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Learning objectives

By the end of this section, you will be able to:

  • Describe the components of Earth’s interior and explain how scientists determined its structure
  • Specify the origin, size, and extent of Earth’s magnetic field

Earth is a medium-size planet with a diameter of approximately 12,760 kilometers ( [link] ). As one of the inner or terrestrial planets, it is composed primarily of heavy elements such as iron, silicon, and oxygen—very different from the composition of the Sun and stars, which are dominated by the light elements hydrogen and helium. Earth’s orbit is nearly circular, and Earth is warm enough to support liquid water on its surface. It is the only planet in our solar system that is neither too hot nor too cold, but “just right” for the development of life as we know it. Some of the basic properties of Earth are summarized in [link] .

Blue marble.

Image of Earth from Space. This photograph shows Africa, the Arabian Peninsula, Madagascar, and Antarctica surrounded by the Atlantic & Indian oceans. Numerous cloud formations are scattered across the globe.
This image of Earth from space, taken by the Apollo 17 astronauts, is known as the “Blue Marble.” This is one of the rare images of a full Earth taken during the Apollo program; most images show only part of Earth’s disk in sunlight. (credit: modification of work by NASA)
Some Properties of Earth
Property Measurement
Semimajor axis 1.00 AU
Period 1.00 year
Mass 5.98 × 10 24 kg
Diameter 12,756 km
Radius 6378 km
Escape velocity 11.2 km/s
Rotational period 23 h 56 m 4 s
Surface area 5.1 × 10 8 km 2
Density 5.514 g/cm 3
Atmospheric pressure 1.00 bar

Earth’s interior

The interior of a planet—even our own Earth—is difficult to study, and its composition and structure must be determined indirectly. Our only direct experience is with the outermost skin of Earth s crust, a layer no more than a few kilometers deep. It is important to remember that, in many ways, we know less about our own planet 5 kilometers beneath our feet than we do about the surfaces of Venus and Mars.

Earth is composed largely of metal and silicate rock (see the Composition and Structure of Planets section). Most of this material is in a solid state, but some of it is hot enough to be molten. The structure of material in Earth’s interior has been probed in considerable detail by measuring the transmission of seismic waves through Earth. These are waves that spread through the interior of Earth from earthquakes or explosion sites.

Seismic waves travel through a planet rather like sound waves through a struck bell. Just as the sound frequencies vary depending on the material the bell is made of and how it is constructed, so a planet s response depends on its composition and structure. By monitoring the seismic waves in different locations, scientists can learn about the layers through which the waves have traveled. Some of these vibrations travel along the surface; others pass directly through the interior. Seismic studies have shown that Earth’s interior consists of several distinct layers with different compositions, illustrated in [link] . As waves travel through different materials in Earth’s interior, the waves—just like light waves in telescope lenses—bend (or refract) so that some seismic stations on Earth receive the waves and others are in “shadows.” Detecting the waves in a network of seismographs helps scientists construct a model of Earth’s interior, showing liquid and solid layers. This type of seismic imaging is not unlike that used in ultrasound, a type of imaging used to see inside the body.

