- The moon is the only natural satellite of the earth.
- Like the origin of the earth, there were many concrete attempts to explain how the moon was formed.
- In 1838, Sir George Darwin suggested that initially, the earth and the moon formed a single rapidly rotating body.
- The whole mass became a dumb-bell-shaped body and eventually it broke.
- It was also suggested that the material forming the moon was separated from what we have at present the depression occupied by the Pacific Ocean.
- It is now generally believed that the formation of the moon, as a satellite of the earth, is an outcome of ‘giant impact’ called as “the big splat”.
- A body of the size of 1:3 times that of mars collided into the earth after the earth was formed. It blasted a large part of the earth into space.
- This portion of blasted material then continued to orbit the earth and eventually formed into the present moon about 4.44 billion years ago.
The Moon- Earth’s Natural Satellite
The Phases of the Moon
- As the Moon moves around the Earth, we see different parts of the near side of the Moon illuminated by the Sun.
- This causes the changes in the shape of the Moon that we notice on a regular basis, called the phases of the Moon.
- As the Moon revolves around Earth, the illuminated portion of the near side of the Moon will change from fully lit to completely dark and back again.
- A full moon is the lunar phase seen when the whole of the Moon’s lit side is facing Earth. This phase happens when Earth is between the Moon and the Sun.
- About one week later, the Moon enters the quarter-moon phase. At this point, the Moon appears as a half-circle, since only half of the Moon’s lit surface is visible from Earth.
- When the Moon moves between Earth and the Sun, the side facing Earth is completely dark. This is called the new moon phase, and we do not usually see the Moon at this point.
- Sometimes one can just barely make out the outline of the new moon in the sky. This is because some sunlight reflects off the Earth and hits the moon.
- Before and after the quarter-moon phases are the gibbous and crescent phases.
- During the gibbous moon phase, the moon is more than half lit but not full.
- During the crescent moon phase, the moon is less than half lit and is seen as only a sliver or crescent shape.
- It takes about 29.5 days for the Moon to revolve around Earth and go through all the phases.
Phases of The Moon
Try yourself:The phase where the Earth happens to be in between the Sun and the Moon is known as:
A full moon is the lunar phase seen when the whole of the Moon’s lit side is facing Earth. This phase happens when Earth is between the Moon and the Sun.
- A lunar eclipse occurs when the full moon moves through the shadow of the Earth.
- This can only happen when the Earth is between the Moon and the Sun and all three are lined up in the same plane, called the ecliptic.
- The ecliptic is the plane of Earth’s orbit around the Sun.
- The Earth’s shadow has two distinct parts: the dark umbra and the lighter penumbra.
- A total lunar eclipse occurs when the Moon travels completely in Earth’s umbra.
- A penumbral eclipse happens when the Moon passes through Earth’s penumbra.
- The Earth’s shadow is quite large, so a lunar eclipse lasts for hours and can be seen by anyone with a view of the Moon at the time of the eclipse.
- Partial lunar eclipses occur at least twice a year, but total lunar eclipses are less common. The moon glows with a dull red coloring during a total lunar eclipse.
- Asteroids are rocky remains left over from the formation of the solar system. Most asteroids orbit the sun in a girdle between Mars & Jupiter. Scientists believe there are possibly millions of asteroids, arraying extensively in size from hundreds of kilometers across to less than 1 kilometer broad.
- Asteroids' orbital paths are inclined by the gravitational haul of planets, which cause their paths to change. Scientists consider wandering asteroids or fragments from past collisions have knocked into Earth in the past, playing a key role in the development of our planet.
Try yourself:Most asteroids orbit the sun in a girdle between:
Most asteroids orbit the sun in a girdle between Mars and Jupiter.
- Comets are comparatively small, flimsy, erratically shaped bodies. They are the leftovers from the solar system formation. Comets are icy balls that form in the outer solar system. The icy surface is entrenched with grit, dust & particles from space.
- Several comets have elliptical orbits that cut across the orbits of planets, taking them very close to the sun & then swinging them far away. The far-off comets may take more than 30 million years to complete 1 orbit.
- Comets are very cold when far from the sun. As they come near the sun, their surfaces begin to warm and fickle materials vaporize.
- Example: Halley's Comet is arguably the most famous comet. It 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.
Meteors, Meteoroids and Meteorites
- While traveling through space, asteroids sometimes collide with each other & break up into minor remains. Comets shack dust as they wander the solar system. These 'break ups' result in frequent small particles & fragments, known as meteoroids, which orbit the sun.
