Author: Fraser Cain

You can’t see it, but there’s an invisible force field around the Earth. Okay, not a force field, exactly, but a gigantic magnetic field surrounding the Earth, and it acts like a force field, protecting the planet – and all the life – from space radiation. Let’s take a look at the Earth’s magnetic field.

The Earth is like a great big magnet. The north pole of the magnet is near the top of the planet, near the geographic north pole, and the south pole is near the geographic south pole. Magnetic field lines extend from these poles for tens of thousands of kilometers into space; this is the Earth’s magneto sphere.

The geographic poles and the magnetic poles are far enough apart that scientists distinguish them differently. If you could draw a line between the magnetic north and south poles, you would get a magnetic axis that’s tilted 11.3 degrees away from the Earth’s axis of rotation. And these magnetic poles are known to move around the surface, wandering as much as 15 km every year.

Scientists think that the Earth’s magnetic field is generated by electrical currents flowing in the liquid outer core deep inside the Earth. Although it’s liquid metal, it moves around through a process called convection. And the movements of metal in the core sets up the currents and magnetic field.

As I mentioned at the top of this article, the magnetic field of the Earth protects the planet from space radiation. The biggest culprit is the Sun’s solar wind. These are highly charged particles blasted out from the Sun like a steady wind. The Earth’s magnetosphere channels the solar wind around the planet, so that it doesn’t impact us. Without the magnetic field, the solar wind would strip away our atmosphere – this is what probably happened to Mars. The Sun also releases enormous amounts of energy and material in coronal mass ejections. These CMEs send a hail of radioactive particles into space. Once again, the Earth’s magnetic field protects us, channeling the particles away from the planet, and sparing us from getting irradiated.

The Earth’s magnetic field reverses itself every 250,000 years or so. The north magnetic pole becomes the south pole, and vice versa. Scientists have no clear theory about why the reversals happen. One interesting note is that we’re long overdue for a reversal. The last one happened about 780,000 years ago.

We have written many articles about Earth for Universe Today. Here’s an article about how the geomagnetic reversal doesn’t mean doomsday in 2012, and here’s an article about how a solar storm compressed the Earth’s atmosphere.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

Reference:
NASA: Is the Earth’s magnetic field changing?

From our perspective, the three objects that have the greatest impact on our lives are the Earth, Sun, and Moon. The Earth, of course, is the planet beneath our feet. Without it, well, we wouldn’t have anything at all. The Sun warms our planet, and with the Moon, creates the tides.

The Moon orbits the Earth and in turn, the Earth orbits the Sun. We see the Universe from a platform that is both rotating on its axis, and traveling in an elliptical orbit around the Sun. The Earth’s rotation on its axis makes the Sun rise in the east and set in the west, and is a big part of why the Moon rises and sets too; although the Moon takes 29 days to complete an orbit around the Earth as well.

The average distance from the Earth to the Moon is 384,403 km. And the average distance from the Earth to the Sun is 149,597,887 km. If you divide these two numbers, you get approximately 389. Now, if you divide the diameter of the Sun (1.4 million km) by the diameter of the Moon (3,474 km), you get 403. Those two numbers are pretty close. This is why the Moon and the Sun appear to be the same size in the sky; it’s a total coincidence.

Because they appear to be the same size in the sky, the Sun, Earth and Moon work together to create eclipses. When the Moon is directly in between the Earth and Sun, we see a solar eclipse. The Moon appears to pass in front of the Sun and darken it completely. And in the opposite situation, when the Earth is in between the Sun and the Moon, the Earth’s shadow darkens the Moon. This is a lunar eclipse. We don’t see eclipses every month because the Moon’s orbit it tilted slightly away from the Earth’s orbit around the Sun. Sometimes the Moon is above this orbit and sometimes it’s below, so it doesn’t block the light from the Sun, or get caught in the Earth’s shadow.

The Sun and the Moon work together to create the tides we experience here on Earth. Most of the rise of the tides comes from the gravitational pull of the Moon, but a small amount comes from the Sun. When the two objects are on the same side of the Earth, we get the highest and lowest tides, and when they’re on opposite sides of the Earth, the tides are less extreme.

The brightest object in the Sky is the Sun. Astronomers measure its apparent magnitude as -26.73. This makes it 449,000 times brighter than the full Moon. The brightness of the Moon is only -12.6. Of course all of the Moon’s brightness is just reflected light from the Sun.

We have written many articles about the Earth for Universe Today. Here’s a more detailed article about the Sun and the Moon.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

Reference:
NASA Earth Observatory

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When I was first asked this question, “who discovered the Earth”, I thought it was ridiculous. If you use your eyes and look down beneath your feet, you’d be able to discover the Earth. This was how the first humans would have done it hundreds of thousands of years ago. But maybe a better question is: who discovered that the Earth is a planet?

In ancient times, people thought the Earth was the center of the Universe, and that the Sun, Moon, planets and stars rotated around us. Although some thought the Earth was flat, the ancient Greeks, like Plato, were convinced that the Earth was a sphere. They thought that each of the worlds and stars were in crystal spheres surrounding us.

