The Universe Handout
The Universe Handout
This is an important topic, on an average, a question appears in CSE prelims, every year.
Let us discuss the following concepts one by one.
1. Stars 2. Solar system 3. The Moon 4. The Earth 5. Longitudes and Latitudes 6. Motions of the Earth 7. The Universe 8. Current updates
1. Stars
The sun, the moon and all objects shining in the night sky are known as Celestial bodies
Some celestial bodies called stars are very big and hot. They are made up of gases. They have their own heat and light, which they emit in large amounts. The sun is a star.
Countless twinkling stars in the night sky are similar to the sun. Since they are very far from us, we do not feel their heat or light, and they look so tiny.
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Source of energy of the stars :
The source of energy of the stars is due to Nuclear fusion (conversion of hydrogen to helium)
Twinkling of Stars:
Stars look like twinkling, when observed from the earth, due to the atmosphere on Earth.
Different layers of atmosphere has different temperatures, hence different densities, So the light ray coming from the star, undergoes Refraction (bending of light ray when it enters from one medium to another medium of different density)
Colour of Stars:
The colour of a star depends on its surface temperature.
Some stars looks dull and red, while others are white and others look bright blue. (More temp – towards Blue, less temp – towards Red) 2 www.eduscreen.in www.aasaanias.com
Colour of the Sun:
The light of the Sun would actually look very white from space. (Our Sun’s surface temperature is about 6,000 Kelvin).
When we see the Sun at sunrise or sunset, when it is low in the sky, it may appear yellow, orange, or red. But that is only because its short-wavelength colors (green, blue, violet) are scattered out by the Earth's atmosphere Hence only the reds, yellows, and oranges get through the thick atmosphere to our eyes.
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Why the Colour of the Sky looks Blue?
The colour of sky looks blue when observed from the Earth, it is due to atmosphere on earth. (When the Sun is high in the sky, the shorter waves, primarily the blue, strike air molecules in the upper atmosphere and bounce around and scatter. Hence the sky looks blue).
If there is no atmosphere on Earth, or if observed from the space, or from the moon, the sky looks dark (Black).
The Sun and the Earth Comparison:
The ratio of the Sun's diameter to the Earth's diameter is 1,392,000/12756 = 109.1
This means the ratio of their volumes is 109.1 x 109.1 x 109.1, which is about 1,300,000,and that means that 1,300,000 Earths should fit inside the Sun.
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Imp. Units to measure distances of astronomical bodies:
Astronomical unit (A.U):
The distance from Earth to the sun is called an astronomical unit, or AU, which is used to measure distances throughout the solar system.
1 AU = 149,597,870,700 meters (app. 150 mn km)
The Sun is the closest star to Earth, about 150 million kms away.
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Light year:
A light year is a measure of distance and not of time. Light travels at a speed of 300,000 km/second. Considering this, the distances the light will travel in one year is taken to be one light year. This equals to 9.461×1012 km.
The mean distance between the sun and the earth is 149,598,000 km. In terms of light years, it is 8.311minutes.
One light-year (ly) is 63,240 A.U
Parsec (Pc):
A parsec is the length of the long leg of a right triangle, whose short leg is one astronomical unit when the angle between the Sun and the Earth, as seen from an object in space (a star for example), is one arcsecond
The word parsec stands for "parallax of one arc second" one parsec (pc) is equal to 3.26 light years.
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Imp. Stars:
Closest stars:
Proxima Centauri claims the honor of being our true nearest neighbor (after Sun) at only 4.24 light years away.
Alpha Centauri is a triple star system (Three stars bound together by gravity) Alpha Centauri A, Alpha Centauri B and Proxima Centauri.
Pole star (Polaris):
Polaris (the North Star or Pole Star) is the brightest star in the constellation Ursa Minor, and the 50th brightest star in the night sky.
It is very close to the north celestial pole, making it the current northern pole star.
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Many recent papers calculate the distance to Polaris at about 434 light-years.
The south celestial pole lacks a bright star like Polaris to mark its position. At present, the naked-eye star nearest to this imaginary point is the faint Sigma Octantis, which is sometimes known as the South Star.
Constellation of stars:
Various patterns formed by different groups of stars are known as Constellations.