Meteors, Meteoroids and Meteorites
- Most meteoroids are rocky and small. When it draws near Earth, it burns up as it goes through the atmosphere of Earth. This is called a meteor.
- Fireballs are bigger meteoroids, approximately ranging in dimension from a basketball to a Volkswagen. They smash into fragments & burn up in their way through Earth's atmosphere. Some meteoroids endure passage through the atmosphere of Earth & hit the ground. These are known as meteorites.
Try yourself:Some rocky fragments that endure passage through the atmosphere of Earth & hit the ground are known as:
Some meteoroids endure passage through the atmosphere of Earth & hit the ground. These are known as meteorites.
- The Goldilocks Zone refers to the habitable zone around a star, where it is not too hot nor too cold for liquid water to exist on the surface of surrounding planets.
- Had Earth been where Pluto is, then the Sun would be barely visible (about the size of a pea) and Earth's ocean and much of its atmosphere would freeze. On the other hand, if Earth took Mercury’s place, it would be too close to the Sun and its water would form a steam atmosphere, quickly boiling off.
- The distance Earth orbits the Sun is just right for water to remain a liquid. This distance from the Sun is called the habitable zone, or the Goldilocks zone. Rocky exoplanets found in the habitable zones of their stars, are more likely targets for detecting liquid water on their surfaces. Water is important because Life on Earth started in water, and water is a necessary ingredient for life.
- A ‘field’ is a region in which a body experiences a force owing to the presence of other bodies. Earth’s Magnetic Field is one such field.
- Magnetic fields determine how moving electric charges exert a force on other charged particles.
Earth’s Magnetism is generated by convection currents of molten iron and nickel in the earth’s core. These currents carry streams of charged particles and generate magnetic fields. This magnetic field deflects ionising charged particles coming from the sun (called solar wind) and prevents them from entering our atmosphere. Without this magnetic shield, the solar wind could have slowly destroyed our atmosphere, preventing life on earth to exist. Mars does not have a strong atmosphere that can sustain life because it does not have a magnetic field protecting it.
Try yourself:Earth’s Magnetism is generated by convection currents of molten iron and nickel in the Earth’s:
Earth’s Magnetism is generated by convection currents of molten iron and nickel in the earth’s core.
Dynamo theory: Generation of Earth’s Magnetic Field
Dynamo theory proposes a mechanism by which a celestial body such as Earth or a star generates a magnetic field and sustains it over astronomical time scales (millions of years). This theory suggests that convection in the outer core, combined with the Coriolis effect (caused due to the rotation of the earth), gives rise to self-sustaining Earth’s magnetic field.
- Earth’s magnetic field is generated in the earth’s outer core which is fluid due to lower pressure when compared to that of the inner core.
- The temperature of the outer core is between 4400 °C and 6000 °C.
Heat sources include:
(a) energy released by the compression of the core
(b) energy released at the inner core boundary as it grows and
(c) radioactivity of potassium, uranium and thorium.
- The differences in temperature, pressure and composition within the outer core cause convection currents in the outer core as cool, dense matter sinks while warm, less dense matter rises.
- This flow of liquid iron (found in the outer core) generates electric currents, which in turn produce magnetic fields.
- The charged metals passing through these fields create electric currents of their own, and so the cycle continues. This self-sustaining loop is known as the geodynamo.
- The spiral movement of the charged particles caused by the Coriolis force means that separate magnetic fields created are roughly aligned in the same direction, their combined effect adding up to produce a single vast magnetic field of the planet.
Convection Currents in The Core
- The Magnetosphere is the region above the ionosphere that is defined by the extent of the Earth’s magnetic field in space.
- It extends several tens of thousands of kms into space, protecting the Earth from the charged particles of the solar wind and cosmic rays which would otherwise strip away the upper atmosphere, including the ozone layer that protects the Earth from harmful ultraviolet radiations.
- Many cosmic rays are kept out of the Solar system by Heliosphere (Sun’s Magnetosphere)
- Earth’s magnetic field is distorted further by the solar wind that exerts a pressure.
- However, it is kept away by the pressure of the Earth’s magnetic field.
- The Magnetopause is the boundary of the magnetosphere.
- The magnetosphere is asymmetric, with the sunward side being about 10 Earth radii out but the other side stretching out in a magnetotail extending beyond 200 Earth radii.
The turbulent magnetic region just outside the magnetopause is known as the magnetosheath.
