is zero and hence both the hemispheres during September as well March will experience neither winter nor Summer but moderate weather conditions in both the Hemispheres. 

• The outcomes will be:

1. Seasons in a different hemisphere

2. Duration of day and night also change throughout the year during different seasons

 

HOW WE DRAW LATITUDES

• When the axis of rotation touches the surface of the Earth on the North, we call it the North Pole and on the South, we call it the South Pole. 

• Both these poles are considered 90 degrees w.r.t. the center of the Earth. 

• Hence, zero degrees angle w.r.t. the center of the Earth drawn w.r.t poles as 90 degrees is known as 0-degree latitude where it touches the Earth surface, this way 0-degree latitude is drawn.

• When we make a line through all 0-degree latitude points on Earth we get a circle called a 0-degree latitude circle. It is the biggest horizontal circle on the Earth’s surface when Earth's axis of rotation is vertical. 

• This 0-degree latitude circle also equally divides the Earth into two half circles known as North Hemisphere and South Hemisphere. Because 0-degree latitude also divides Earth into equal parts it is also called the Equator.

• W.r.t. 0-degree latitude the other latitudes are drawn up to 90 degrees and we can see that all these latitude circles are parallel circles w.r.t each other on the Earth surface. On both sides of the equator, we can draw up to 90 degrees. 

• On the North of the equator, they are North latitudes and on the South of the equator, they are  South latitudes. 

• Every one-degree latitude on the Earth’s surface would be separated from others by approximately 111 km. But, because the Earth is flatter towards the pole, the distance between latitudes increases slightly towards the pole.

• 1 degree = 60' minutes, and 1 minute= 60'' seconds of angle

• So every 1 minute of Earth angle will be about 2 km and 1 second is approximately 30 meters. 

• This division is done for the accurate, better, and faster location of a place.

IMPORTANCE OF TROPICS AND POLAR CIRCLES 

• If Earth's axis of rotation was vertical then the equatorial circle would have been horizontal and afternoon vertical sunlight on Earth’s surface would have fallen on the equator. 

• The vertical sunlight has the highest intensity and maximum heating capacity. This is the case in reality on the equinox days when at noon the sun rays fall perpendicular over the equator region.

• On June Solstice day, the noon overhead vertical rays fall over the 23.5 degrees North latitude because the Earth is maximum tilted towards the Sun in June Solstice.

• After this towards September equinox, Earth's tilt becomes lesser and becomes zero in September Equinox. 

• Similarly, on December Solstice day the noon sun rays will fall perpendicular over the Earth surface at noon at the 23.5 degrees South latitude. I.e. December Solstice.

• After this Earth's tilt will again start becoming less and on March Equinox there will be no tilt w.r.t. the Sun.

• Hence, as we see that throughout the year the Sun's vertical overhead position at noon moves between 23.5 degrees North to 23.5 degrees South like a sine wave returning back each time from the 23.5 degrees latitude. 

• This point of return is known as Tropic in Greek and the Sun returns from the Northern Tropic when Zodiac is Cancer hence known as Tropic of Cancer. And the Sun returns from the Southern Tropic in December the Zodiac is Capricon hence, the Southern Tropic is known as the Tropic of Capricorn.

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HOW DURATION OF DAY AND NIGHT CHANGES- WITH REVOLUTION OF EARTH

• Tilt of the Earth w.r.t. the sun keeps changing all the time as the Earth revolves around the sun ranging between 23.5 degrees to zero degrees between the Solstice and Equinox days respectively. When the Earth is not tilted w.r.t. the Sun, all the places on the Earth experience 12 hour day and 12 hour night.

• But, when the Earth is tilted w.r.t. The Sun duration of day and night changes(except on the Equator where day and night are equal and are of 12 hours) in such a manner that days are longer in the summer hemisphere and nights are longer in winter Hemisphere.

• If we see latitude wise, we see the equator gets a 12 hour day and 12 hour night. Whereas 23.5-degree latitude in the Summer Hemisphere gets a more than 13 hour day and less than 11 hour night. Similarly, 40-degree latitude gets a nearly 15 hour day.

• Further, when we see the latitude which is completely in the day Hemisphere ( never going to the night side) during rotation.

• Hence, by this logic, the higher latitudes in the Summer Hemisphere have longer daylight. The 66.5 degree in the Summer Hemisphere remains for 24 hours on the daylight side, hence experiencing a 24 hour day.