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Some important constellations are Ursa major (also known as Big Bear/ Big Dipper/Plough), Saptarishi, Ursa minor (also known as Little Bear / Little Dipper) etc.
Dubhe and Merak are important stars of Ursa major constellation
Ursa minor is the most northern constellation. Polaris (North Pole star) is a part of Ursa minor and infact, it is the brightest star in the constellation.
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Zodiac:
Zodiac is the Imaginary region that encompass the path of the Sun and Planets. the zodiac is divided into twelve signs, each occupying 30° of celestial longitude and roughly corresponding to the constellations (Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius and Pisces)
Neutron stars:
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Neutron stars are city-size stellar objects with a mass about 1.4 to 3 times that of the sun. Born from the explosive death of another larger stars, these tiny objects pack quite a punch.
Neutron stars pack their mass inside a 20-kilometer diameter. They are so dense that a single teaspoon would weigh a billion tons
When stars four to eight times as massive as the sun explode in a violent supernova, their outer layers can blow off in an often-spectacular display, leaving behind a small, dense core that continues to collapse. Gravity presses the material in on itself so tightly that protons and electrons combine to make neutrons, yielding the name "neutron star
Types of neutron stars:
Magnetar – Neutron star with powerful magnetic field:
Magnetars have magnetic fields a thousand times stronger than the average neutron star. The resulting drag causes the star to take longer to rotate
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PULSAR – A flashing Neutron star:
Some neutron stars have jets of materials streaming out of them at nearly the speed of light. As these beams pan past Earth, they flash like the bulb of a lighthouse. This pulsing appearance led them to be called pulsars.
2. Solar system
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Our solar system consists of the sun, eight planets, moons, many dwarf planets (or plutoids), an asteroid belt, comets, meteors, and others. The sun is the center of our solar system. the planets, their moons, a belt of asteroids, comets, orbit the sun.
Planets:
Planets do not have their own heat and light. They are lit by the light of the stars.
If we look at the Earth from a great distance, say the moon, it will appear to be shining just as the moon.
(The word ‘planet’ comes from the Greek word “Planetai” which means ‘wanderers’).
The eight planets that orbit the sun are (in order from the Sun) are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
Another large body is Pluto, now classified as a dwarf planet or plutoid.
All the eight planets of the solar system move around the sun in fixed paths called orbits. These paths are elongated.
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(Easy ways to remember the order of the planets (plus Pluto) are the mnemonics: "My Very Excellent Mother Just Served Us Nuts")
A belt of asteroids (minor planets made of rock and metal) lies between Mars and Jupiter.
Inner Planets or Terrestrial planets:
The first four planets, i.e. Mercury, Venus, Earth, and Mars are known as the inner planets
These planets are, in terms of composition, "Earth-like. (that is composed primarily of silicate rocks or metals). So they are also known as terrestrial planets, telluric planets, or rocky planets
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Giant Planets:
A giant planet is any massive planet. They are usually primarily composed of low-boiling-point materials (gases or ices), rather than rock or other solid matter, but massive solid planets can also exist. There are four known giant planets in the Solar System: Jupiter, Saturn, Uranus and Neptune.
planets such as Jupiter and Saturn are called the gas giants, hydrogen and helium constitute most of the mass of the planet, whereas they only make up an outer envelope on Uranus and Neptune, which are instead mostly composed of water, ammonia, and methane and therefore increasingly referred to as "ice giants".
Ice giants consist of only about 20% hydrogen and helium in mass, as opposed to the Solar System's gas giants (Jupiter and Saturn), which are both more than 90% hydrogen and helium in mass
Size of the Planets:
The largest planet is Jupiter. Jupiter is so big that all the other planets could fit inside it.
It is followed by Saturn, Uranus, Neptune, Earth, Venus, Mars, Mercury (smallest).
Pluto is the largest of the dwarf planets.
Earth is the fifth largest planet in our solar system (Largest among inner planets)
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The temperature of the Planets:
Generally, the farther from the Sun, the cooler the planet. Differences occur when the greenhouse effect warms a planet (like Venus) surrounded by a thick atmosphere.
Venus is the hottest and Uranus is the coldest.
Uranus seems to have been knocked on its side, possibly by a giant impact long ago, which has caused the heat from within its core to spill out into space.