Sunward of the magnetopause is the bow shock, the area where the solar wind slows abruptly
- Inside the magnetosphere is the plasmasphere.
- This is a region that contains low-energy charged particles.
- It begins at the height of 60 km, and extends up to 3 or 4 Earth radii, and also includes the ionosphere.
- This region is found to rotate with the Earth.
Try yourself:The area where the solar wind slows abruptly is known as:
The area where the solar wind slows abruptly is known as Bow Shock.
- Aurora is the name given to the luminous glow in the upper atmosphere of the Earth which is produced by charged particles descending from the planet’s magnetosphere.
- Some of these particles penetrate the ionosphere and collide with the atoms there.
- This results in an excitation of the oxygen and nitrogen molecular electrons. The molecules get back to their original state by emitting photons of light which are the aurorae.
- They result from emissions of photons in the Earth’s upper atmosphere (above 80 km), from ionized nitrogen atoms regaining an electron, and from electrons from oxygen and nitrogen atoms returning from an excited state to ground state.
- The charged particles follow magnetic field lines which are oriented in and out of our planet and its atmosphere near the magnetic poles.
- Therefore, aurorae mostly are seen to occur at high latitudes (Arctic and Antarctic regions)
- The aurora’s color depends on the type of atom that is excited and how its electrons return from those excited states to the ground state.
Classification of Aurorae
An aurora is classified as either a diffuse or a discrete aurora.
- A diffuse aurora is a featureless glow in the sky that may not be visible to the naked eye even on a dark night and defines the extent of the auroral zone (the area in which auroras are visible).
- Discrete auroras are sharply-defined features within the diffuse aurora; they vary in brightness from barely visible to bright enough for reading a newspaper at night. Discrete aurorae often display magnetic field lines or curtain-like structures. They can change within seconds or glow unchanging for hours, most often in fluorescent green
In northern latitudes, the effect is known as the aurora borealis (or the northern lights), named in 1621 after the Roman goddess of dawn, Aurora, and the Greek name for the north wind, Boreas. The aurora borealis most often occurs near the winter equinox when it is dark for long periods of time.
Aurora BorealisThe aurora borealis’ southern counterpart, the Aurora Australis (or the southern lights), has almost identical features. It changes simultaneously with the northern auroral zone and is visible from high southern latitudes in Antarctica, South America, New Zealand, and Australia.
- The varying conditions in the magnetosphere, known as space weather, are largely driven by solar activity.
- If the solar wind is weak, the magnetosphere expands; while if it is strong, it compresses the magnetosphere.
- Periods of intense activity, called Geomagnetic storms, can occur when a coronal mass ejection erupts above the Sun and sends a shock wave through the Solar System.
- It takes two days to reach the Earth.
- At the Earth’s surface, a magnetic storm is seen as a rapid drop in the Earth’s magnetic field strength.
- The ionosphere gets heated and distorted, which makes long-range radio communication difficult.
- Ionospheric expansion can increase satellite drag, and it may become difficult to control their orbits.
- Geomagnetic storms disrupt satellite communication systems like GPS.
- Astronauts and high-altitude pilots would face high radiation levels.
- Electric power grids would see a high increase in voltage that would cause blackouts.
Van Allen Radiation Belt
- The Van Allen radiation belt is a zone of energetic charged particles, which originate from the solar wind, that are captured and held around a planet by that planet’s magnetic field.
- There are two such concentric tire-shaped regions. The inner belt is 1–2 Earth radii out while the outer belt is at 4–7 Earth radii.
- By trapping the solar wind, the belts deflect the energetic particles and protect the atmosphere.
- The belts endanger satellites, which must have their sensitive components protected with adequate shielding if they spend significant time in that zone.
- Beyond the belts, they face additional hazards from cosmic rays and solar particle events.
Van Allen Radiation Belt
Van Allen Radiation Belt
1. Briefly explain the different phases of the moon. (150 words)
Introduction: Describe about the moon in 2 lines.
Body: Explain the various phases of the moon with diagrams
Conclusion: mention its significance in 2 lines.
Discuss the Dynamo theory of Geomagnetism. What are the effects of Geomagnetic storms? (250 words)
Introduction: Write two lines about Earth’s magnetic field.
Body: (1) Explain the dynamo theory. (2) List down the effects of geomagnetic storms after defining the concept.
Conclusion: Write about the importance of the magnetic field in 2 sentences to conclude.