• Any latitude higher than 66.5-degrees latitude in the summer Hemisphere will similarly experience 24-hour daylight. Hence, this is the importance of 66.5-degree latitude.

• Similarly, when we see the winter Hemisphere, the latitude from where a 24 hour night starts to be experienced is 66.5-degree latitude in that Hemisphere and any higher latitudes in that Hemisphere will also experience a 24 hour night.

• Hence, we can conclude that 66.5-degree latitude is where we start to experience 24-hour daylight or 24 hour night at least on one day of the year.

• As we go to higher latitudes the 24 hour day or night phenomenon starts to be experienced for more than one day. This trend gradually increases towards the pole(90-degree latitude) in such a way that at the pole we experience a 6 month 24 hour day and 6 months 24 hour night.

• Hence, a 24 hour day or 24 hour night is a phenomenon we associate normally with the pole (90 degrees North or 90 degrees South) because this 24 hour day/24-night phenomenon is experienced from up to 66.5-degree latitudes and in between poles, hence 66.5-degree latitude is also known as Polar circle.

IMPACT OF REVOLUTION ON TILTED AXIS

• Outcomes

• Changing seasons

• Changing duration of day and night

• 6 month day 6 month night

ROTATION OF EARTH

• All the planets in the solar system tend to rotate, all at different speeds. Venus is slowest all others are relatively fast.

TYPES OF ROTATION OF THE EARTH

• ABSOLUTE ROTATION- absolute rotation of the Earth which is known as 'sidereal day'(or day w.r.t. other stars) is the rotation of the Earth when the revolution effect of the earth is not considered. The Earth takes 23 hours 56 minutes and 4 seconds to do that rotation.

• At times we also say it w.r.t. other stars because w.r.t. other stars the Earth not revolving.

• ROTATION W.R.T. SUN  (considering the effect of revolution): When we consider rotation and revolution the Earth completes 360-degree rotation w.r.t. Sun in 24 hours.

MEASUREMENT OF EARTH ROTATION( w.r.t. earth)

• For measuring the rotation of the Earth we use the longitudes drawn over the Earth Surface from pole to pole as the reference lines. Because the longitudes start from the poles and go to the other poles, one such line or longitude which passes through the Royal Astronomy Laboratory, Greenwich, London has been taken as 0-degree longitude reference, also known as Prime Meridian. As we can see that around the Earth we can draw 360-degree longitudes, separated from each other by one degree.

• So, there are 360 longitudes on Earth, the Prime Meridian is considered 0 Degrees all other longitudes are measured East or West of the Prime Meridian or 0 degrees and measured up to 180 degrees on the other side. Hence, we have marked up to 179 degrees East longitudes and 179 degrees west longitudes respectively.

• The 180-degree longitude is neither called East nor West rather it is known as International Date Line( will be discussed later)

• Unlike the latitudes, all the longitudes have the same length and all of them converge at poles. Hence, the distance between two longitudes is not constant and keeps reducing as we move from equator to pole.

• Distance between 1-degree longitude at equator is approximately 111km which keeps on reducing till convergence at poles.

• We use longitudes for understanding the effect of rotation like time etc. Longitudes are divided into degrees, minutes, seconds.

EFFECT OF THE LONGITUDES

• Earth effectively rotates in 24 hours w.r.t. the sun which implies that for 360-degree longitude rotation the Earth takes 24x60 minutes, this implies for 1-degree rotation Earth takes 4 minutes. In 4 minutes it does 1-degree rotation, so, in 1 hour it does 15-degree rotation. Which simply means 1 hour = 15-degree rotation as the relation.

• The global time is measured w.r.t. time at Prime Meridian which is 0-degree longitude. The eastern longitudes up to 179 degrees are ahead in time from the Prime Meridian. The Western  longitudes up to the International date line will be lagging in time. Every 1 degree longitude differs in time by 4 minutes.

• Hence, for e.g. If the time at Prime Meridian is 12 noon. The time at 1 degree East will be 4 minutes ahead i.e. 12.04 pm, time at 15 degrees East longitude will be 1 hour ahead i.e. 10 pm, 150 degree East will be 10 hours ahead i.e. 10 pm, at 179 degree east time will be 11. 56 pm of night.

• Hence, the longitude 179 degrees East here, is just a few minutes away from the next date. For e.g., If at Prime Meridian it is 12 noon on 22 Feb then at 179 degrees East will be just 4 minutes away from the next day i.e. 23 Feb.

