About The Universe 2: Our Neighbourhood: The Earth and The Nature of Planets

When I was a kid, I had a Time Life book called The Universe. It was published in 1967, two years before man first set foot on the moon. The Universe went into great detail about the neighbouring planets in our Solar System, but hardly talked at all about what lay outside. Without the internet or the money to buy books to find out more, I wondered about what is out there beyond the Solar System for many years. If you’ve been wondering too, then I’m going to tell you soon enough!

But first, let’s start at home with our own planet: Earth. In this chapter, I’m not going to try and explain the ‘how’ or ‘why’ of Earth, just the ‘what’; I’ll explain the overall behaviour of the Earth without explaining why it came to be this way. We’ll go into that later.

Earth and The Conditions for Life

Earth is a fairly small rocky planet with a radius of about 6,400km (4,000 miles). That means the distance from the centre of the Earth to any point on the surface is 6,400km; if you like, the Earth is 6,400km ‘deep’ – if you dig straight down for 6,400km, you’ll find yourself exactly in the centre of the Earth – if you dig down twice as much, or 12,800km (8,000 miles), you’ll pop out on the other side! It’s not important to know the radius of the Earth, but it’s helpful to have a yardstick to use when comparing the size of our home planet with the other ones we’ve discovered.

Earth is remarkable in that it has just the right conditions to support the development of life as we know it. In order to support life, a planet must satisfy many criteria, such as:

• Water. Water is essential for most organisms to survive, and the oceans are generally considered to be the place where life initially forms on planets capable of sustaining life. Also, oceans help a planet to store heat from sunlight, which is needed to sustain life. When scientists look for planets that can sustain life, the presence of water is one of the first things they look for.
• Heat. If the planet is too hot, vegetation cannot survive and water will boil away. If it’s too cold, water will freeze and become unusable. The only heat source of any planet is its nearest star (the Sun in our case), so the heat of any planet is mainly dependent on its distance from the star – the further away it is, the colder the planet will be. Earth is just at the right distance from our Sun to create a mild, temperate climate.
• Atmosphere. The atmosphere – in broad terms – is a blanket of chemicals that surrounds a planet. Our atmosphere is rich in oxygen, which we need to breathe into our lungs to survive. The atmosphere also has to have certain properties that help it retain the heat from the nearest star – like the glass of a greenhouse. Greenhouse glass lets the heat from the Sun in, and prevents it from escaping again. Without these properties, a planet would be too cold to support life.
• Light. Plants use a process called photosynthesis to survive. This process lets plants capture sunlight through their leaves by turning them to face the Sun. The leaves contain a chemical called chlorophyll which absorbs the sunlight and turns it into energy that can be used to help the plant draw water from the soil and keep itself alive [Note: this is a major simplification of what actually happens]. Without sunlight, most plants could not exist. In order to have sunlight, you need to be both the right distance from the Sun, and to have an atmosphere which keeps the sunlight in.
• Composition. Animals – including humans – eat vegetation to survive and grow. Vegetation can only grow in certain types of soil. The soil on Earth has just the right mix of chemicals to support a rich variety of vegetation and plant life. The composition of a planet’s soil (upper surface layer) depends mainly on what chemical elements the planet was originally made from (I’ll explain how planets are formed later) and what weather systems are in place. Some planets don’t have any soil at all. Some planets don’t even have a solid ground to walk on!
• Gravity. There is a certain narrow range of gravitational strength in which organisms can survive. The larger a planet is, the stronger gravity feels if you try to walk on it. If gravity is too weak (a planet is too small), soil will not condense properly, and people will float away if they jump too high. If gravity is too strong (a planet is too big), anything that tries to walk around will be crushed by its own weight. Earth has just the right size to provide about the right amount of gravity life needs to develop. When looking for planets that can sustain life, we look for planets that are roughly Earth-sized for precisely this reason.
• Weather. Weather is caused by atmospheric conditions. In order for life to flourish, a planet needs a certain amount of rain and a certain amount of dryness. On some planets, it is always completely dry, or there are huge thunderstorms raging 24 hours a day every day. As far as we know, life cannot survive under such prolonged weather conditions. Also, soil is created by bombarding rocks with dust and rain, causing them to erode into small grains over a long period of time. Without weather, rocks would not erode into soil in the same way.