Density of the Planets:
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The outer, gaseous planets are much less dense than the inner, rocky planets.
The Earth is the densest planet. Saturn is the least dense planet; it would float on water.
Mass of the Planets:
Jupiter is by far the most massive planet; Saturn trails it. Neptune, Uranus, Earth, Venus, Mars, and Mercury are orders of magnitude less massive
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Gravity on the Planets:
The planet with the strongest gravitational attraction at its surface is Jupiter.
(Although Saturn, Uranus, and Neptune are also very massive planets, their gravitational forces are about the same as Earth. This is because the gravitational force a planet exerts upon an object at the planet's surface is proportional to its mass and to the inverse of the planet's radius squared).
Out of the 8 planets, Mercury has the lowest gravitational force
The Earth has fourth strongest gravitational force among the planets of our solar system and strongest among inner planets.
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Duration of a day and an year:
A day is the length of time that it takes a planet to rotate on its axis (360°). A day on Earth takes almost 24 hours.
The planet with the longest day is Venus; a day on Venus takes 243 Earth days. (A day on Venus is longer than its year; a year on Venus takes only 224.7 Earth days).
The planet with the shortest day is Jupiter; a day on Jupiter only takes 9.8 Earth hours
The farther the planet from the sun, the longer the duration of an year.
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Number of Moons:
Jupiter has the highest number of moons (67), followed by Saturn (62). Mercury and Venus do not have any moons. We all know that the Earth has one moon, where as mars has two moons.
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Albedo:
Nearly 30% of the solar radiation return back to the space without contributing anything to the earth’s surface temperature. This amount of radiation is known as earth’s albedo.
Venus has the highest albedo of any major planet in our solar system. Its albedo is close to .7, meaning it reflects about 70 percent of the sunlight striking it.
Mercury has the lowest albedo. The planetary albedo of Mercury is just 6 percent.
Insolation
Insolation is the solar radiation that reaches the earth's surface. It is measured by the amount of solar energy received per unit area (say, square centimeter) per unit time (say, minute).
Insolation affects temperature. The more the insolation, the higher the temperature.
Factors affect insolation are Angle of the sun, Distance between the sun and the earth
Duration of daylight etc. (the strongest insolation is received when the sun is vertically over head).
The longer the duration of daylight, the more the insolation received per day.
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Colour of the planets:
Mercury: Gray (or slightly brownish). Mercury has practically no atmosphere, so we just see the rocky surface.
Venus: Pale yellow. It is due to the thick atmosphere (We can only see the thick layer of featureless sulfuric acid clouds).
Earth: Mostly blue with white clouds. Oceans and light scattered by the atmosphere make Earth prevailingly blue.
Mars: Mostly reddish brown, though with some darker regions, and also white ice caps. The dominant reddish color comes from rusty rocks on the surface, since the clouds are rare and thin.
Jupitar: Orange and white bands. The white bands are colored by ammonia clouds, while the orange comes from ammonium hydrosulfide clouds. None of the four "gas giant" planets (Jupiter, Saturn, Uranus, and Neptune) has a solid surface, so all we see are just clouds in their atmospheres.
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Saturn: Pale yellow. White ammonia haze covers the whole planet and partially obscures redder clouds below. Clouds in Saturn's winter hemisphere are pale blue. Scientists think that because the rings are blocking the Sun in the winter hemisphere, things are colder there and the ammonia clouds are lower down than normal. This gives the rest of the atmosphere more of a chance to scatter light, just like the Earth's atmosphere does.
Uranus: Pale blue. The color comes from methane clouds (which absorb red end of spectrum and reflect blue)..
Neptune: Pale blue. As in the case of Uranus, the color is due to methane. Neptune would appear darker than Uranus due to dimmer illumination (greater distance from the Sun).
Venus – Earth’s twin planet:
Venus is considered as ‘Earth’s-twin’ because, they are almost the same size, have about the same mass (they weigh about the same), and have a very similar composition (are made of the same material).
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Venus is the brightest object in the sky after the sun.
Venus is called both the morning star as well as evening star
During the venus transit, the planet appeared as a tiny black circle moving on the sun. This black colour on the sun is because the planet obstructs all light from sun.
Venus rotates in the opposite direction of Earth (Retrograde rotation)
The planet with the longest day is Venus (243 Earth days).