• At 12 noon of 22nd Feb at Prime Meridian the time and date at different longitudes will be 1 degree West 11.56 am of 22nd Feb, 15 degrees West 1 hour behind 11 am, 150 degrees West will be 10 hours behind, so, 2 am of 22nd Feb and 179 degrees West it will lag by 11 hours 56 minutes, so time is 12.04 am and date is 22nd Feb. This means that 179 degrees W has just a few minutes earlier has just entered the date of 22nd Feb.

• Hence, we move from East longitude to West longitude across the International Date Line we gain a day. Whereas when we go from Western longitude to Eastern longitude on the International Date Line we lose a day.

• If in a country having wide longitudinal spread, different time is kept for different longitude regions then it will be administratively very difficult to manage. Hence, to overcome such administrative difficulties countries try to manage with as few official time zones as possible.

• India takes the time of 82degree 30' time zone as our Standard Time, which is almost mid-way between Westernmost and Easternmost longitudes, which is 5 hours 30 minutes ahead of Greenwich Mean Time(GMT).

• But, if the country has very vast longitudinal expanse like for eg. Russia has longitudinal expanse of nearly 180-degree longitudes, which means 12 hours difference between its Eastern most and Western most location

• Even such countries keep the middle time as the national time yet the Easternmost and Westernmost locations will lead and lag by 6 hours respectively. Practically this will mean if the whole country has 6 am as its time, and in reality it is sunrise in the middle of the country then the eastern place has seen sunrise 6 hours earlier and the westernmost place will see sunrise 6 hours later. So, this gives a confusing picture of time.

• So, for administrative convenience such countries divide into different time zones for e.g. Russia in between its Western most location and Eastern most location is divided into 11 time zones, each experiencing their local standard longitude time as their Official Time. So, there is no confusion between time and day, evening morning experiences.

• Similarly, the US has 9 time zones across its islands and mainland between the Atlantic and Pacific. When other Islands of the US are considered they have a total 11 time zones. Maximum time zones are of France not because of the main country which has one time zone, but because of the French Islands across the world.

• In India also, some challenges are experienced especially in the North-East states because morning happens earlier but it is still not official time to open and sunsets earlier even if it Is not official time to close work. Hence, there have been demands for dividing the country into two time zones, where North East time will be one hour ahead. But, the government has not agreed due to practical challenges. But, to overcome such practical problems unofficially at many places, especially in tea gardens, workers keep their clocks ahead of one hour, so they don’t lose one hour of morning light.

DAYLIGHT SAVING TIME

• It is a practical adjustment also done officially in the higher latitude countries so as to better utilize evening light during their working hours, this concept is applied by putting the clock one hour ahead during spring, so the Sun will set one hour ahead as per clock. So, this way they gain one additional hour of sunlight in the evening.

• This also causes the Sun to rise one hour later in the morning as per clock. They put their clock back during winter to be back to normal, but this is not a very convenient system and countries don't have universal following for this.

CONCEPT OF ECLIPSES ON EARTH

• ECLIPSE ON EARTH: Meaning of eclipse is that shadow of Earth or Moon is following on the other. And is creating Lunar or Solar Eclipse respectively.

• Because, the Moon revolves around the Earth, w.r.t. the Earth almost daily, hence, we might feel that shadow of the one might follow on the other frequently. Here, it should be noted that the Moon takes around 4 weeks to complete one revolution around the Earth. The moon also takes the same time to do Rotation around its axis. Hence, normally, twice the moon and Earth are on the same line with the Sun.

• But, because the Moon's revolution orbit is tilted w.r.t. Earth's revolution orbit, hence, even when the Moon is between the Earth and the Sun, still the shadow of the Moon doesn't fall on the Earth.

• Even in the second position of the moon the Earth's shadow doesn’t fall on the moon, hence, we don't get an eclipse.

• The eclipse happens only when the Earth's revolution plane and Moon's revolution plane intersect and they fall exactly in the same line. So, that shadow of Earth or Moon falls on the other.

• Solar Eclipse: When the shadow of the Moon falls on the Earth, we can't see the Sun3 from that location and it is known as Solar Eclipse for that location.

• Lunar Eclipse: When the shadow of the Earth falls on the Moon, the Moon is not visible and it is called Lunar Eclipse.