Category Physics

WHAT IS THE PRIME MERIDIAN?

This is an imaginary line of 0° longitude that is perpendicular to the equator, and parallel to the axis. It passes through Greenwich in the UK, and divides Earth into eastern and western hemispheres. As it crosses the poles to the opposite side of the globe, the line becomes 180° longitude and is also known as the International Date Line.

The prime meridian is arbitrary, meaning it could be chosen to be anywhere. Any line of longitude (a meridian) can serve as the 0 longitude line. However, there is an international agreement that the meridian that runs through Greenwich, England, is considered the official prime meridian.

Governments did not always agree that the Greenwich meridian was the prime meridian, making navigation over long distances very difficult. Different countries published maps and charts with longitude based on the meridian passing through their capital city. France would publish maps with 0 longitude running through Paris. Cartographers in China would publish maps with 0 longitude running through Beijing. Even different parts of the same country published materials based on local meridians.

Finally, at an international convention called by U.S. President Chester Arthur in 1884, representatives from 25 countries agreed to pick a single, standard meridian. They chose the meridian passing through the Royal Observatory in Greenwich, England. The Greenwich Meridian became the international standard for the prime meridian.

Credit: National geographic

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WHAT ARE TIME ZONES?

As Earth spins, different parts of its surface turn towards the Sun at different times – the Sun is always rising in one place and setting in another. So, the time of day varies around the world. When it’s dawn where you live, it’s sunset on the other side of the world. To make it easier to set clocks, the world is split into 24 time zones, one for each hour of the day. As you go east around the world, you put clocks forward by one hour for each zone – until you reach an imaginary line called the International Date Line. If you go further on across the Date Line, you carry on adding hours, but put the calendar back by a day.

A time zone is a region on Earth that uses a uniform time. They are often based on the boundaries of countries or lines of longitude. Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory located in Greenwich, London, considered to be located at a longitude of zero degrees. Although GMT and Coordinated Universal Time (UTC) essentially reflect the same time, GMT is a time zone, while UTC is a time standard that is used as a basis for civil time and time zones worldwide. Although GMT used to be a time standard, it is now mainly used as the time zone for certain countries in Africa and Western Europe. UTC, which is based on highly precise atomic clocks and the Earth’s rotation, is the new standard of today.

UTC is not dependent on daylight saving time (DST), though some countries switch between time zones during their DST period, such as the United Kingdom using British Summer Time in the summer months.

Most time zones that are on land are offset from UTC. UTC breaks time into days, hours, minutes, and seconds, where days are usually defined in terms of the Gregorian calendar. Generally, time zones are defined as + or – an integer number of hours in relation to UTC; for example, UTC-05:00, UTC+08:00, and so on. UTC offset can range from UTC-12:00 to UTC+14:00. Most commonly, UTC is offset by an hour, but in some cases, the offset can be a half-hour or quarter-hour, such as in the case of UTC+06:30 and UTC+12:45

Credit: calculator.net

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HOW DOES EARTH GET ITS MAGNETIC FIELD?

The Earth’s outer core is in a state of turbulent convection as the result of radioactive heating and chemical differentiation. This sets up a process that is a bit like a naturally occurring electrical generator, where the convective kinetic energy is converted to electrical and magnetic energy. Basically, the motion of the electrically conducting iron in the presence of the Earth’s magnetic field induces electric currents. Those electric currents generate their own magnetic field, and as the result of this internal feedback, the process is self-sustaining so long as there is an energy source sufficient to maintain convection.

Unlike Mercury, Venus, and Mars, Earth is surrounded by an immense magnetic field called the magnetosphere. Generated by powerful, dynamic forces at the center of our world, our magnetosphere shields us from erosion of our atmosphere by the solar wind (charged particles our Sun continually spews at us), erosion and particle radiation from coronal mass ejections (massive clouds of energetic and magnetized solar plasma and radiation), and cosmic rays from deep space. Our magnetosphere plays the role of gatekeeper, repelling this unwanted energy that’s harmful to life on Earth, trapping most of it a safe distance from Earth’s surface in twin doughnut-shaped zones called the Van Allen Belts.