This idea is natural and intuitive. Anyone who stands outside and looks up can clearly see that the stars and the planets are turning around the Earth. But ancients astronomers who studied the heavens found a few problems. Instead of following a straight path in the sky, some of the planets would appear to stop, move backwards, stop again, and then move forwards. To explain this, the Greek astronomer Ptolemy said that the planets were in tiny spheres and made little circles as they orbited around the Earth.

It wasn’t until the 16th century that the Polish mathematician and astronomer Nicolaus Copernicus presented the heliocentric model of the Solar System, where the Earth and the other planets orbited around the Sun. His model of the Solar System was backed up by observations by Galileo, who saw that Jupiter had moons of its own, and that Venus went through phases like the Moon.

It took a few years for the ideas to catch on, and for the scientific establishment to agree that yes, the Earth is just another planet, orbiting the Sun, and it’s not the center of the Universe.

We have written many articles about the Earth for Universe Today. Here’s an article about a new telescope that will let you see what Galileo saw.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

Reference:
NASA Earth Observatory: Planetary Motion

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Take a look down beneath your feet. You’re standing on the Earth’s crust. Although it seems limitless, the Earth’s crust only accounts for less than 1% of the Earth’s interior. Let’s take a look at everything that’s inside the Earth.

The Earth’s crust is the outer shell of the Earth. This is the part that has cooled down enough to solidify into rock. The crust extends down 30 km to 80 km underneath the continents, and only 5 km beneath the oceans. As you travel down through the crust, temperatures increase. The crust is broken up into several tectonic plates which “float” on top of the Earth’s mantle. In some regions, plates are sliding underneath one another, recycling rocks into the Earth. The crust beneath the middle of the oceans is spreading apart, and new material is welling up.

Beneath the crust is the largest part of the Earth’s interior: the mantle, which makes up about 84% of the Earth’s volume. This region extends down to a depth of 2,890 km. As you travel down through the mantle, temperatures increase immensely; they start at 500 C near the crust, and get to well over 4000 C at the boundary to the core. The mantle is mostly solid, but it acts like a viscous fluid, and experiences convection. Hot blobs of rock rise up from regions around the core through the mantle, give up their heat, and then sink back down.

At the very center of the Earth lies the core. This is a solid sphere of metal 2,440 km across surrounded by a layer of liquid metal. Scientists think that mostly made of iron (80%), with the rest composed of other heavy metals, like nickel, gold, platinum and even uranium. The core is slowly rotating compared to the crust, so that the core completes one rotation every 1000 years or so. The Earth’s magnetic field is though to be generated by the convection of hot metal in the Earth’s outer core. This field protects the Earth from the Sun’s solar wind; there probably wouldn’t be life on Earth without this field.

We have written many articles about the Earth for Universe Today. Here’s an article about the discovery of the Earth’s inner, inner core.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

Sources:
http://www.portal.gsi.gov.in/portal/page?_pageid=127,687643&_dad=portal&_schema=PORTAL
http://en.wikipedia.org/wiki/Mantle_%28geology%29
http://www.windows2universe.org/earth/Interior_Structure/interior.html

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Don’t listen to the Flat Earth Society, they’re wrong; the Earth is round. But did you ever wonder why the Earth is round? It all comes down to gravity.

One of the effects of mass is that it attracts other mass. For small objects, like your computer, your car, and even a building, the force of gravity is tiny. But when you have millions, and even trillions of tonnes of mass, the effect of the gravity really builds up. All of the mass pulls on all the other mass, and it tries to create the most efficient shape… a sphere.

For smaller objects, like asteroids, the force of gravity trying to pull the object into a sphere isn’t enough to overcome the strength of the rock keeping it in shape. But once you get above a certain mass and size, the strength of the object can’t stop the force of gravity from pulling it into a sphere. Objects larger than about 1,000 km in size are able to pull themselves into a sphere.

In fact, the International Astronomical Union decided in 2006 that this ability was one of the requirements for an object to be considered a planet. They must orbit the Sun, they need to have cleared out all the smaller objects in their orbit, and they need to have enough gravity to pull themselves into a sphere.

When an object has the gravity to pull itself into a sphere, astronomers say that it’s in hydrostatic equilibrium. And that’s why the Earth is round.

Of course, the Earth isn’t perfectly round. Because it’s turning on its axis approximately once every 24 hours, the Earth’s equator bulges outwards. And there are mountains and valleys that make the Earth’s surface rough.

We have written many articles about the Earth for Universe Today. Here’s an article about how round the Earth really is.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, answering a few questions, like why is Earth round. This was part of our tour through the Solar System – Episode 51: Earth.

You might not realize it, but you’re standing on a thin shell of solid rock encasing a vast quantity of molten rock. This is the Earth’s crust, and it’s the part of the planet that has cooled down enough to solidify. But just a few kilometers below your feet, it’s molten rock, extending for thousands of kilometers down to the planet’s superheated iron core.

Here on solid ground, on the continental shelves, the crust of the Earth is about 30 km thick. In the mid-ocean, the thickness of the crust can be as little as 5 km. The entire crust occupies just 1% of the Earth’s volume.