A day on Venus is longer than its year (a year on Venus takes only 224.7 Earth days).
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Asteroids:
Asteroids are small rocky bodies orbiting the Sun. They are also called as planetoids.
Individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups: C-type, M-type, and S-type. These were named after and are generally identified with carbon-rich, metallic, and silicate (stony) compositions, respectively.
The large majority of known asteroids orbit in the asteroid belt between the orbits of Mars and Jupiter, or are co-orbital with Jupiter (the Jupiter Trojans).
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(Trojans are populations that share an orbit with a larger planet or moon. The most significant population of trojans are the Jupiter trojans)
Imp. Asteroids:
Ceres Largest
Vesta 2nd largest (Brightest)
Pallas 3rd largest
Although their location in the asteroid belt excludes them from planet status, the three largest objects, Ceres, Vesta, and Pallas, are intact proto-planets that share many characteristics common to planets.
Vesta is the brightest asteroid. It has a relatively reflective surface, So it is normally visible to the naked eye (only in very dark skies when it is favorably positioned).
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Comets:
A comet is an icy body (frozen gases held together by rocky and metallic material)
They are often compared to dirty snowballs, that releases gas or dust
Comets contain dust, ice, carbon dioxide, ammonia, methane and more.
Comets revolve around the sun in elliptical orbits
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The streams of dust and gas thus released form a huge and extremely thin atmosphere around the comet called the "coma", and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous "tail" to form pointing away from the Sun.
A comet develops a tail when it gets close to the sun.
Approximately once a decade, a comet becomes bright enough to be noticed by a casual observer, leading such comets to be designated as great comets
Halley's Comet is a "periodic" comet and returns to Earth's vicinity about every 75 years, making it possible for a human to see it twice in his or her lifetime.
The last time it was here was in 1986, and it is projected to return in 2061.
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Meteoroid:
A meteoroid is a small rocky or metallic body travelling through space. Meteoroids are significantly smaller than asteroids, and range in size from small grains to 1 meter-wide objects. Objects smaller than this are classified as micrometeoroids or space dust. Most are fragments from comets or asteroids, whereas others are collision impact debris ejected from bodies such as the Moon or Mars.
Metors are those which have entered earths atmosphere.
If any part of a meteoroid survives the fall through the atmosphere and lands on Earth, it is called a meteorite
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4. The Moon
The moon that we see in the sky is Earth’s natural satellite. It is a companion of our earth and moves round it.
Stars twinkle (as they have their own source of energy), where as the moon just shines.
The diameter of moon is just more than 1/4th that of the earth. (The diameter of the Earth is around 3.7 times that of moon, hence the Earth is approx. 50 times larger to the moon)
It is about 3,84,400 km away from us. It appears so big because it is nearer to our planet than other celestial bodies.
The moon does not have conditions favourable for life. It has neither water nor air. It has mountains plains and depressions on its surface
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Gravity:
The gravity on Moon is 1/6th to that on Earth. So the weight of a person on moon will be only 1/6th of that on Earth
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Phases of Moon:
The phases of the Moon are the different ways the Moon looks from Earth. (Moon appear at different times, in different shapes and at different positions).
As the Moon orbits around the Earth, the half of the Moon that faces the Sun will be lit up. The different shapes of the lit portion of the Moon that can be seen from Earth are known as phases of the Moon. Each phase repeats itself every 29.5 days. This duration is known as Synodic or Lunar month.
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Sideral and Synodic months:
The Moon's sidereal orbital period (the sidereal month) is app. 27.3 days; this is the time interval that the Moon takes to orbit 360° around the Earth relative to the "fixed" stars.
However, because the Earth is constantly moving along its orbit about the Sun, the Moon must travel slightly more than 360° to get from one new moon to the next. Thus, the synodic month, or lunar month, is longer than the sidereal month. A sidereal month lasts 27.322 days, while a synodic month lasts 29.531 days
Tidal locking:
Moon takes same time (27.32 days) for revolution around the earth and rotation on its own axis. So we see the same face of the moon. This is known as Tidal locking (or captured rotation) or synchronous rotation.
This is the case between Pluto and Charon also.