But Earth’s magnetosphere isn’t a perfect defense. Solar wind variations can disturb it, leading to “space weather” — geomagnetic storms that can penetrate our atmosphere, threatening spacecraft and astronauts, disrupting navigation systems and wreaking havoc on power grids. On the positive side, these storms also produce Earth’s spectacular aurora. The solar wind creates temporary cracks in the shield, allowing some energy to penetrate down to Earth’s surface daily. Since these intrusions are brief, however, they don’t cause significant issues.

Credit: NASA

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WHY IS THE CORE OF EARTH MADE OF IRON?

Because as Earth cooled, dense metal like iron sank to the centre, while lighter rock-forming materials floated to the top.

At the center of Earth is a solid iron inner core. The hot dense core has a radius of about 759 miles (1,221 kilometers) and a pressure of about 3.6 million atmospheres (atm).

Temperatures in the inner core are about as hot as the surface of the sun (about 9,392 degrees F or 5,200 degrees C) — more than hot enough to melt iron — but the immense pressure from the rest of the planet keeps the inner core solid, according to National Geographic.

The primary contributors to the inner core’s heat are the decay of radioactive elements such as uranium, thorium and potassium in Earth’s crust and mantle, residual heat from planetary formation, and heat emitted by the solidification of the outer core.

Earth’s inner core rotates in the same direction as the surface of the planet but rotates ever so slightly faster, completing one extra rotation every 1,000 years or so.

Credit: space.com

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HOW THICK IS EARTH’S CRUST?

Earth’s Crust

The crust is what we live on and is by far the thinnest of the layers of earth. The thickness varies depending on where you are on earth, with oceanic crust being 5-10 km and continental mountain ranges being up to 30-45 km thick. Thin oceanic crust is denser than the thicker continental crust and therefore ‘floats’ lower in the mantle as compared to continental crust. You will find some of the thinnest oceanic crust along mid ocean ridges where new crust is actively being formed. In comparison, when two continents collide as in the case of the India Plate and Eurasia Plate, you get some of the thickest sections of crust as it is crumpled together.

The temperatures within Earth’s crust will vary from air temperatures at the surface to approximately 870 degrees Celsius in deeper sections. At this temperature, you begin to melt rock and form the below-lying mantle. Geologists subdivide Earth’s crust into different plates that move about in relation to one another.

Given that Earth’s surface is mostly constant in area, you cannot make crust without destroying a comparable amount of crust. With convection of the underlying mantle, we see insertion of mantle magma along mid ocean ridges, constantly forming new oceanic crust. However, to make room for this, oceanic crust must subduct (sink below) continental crust.  Geologists have studied extensively the history of this plate movement, but we are sorely lacking in determining why and how these plates move the way they do.

Earth’s crust “floats” on top of the soft plastic-like mantle below. In some instances mantle clearly drives changes in the crust, as in the Hawaiian Islands. However, there is ongoing debate whether oceanic crust subduction and mid ocean ridge spreading is driven by a push or pull mechanism.

In very broad terms, oceanic crust is made up of basalt and continental crust is made up of rocks similar to granite. Below the crust is a solid relatively cooler portion of the upper mantle that is combined with the crust to make the  lithosphere layer. The lithosphere is physically distinct from the below-lying layers due to its cool temperatures and typically extends 70-100 km in depth.

Below the lithosphere is the asthenosphere layer, a much hotter and malleable portion of the upper mantle. The asthenosphere begins at the bottom of the lithosphere and extends approximately 700 km into the Earth. The asthenosphere acts as the lubricating layer below the lithosphere that allows the lithosphere to move over the Earth’s surface.

Credit: Forbes

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WHAT HAPPENS WHEN PLATES COLLIDE?

Plates move at an average of between four and seven centimetres in a year. If plates collide along a deep trench beneath an ocean, one plate is pulled beneath another and melts and is recycled. On land, when continents collide, their edges are pushed up into new mountain ranges.

When two tectonic plates collide, they form a convergent plate boundary.

  • A convergent plate boundary such as the one between the Indian Plate and the Eurasian Plate forms towering mountain ranges, like the Himalayas, as Earth’s crust is crumpled and pushed upward.
  • In some cases, however, a convergent plate boundary can result in one tectonic plate diving underneath another. This process is called subduction. It involves an older, denser tectonic plate being forced deep into the planet underneath a younger, less-dense tectonic plate. When this process occurs in the ocean, a trench can be formed.
  • When subduction occurs, a chain of volcanoes often develops near the convergent plate boundary.

Credit: Byju’s

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