Questions & Answers

why the dark side of moon never face us? because moon dont rotate? why tho
Rishabh Reply
im in 8th standard and my school teach us nothing about astronomy but i want to be an astronomer so i study from youtube and apps like this but sometimes i get language problems
The Moon does actually rotate. It takes 27 days for the Moon to make a full rotation. Because it takes it so long and the Earth is rotating around the sun, it seems like the Moon is pretty much standing still. That's called "synchronous rotation".
I ment to say "The Earth is orbiting around the sun"
Tidal Locked.....
thats cool
ya gives you a look at how the moon orbits around us too and a perspective of how fast we are going. also gives the moon phases for the year! enjoy 😁
yes you are absolutely correct, it just like something like the milk of heaven
the far side of the moon is not the dark side of the moon.
what phases of the moon occurs when spring tide?
Florence Reply
the moon has a 28 day cycle. you can download a moon calander or follow the pattern and make your own calendar. 1st full moon of the year 21/22nd Jan, its also a blood moon and a wolf moon. if this info helps at all. 😊
How to determine the velocity of light?
using this formula c=1/√absolute permeability×absolute permittivity of free space i.e. c=1/√8.854×10^-12×4π×10^-7~3×10^8ms^-1
what is mean by Big Bang
block Reply
What it means by Big Bang is how the world was made and came in to exist
The Big Bang Is The Best Explanation For What We See Around Us, And For The Origin Of The Universe, All Our Evidence Points To The Big Bang Theory..... And Was Proposed By A Belgian Priest.....
if the big bang theory is really true than i would like to know what exploded and where did it came from and where did the energy came from and where come from for the matter to expand into amd where did the organization come from and where did the information come from.........
Some say that on the otherside of the singularity was a black hole. They say that at the other end of all black holes is another universe. Hence the multiverse. It's all theoretical at this point but less than a 100 years ago people believed that there was only 1 galaxy in the universe. So why
should there be only 1 universe?
is astronomy a hard subject to learn because i want to be an astronomer
FNAF Reply
No it not it just takes time and effort to learn
if your already interested/observing, your already an astronomer ! The more you look and read the more you'll learn.
I want to become a scientist but my knowledge is very low. How I can improve my knowledge?
any form of science courses, there's free crash course channel on you tube. I've learned alot from this channel. covers most topics, science: biology chemistry physics, astronomy, ranges to anatomy, history and many many more. it's a never ending subject!
suggest a channel as a example
CrashCourse is the name of the channel and there is a picture of an apple. look through their play lists. They will have other related channels they follow also.
thank you so much
your very welcome
Why are they so far away that their light takes hundreds of millions of years to reach us on earth?
LadyCatTM Reply
what is comets , astroids ,
Sumit Reply
comets are broken pieces of dust frozen debris Asteroids are big soiled pieces of Ice that burst into flames when it in to earth's atmosphere
hi, I am yamini, I am in class 8 but very much interested in astronomy and go to NASA, what are the subjects in which I can master and lead to NASA.
yamini Reply
Hello Yamini, Im Ken and I'm avery intrested in joining the NASA too. Is nice to meet you.
nice to meet u ken
What I know about the subjects is that you have to be a master on science and math, also if you know about aviation is better too
I read that the Russian language is very important, is not a requirement but it's like an extra point!
yeah like physics, chemistry and maths, they are my most favorite.
than you can go free of cost
Yes, That's right!
Oh! Free of cost?
hello yamini nice to meet you
wow from NASA ur so lucky
hello yamini nice to meet you
what is time
Abdul Reply
Time is relative
pls elaborate
the clear defination.I know that.
In planet mars there the life exits or not and is there water there
Eshwarsa Reply
see till now nothing can be found as u know that the curiosity rover has struck in mars
It has been proven that there are water molecules on Mars but not enough that most lifeforms could thrive upon.
There are huge amounts of water in the ice caps and under the surface. The surface and chemistry indicate that Mars had cosiderable amounts of water on its surface in the past.
what is your opinion about the theory of Vedas about modern physics..
Manish Reply
i think in some ways vedas are also correct but not everytime
I agree
hmm even I agree
Is there any patened theory about time relativitg in growth and development?
donot Reply
some astronomer's says that there is no alien exist but why search for extra terrestrial intelligence center is established
Eshwarsa Reply
No One Knows That For Absolute Fact, The Universe Is Too Huge To Have Any Type Of Idea About What Exist In The Far Reaches Of Our Universe.....
Check Out The Drake Equation.....
their should be aliens as like ours there would be another planet
which could have existed life on it
adam i want to ask a question
can kepler 1st law be applied on all the planets of the universe
hello, anyone home?
guys listen we cannot ever find aliens because our technology is not that great.we don't know whether they exist or no because our universe is very large . Just for an example;even if we spot out aliens we cannot reach there because lot of time must have been passed and before us reaching there
what if their species or existence might get vanished our have been extinct!! getting my point
sjskskfhjkkktewqqw and try?
Lanika Reply
what is this ?
hi I am Samuel from India mumbai
nice to meet you
thats my question, what is this?
penzias and wilson's a discovery of the cosmic microwave background is a nice example of scientific serendipity-something that is found by chance but turns out to have a positive outcome
Jacqueline Reply
Practice Key Terms 7

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Source:  OpenStax, Astronomy. OpenStax CNX. Apr 12, 2017 Download for free at http://cnx.org/content/col11992/1.13
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