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Eclipse:
An eclipse happens when a planet or a moon gets in the way of the sun’s light. Here on Earth, we can experience two kinds of eclipses: solar eclipses and lunar eclipses
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Solar Eclipse:
A solar eclipse happens when the moon gets in the way of the sun’s light and casts its shadow on Earth. That means during the day, the moon moves over the sun and it gets dark.
This total eclipse happens about every year and a half somewhere on Earth. A partial eclipse, when the moon doesn’t completely cover the sun, happens at least twice a year somewhere on Earth.
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But not everyone experiences every solar eclipse. Getting a chance to see a total solar eclipse is rare. The moon’s shadow on Earth isn’t very big, so only a small portion of places on Earth will see it. You have to be on the sunny side of the planet when it happens. You also have to be in the path of the moon’s shadow.
On average, the same spot on Earth only gets to see a solar eclipse for a few minutes about every 375 years!
Lunar Eclipse:
During a lunar eclipse, Earth gets in the way of the sun’s light hitting the moon. That means that during the night, a full moon fades away as Earth’s shadow covers it up.
The moon can also look reddish because Earth’s atmosphere absorbs the other colors while it bends some sunlight toward the moon. (Sunlight bending through the atmosphere and absorbing other colors is also why sunsets are orange and red).
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Why don’t we have a lunar eclipse every month?
You might be wondering why we don’t have a lunar eclipse every month as the moon orbits Earth. It’s true that the moon goes around Earth every month, but it doesn’t always get in Earth’s shadow. The moon’s path around Earth is tilted compared to Earth’s orbit around the sun. The moon can be behind Earth but still get hit by light from the sun. Unlike solar eclipses, lots of people get to see each lunar eclipse. If you live on the nighttime half of Earth when the eclipse happens, you’ll be able to see it.
Blue moon phenomenon:
The blue moon phenomenon occurs when two full moons occur in the same month
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Super Moon:
A supermoon is the coincidence of a full moon with the closest approach the Moon makes to the Earth on its elliptical orbit, resulting in the largest apparent size of the lunar disk as seen from Earth.
The technical name is the perigee-syzygy
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The Earth:
The earth is called a blue planet. (From the outer space, the earth appears blue because about 71% of its surface is covered by water).
The earth is slightly flattened at the poles. Its shape is described as a Geoid. (Geoid means an earth-like shape)
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Conditions favourable to support life are probably found only on the earth. The earth is neither too hot nor too cold. It has water and air. The air has life-supporting gases like oxygen. Because of these reasons, the earth is a unique planet in the solar system.
Goldilocks Zone:
The Goldilocks Zone refers to the habitable zone around a star where the temperature is just right (not too hot and not too cold) for liquid water to exist on an planet.
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Liquid water is essential for life as we know it. Where we find liquid water on Earth we also find life.
(Here the assumption is that if it's possible there may be liquid water on the planet, then it's also possible that the planet may be habitable)
Gravity on Earth:
Near Earth's surface, gravitational acceleration is approx. 9.8 m/s2
It is more at poles (app. 9.83 m/s2 and less at equator (app. 9.78 m/s2)
Gravity decreases with altitude. (Gravity is inversely proportional to the square of distance)
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Axis of Earth:
An imaginary line passing through the north and south pole is known as Axis
The axis of the earth makes an angle of 66½° with its orbital plane (the plane formed by the orbit). This is called the axial tilt
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Axial tilt:
The axis of the earth makes an angle of 66½° with its orbital plane (the plane formed by the orbit)
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Geographic poles & Magnetic poles:
The geographic north and south poles indicate the points where the earth's rotation axis intercepts earth's surface
Earth's magnetic field is caused by circulating currents of liquid iron in the outer core.
Earth's magnetic poles are constantly changing location relative to earth's geographic poles. Currently tilted at an angle of about 11 degrees with respect to Earth's rotational axis
5. Longitudes and Latitudes
Latitudes (Parallels):
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Latitude is the angular distance of a point on the earth’s surface, measured in degrees from the center of the earth.
The equator represents the 0o latitude. It divides the globe into two equal halves.
The northern half of the earth is known as the Northern Hemisphere and the southern half is known as the Southern Hemisphere
all parallels north of the equator are called ‘north latitudes.’ Similarly all parallels south of the equator are called ‘south latitudes.’
90o north latitude marks the North Pole and 900 south latitude marks the South Pole.