The crust is composed of a variety of igneous, metamorphic and sedimentary rocks gathered together into tectonic plates. These plates float above the Earth’s mantle, and it’s believed that convection of rock in the mantle causes the plates to slide around. On average, rocks in the crust last about 2 billion years before they slide underneath another plate and are returned to the Earth’s mantle. New rocks are formed in the mid-ocean regions where new material wells out of the Earth in between spreading plates. In comparison, rocks in the oceans are only 200 million years old.

The temperature of the crust increases as you go deeper into the Earth. It starts out cool, but can get up to 400 degrees C at the boundary between the crust and the mantle.

Scientists really know very little about internal structure of the Earth. The crust is the only part that we have any information about. And we’ve barely explored it at all. The deepest hole ever dug was the Russian Kola Superdeep Borehole. Started in 1970, the hole eventually reached a depth of 12.3 km. They eventually had to quit because temperatures in the hole became too hot to go any further. Other plans are in the works to bore into the crust in the ocean, where the thickness is much less.

We have written many articles about the Earth for Universe Today. Here’s an article about how the Earth’s core rotates faster than the crust, and here’s an article about how potassium could be heating up the Earth’s interior.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

Sources:
http://earthquake.usgs.gov/research/structure/crust/index.php
http://en.wikipedia.org/wiki/Crust_%28geology%29

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The ground under your feet might seem solid, but you’re standing on a relatively thin crust of rock above a vast ocean of rock. This molten rock is the Earth’s mantle, and it comprises the largest part of the Earth’s volume.

The crust we stand on is only about 30 km thick. Out in the oceans, it’s even thinner, getting down to 5 km in places. Beneath this crust is the mantle of the Earth; a region that extends down a depth of almost 2,900 km.

Although the mantle is largely hidden from our view, we do see it in places where cracks open up, allowing the molten rock to escape. These are volcanos, of course, and the liquid rock we see pouring out is the same as you’d find in the mantle.

The Earth’s mantle is mostly composed of silicate rocks that are rich in iron and magnesium. Although it’s mostly solid, it’s hot enough that it can flow over long timescales. The upper mantle flows more easily than the lower mantle because of the increasing temperature and pressures as you descend into the Earth.

The Earth’s tectonic plates float on top of the mantle. In some places, the plates are sliding under one another, returning rock back to the interior of the Earth. In other places, the plates are spreading apart, and fresh volcanic material is welling up to fill the cracks.

Inside the mantle, convection is slowly taking place – like in a lava lamp. Hotter material, heated by the core of the Earth rise slowly to the surface of the mantle. Material cools near the crust and then sinks back down to the core, to repeat the process all over again. It’s believed that this convection helps drive the motions of Earth’s tectonic plates.

We have written many articles about the Earth for Universe Today. Here’s an article that talks about how scientists might study the interior of the Earth using neutrinos, and here’s one about how rising temperatures could shut down plate tectonics.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

Sources:
http://en.wikipedia.org/wiki/Mantle_%28geology%29
http://www.schools.utah.gov/curr/science/sciber00/7th/earth/sciber/erlayers.htm

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Deep within the Earth, thousands of kilometers below your feet is the core of the Earth. Once thought to be a single ball of iron, scientists now know that the Earth’s core contains a solid inner core, surrounded by a liquid outer core. Let’s take a look at the outer core of Earth.

The discovery that the core of the Earth contains a solid inner core surrounded by a liquid outer core was made by seismologist Inge Lehmann, who was studying how seismic waves bounce off the interior of the Earth. Instead of bouncing off a solid core, Lehmann observed that the liquid outer core caused the waves to reflect differently from how they bounced off the inner core.

Further studies have refined the size of the outer core. The inner core is thought to be 2,440 km across, and when you include the liquid outer core of the Earth, the entire core measures 6,800 km across; about twice as big as the Moon.

It’s believed that the core of the Earth formed early on in our planet’s history, when the entire planet was made of molten rock and metal. Since it was a liquid, the heaviest elements, like iron, nickel, gold and platinum sunk down into the center, leaving the less dense elements on top.

Without the outer core, life on Earth would be very different. Scientists believe that convection of liquid metals in the outer core create the Earth’s magnetic field. This magnetic field extends outward from the Earth for several thousand kilometers, and creates a protective bubble around the Earth that deflects the Sun’s solar wind. Without this field, the solar wind would have blasted away our atmosphere, and Earth would be dead and lifeless like Mars.

The inner core is also known to rotate, turning approximately 0.3 to 0.5 degrees per year relative to the rotation of the surface. In other words, the inner core makes an extra rotation every 700-1000 years compared to the surface.

We have written many articles about the Earth for Universe Today. Here’s an article about the recent discovery that the Earth has an inner, inner core.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

Sources:
http://www.windows2universe.org/earth/Interior_Structure/interior.html
http://www.amnh.org/education/resources/rfl/web/essaybooks/earth/p_lehmann.html
http://en.wikipedia.org/wiki/Outer_core