The distance between two latitudes of 10 difference is 69 miles (111kms)
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Important parallels of latitudes:
Tropic of Cancer (23½° N) in the Northern Hemisphere.
Tropic of Capricorn (23½° S) in the Southern Hemisphere.
Arctic Circle at 66½° north of the equator.
Antarctic Circle at 66½° south of the equator.
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Heat zones of the Earth:
The mid-day sun is exactly overhead at least once a year on all latitudes in between the Tropic of Cancer and the Tropic of Capricorn. This area, therefore, receives the maximum heat and is called the Torrid Zone.
The mid-day sun never shines overhead on any latitude beyond the Tropic of Cancer and the Tropic of Capricorn.
The angle of the sun’s rays goes on decreasing towards the poles.
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Longitudes (Meredians):
Longitude is the angular distance measured in degrees along the equator east or west of the prime meridian.
Since degrees of longitude are farthest apart at the equator and converge at the poles, their distance varies greatly.
A degree of longitude is widest at the equator at 69.172 miles (111.321) and gradually shrinks to zero at the poles
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Greenwich Line (Prime Meredian):
Unlike parallels of latitude, all meridians are of equal length. Thus, it was difficult to number the meridians. Hence, all countries decided that the count should begin from the meridian which passed through Greenwich, where the British Royal Observatory is located. This meridian is called the Prime Meridian.
Its value is 0° longitude and from it we count 180° eastward as well as 180° westward. The Prime Meridian and 180° meridian divide the earth into two equal halves, the Eastern Hemisphere and the Western Hemisphere.
This meridian passes through the continents of Europe and Africa.
International Date line:
The International Date Line (IDL) is an imaginary line on Earth's surface defining the boundary between one day and the next.
The International Date Line is located halfway around the world from the prime meridian (0° longitude) or about 180° east (or west) of Greenwich. It is also known as the line of demarcation.
The dateline runs from the North Pole to the South Pole and marks the divide between the Western and Eastern Hemisphere. It is not straight but zigzags to avoid political and country borders and to not cut some countries in half.
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Gaining & Losing a day:
Moving from West to east – Gains a day
Moving from East to West – Loses a day
When you cross the International Date Line from west to east, you subtract a day, and if you cross the line from east to west, you add a day.
Japan is known as the Sunrise Country
Newzealand has the Earliest clock
There is no Sunset country 50 www.eduscreen.in www.aasaanias.com
Longitude Vs Time:
10 longitude = 4 mins time (East – Ahead and West – Behind)
(Earth takes 24 hours for one retation. So 3600 in 24 hours, it means 10 in 4 minutes.
Countries with many time zones:
France has 12 Time Zones (highest)
US has 11 time zones
Russia has 11 time zones
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Indian Standard Meredian:
82.5o E longitude is considered as Indian Standard Meredian (Average of 68 & 97)
ISM passes through five states: UP, MP, Chattisgarh, Odissa and AP
6. Motions of the Earth
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The Earth has two types of motions, rotation and revolution
Rotation:
Rotation is the movement of the earth on its axis.
Earth rotates eastward, in prograde motion. As viewed from the north pole star Polaris, Earth turns counter clockwise.
Earth rotates once in about 24 hours with respect to the Sun (The period of rotation is known as the earthday). But with respect to the stars, earth rotates once every 23 hours, 56 minutes, and 4 seconds
Earth's rotation is slowing slightly with time; thus, a day was shorter in the past. This is due to the tidal effects the Moon has on Earth's rotation. Atomic clocks show that a modern day is longer by about 1.7 milliseconds than a century ago.
The day and night are due to the rotation of the Earth. (Due to the spherical shape of the earth, only half of it gets light from the sun at a time. The portion facing the sun experiences day while the other half away from the sun experiences night). The circle that divides the day from night on the globe is known as Circle of illumination
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Revolution:
The movement of the earth around the sun in a fixed path or orbit is called Revolution the earth revolves around the sun in an elliptical orbit.
It takes 365¼ days (Exactly 365.2425 days) to revolve around the sun. We consider a year as consisting of 365 days only and ignore six hours for the sake of convenience
The variations in the duration of day and night in various seasons is due to the revolution of erarth around the sun on a tilted axis.
Summer 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. The areas near the poles receive less heat as the rays of the sun are slanting. The North Pole is inclined
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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.
Winter 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 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
Orbital variations:
The three types of orbital variations are variations in Earth's eccentricity, changes in the tilt angle of Earth's axis of rotation, and precession of Earth's axis. Combined together, these produce Milankovitch cycles.
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Orbital shape (eccentricity):
The Earth's orbit is an ellipse. The eccentricity is a measure of the departure of this ellipse from circularity. The shape of the Earth's orbit varies in time between nearly circular (low eccentricity of 0.000055) and mildly elliptical (high eccentricity of 0.0679) with the mean eccentricity of 0.0019.
The major component of these variations occurs on a period of 413,000 years. The Earth's eccentricity varies primarily due to interactions with the gravitational fields of Jupiter and Saturn 56 www.eduscreen.in www.aasaanias.com
Axial tilt (obliquity):
The angle of the Earth's axial tilt (obliquity of the ecliptic) varies with respect to the plane of the Earth's orbit. These slow 2.4° obliquity variations are roughly periodic, taking approximately 41,000 years to shift between a tilt of 22.1° and 24.5° and back again
Currently the Earth is tilted at 23.44 degrees from its orbital plane, roughly halfway between its extreme values. The tilt is in the decreasing phase of its cycle, and will reach its minimum value around the year 11,800 CE ; the last maximum was reached in 8,700 BCE.
Axial precession:
Precession is the trend in the direction of the Earth's axis of rotation relative to the fixed stars, with a period of roughly 26,000 years. This gyroscopic motion is due to the tidal forces exerted by the Sun and the Moon on the solid Earth
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6. The Universe
The Universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The spatial size of the entire Universe is still unknown. our Sun is one of hundreds of billions of stars in a galaxy, we call the Milky Way, which is one of at least hundreds of billions of galaxies in the Universe. Many of the stars in our galaxy have planets. At the largest scale galaxies are distributed uniformly and the same in all directions, meaning that the Universe has neither an edge nor a center.
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Geocentric model of the Universe:
The earliest scientific models of the Universe were developed by ancient Greek and Indian philosophers and were geocentric (Earth at the center of the Universe).
Under the geocentric model, the Sun, Moon, stars, and planets all orbited Earth. The geocentric model served as the predominant description of the cosmos in many ancient civilizations, such as those of Aristotle and Ptolemy.
The astronomical predictions of Ptolemy's geocentric model were used to prepare astrological and astronomical charts for over 1500 years. The geocentric model held sway into the early modern age, but from the late 16th century onward, it was gradually superseded by the heliocentric model of Copernicus, Galileo, and Kepler.
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Heliocentric model of the Universe:
In Heliocentric model, the Earth and planets revolve around the Sun. Sun is at the center of the Solar System
The notion that the Earth revolves around the Sun had been proposed as early as the 3rd century BC by Aristarchus of Samos, but in the medieval world, Aristarchus's Heliocentrism attracted little attention
It was not until the 16th century that a mathematical model of a heliocentric system was presented, by Nicolaus Copernicus, leading to the Copernican Revolution. In the following century, Johannes Kepler introduced elliptical orbits, and Galileo Galilei presented supporting observations made using a telescope.
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Origin of the Universe:
The most popular argument regarding the origin of the universe is the Big Bang Theory.
Big Bang:
(i) In the beginning, all matter forming the universe existed in one place in the form of a “tiny ball” (singular atom) with an unimaginably small volume, infinite temperature and infinite density.
(ii) At the Big Bang the “tiny ball” exploded violently. This led to a huge expansion. the event of big bang took place 13.7 billion years before the present. The expansion continues even to the present day. As it grew, some energy was converted into matter.
There was particularly rapid expansion within fractions of a second after the bang. Thereafter, the expansion has slowed down. Within first three minutes from the Big Bang event, the first atom began to form.
(iii) Within 300,000 years from the Big Bang, temperature dropped to 4,500 K (Kelvin) and gave rise to atomic matter.
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Expansion of the Universe:
Big Bang theory is also called expanding universe hypothesis.
Edwin Hubble, in 1920, provided evidence that the universe is expanding. As time passes, galaxies move further and further apart.
Scientists believe that though the space between the galaxies is increasing, observations do not support the expansion of galaxies.
The expansion of universe means increase in space between the galaxies. An alternative to this was Hoyle’s concept of steady state. It considered the universe to be roughly the same at any point of time. However, with greater evidence becoming available about the expanding universe, scientific community at present favours argument of expanding universe. the scientists gave the following evidences for the continued expansion of universe are i) Detection of microwaves in space:
Since the expansion of space causes their wavelength to increase over time (wavelength is inversely proportional to energy) ii) Observation of redshirt phenomenon in space.
Redshift and blueshift describe how light shifts toward shorter or longer wavelengths as objects in space (such as stars or galaxies) move closer or farther away from us. The concept is key to charting the universe's expansion.
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(When an object moves away from us, the light is shifted to the red end of the spectrum, as its wavelengths get longer. If an object moves closer, the light moves to the blue end of the spectrum, as its wavelengths get shorter).
American astronomer Edwin Hubble was the first to describe the redshift phenomenon and tie it to an expanding universe. His observations, revealed in 1929, showed that nearly all galaxies he observed are moving away.
Big Crunch:
The Big Crunch is one possible scenario for the ultimate fate of the universe, in which the metric expansion of space eventually reverses and the universe recollapses, ultimately causing the cosmic scale factor to reach zero or causing a reformation of the universe starting with another Big Bang.
The idea behind the theory is that the expansion of the universe is linked to the energy released in the Big Bang, therefore the outward speed of the matter would decrease over time due to gravity (mutual attraction). This would act as ballast and would eventually lead to a halt of the expansion. As matter attracts and there is no matter beyond the maximum expansion point, eventually all matter would begin to travel inwards again, accelerating as time passes.
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Formation of Galaxies and Stars:
The distribution of matter and energy was not even in the early universe. These initial density differences gave rise to differences in gravitational forces and it caused the matter to get drawn together. These formed the bases for development of galaxies.
A galaxy contains a large number of stars. Galaxies spread over vast distances that are measured in thousands of light-years. The diameters of individual galaxies range from 80,000- 150,000 light years.
A galaxy starts to form by accumulation of hydrogen gas in the form of a very large cloud called nebula. Eventually, growing nebula develops localised clumps of gas. These clumps continue to grow into even denser gaseous bodies, giving rise to formation of stars. The formation of stars is believed to have taken place some 5-6 billion years ago
Formation of Planets:
The stars are localised lumps of gas within a nebula. The gravitational force within the lumps leads to the formation of a core to the gas cloud and a huge rotating disc of gas and dust develops around the gas core.
In the next stage, the gas cloud starts getting condensed and the matter around the core develops into small rounded objects. These small-rounded objects by the process of cohesion develop into what is called planetesimals. Larger bodies start forming by collision, and
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gravitational attraction causes the material to stick together. Planetesimals are a large number of smaller bodies.
In the final stage, these large number of small planetesimals accrete to form a fewer large bodies in the form of planets.
Black hole:
Black holes are some of the strangest and most fascinating objects found in outer space. They are objects of extreme density, with such strong gravitational attraction that even light cannot escape from their grasp if it comes near enough.
Albert Einstein first predicted black holes in 1916 with his general theory of relativity. The term "black hole" was coined in 1967 by American astronomer John Wheeler, and the first one was discovered in 1971.
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Quasar:
Quasar or quasi-stellar object is a massive and extremely remote celestial object, emitting exceptionally large amounts of energy, which typically has a starlike image in a telescope. It has been suggested that quasars contain massive black holes and may represent a stage in the evolution of some galaxies.
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Blazars:
A blazar is a very compact quasar (quasi-stellar radio source) associated with a presumed supermassive black hole at the center of an active, giant elliptical galaxy. Blazars are among the most energetic phenomena in the universe.
Einstein Ring:
An Einstein ring, also known as an Einstein–Chwolson ring or Chwolson ring, is the deformation of the light from a source (such as a galaxy or star) into a ring through gravitational lensing of the source's light by an object with an extremely large mass (such as another galaxy or a black hole).
This occurs when the source, lens, and observer are all aligned — a syzygy. The first complete Einstein ring, designated B1938+666, was discovered by collaboration between astronomers at the University of Manchester and NASA's Hubble Space Telescope in 1998
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