There are many other factors which influence the development of life. It may also be possible that other forms of life that we don’t know about can survive on planets that don’t satisfy all the criteria above. We can’t speculate on this as we have no evidence for or against it. We can only look at life on our own planet, determine the conditions that were necessary for its creation, and look for other planets with similar conditions. This is where the phrase “life as we know it” comes from – some forms of life may be able to thrive without water or atmosphere, or in strong gravity, but we have not yet come across them.

To date, we have not yet found any signs of life on other planets, however the likelihood that life exists elsewhere is extremely high and I will discuss this in more detail later.

Time and Orbits

Every planet has its own measure of time. The length of a day and the length of a year is different on every planet! Why?

A planet orbits (travels) around its nearest star (for now we’ll say it goes around in a circle and I’ll talk more about orbits later). The Earth orbits around our nearest star, which is the Sun. The time it takes for a planet to travel around its star once in a complete orbit determines how long a year is on that planet. This is because our seasons change depending on where we are in our travels around the Sun. One complete orbit exposes the Earth to each season exactly once, and thus we complete a year. For the Earth, it takes 365 and a quarter days to orbit the Sun once (this is the origin of leap years; we treat each year as 365 days, and every 4 years, we add a day to make up for the four quarter days we missed; we just do this for convenience because our calendar makes it awkward to have quarter days!).

As well as orbiting its nearest star, each planet also spins on its own axis of rotation. Imagine a spinning top on a table. When you start it spinning, it rotates around its centre repeatedly. This is exactly what the Earth and other planets do. Visualise an imaginary line right down the centre of the Earth, from the North Pole to the South Pole. Now imagine that you’re a giant; put your fingertips on the Earth and make it spin around the line, just like a spinning top. The Earth and other planets make this motion at a constant speed all the time. The time it takes a planet to revolve around its axis once determines the length of a day. This is because the rotation of the planet causes everywhere on the planet to be exposed to some period of sunlight followed by some period of darkness exactly once. As the Earth spins, some parts of it start to face the Sun (which you can consider to be sitting still relative to the Earth), the Sun appears to rise in the sky and daylight takes hold. After a while, as the rotation continues, the same point on Earth starts to face away from the Sun, the Sun appears to set and – since the Sun is now behind us – it becomes dark. It takes the Earth 24 hours to revolve on its axis exactly once.

Read more about the revolution and orbits of the Earth, Moon, Sun and galaxy at NASA’s Imagine The Universe.

Sunrise and Sunset

Why does the Sun always rise in the east and set in the west? Because the Earth rotates anticlockwise around its axis, if you look down on it from the North Pole. This means we are always ‘revolving East’ as we go around and around, so as we start to face towards the Sun, we are rotating Eastwards, and so the Sun appears on the eastern horizon. The Sun appears to move West across the sky during the day because we are still revolving East, and eventually we rotate away from the Sun – still revolving East – and the Sun sets on the western horizon.

In the real world, the sun doesn’t rise and set precisely East and West, but slightly to the North or South of these directions depending on the time of year.

Read more details about sunrise and sunset at Where Do the Sun and Stars Rise?.

Seasons and Daylight

Why do we have seasons? Why is it that when it’s summer in the US, it’s winter in Australia?

First, the Earth’s rotation around the Sun is not circular, but elliptical (an ellipse is an elongated circle, just like a rectangle is an elongated square). This means that at different times of year, we are different distances away from the Sun. This affects the amount of sunlight we get each day, but not in the way you might think – keep reading!

Secondly, my explanation about the Earth’s axis of rotation above was simplified. In fact, Earth is not “standing upright”, but has a ’tilt’ of about 23 degrees west of North (360 degrees is a complete circle; so if 0 degrees is North, 90 degrees is East, 180 degrees is South and 270 degrees is West). This tilt stays the same as the Earth orbits the Sun over the course of a year. The axis of rotation still runs through the North and South poles, but the poles are not pointing “up” and “down” as you might imagine, but are tilted sideways; the North Pole is tilted West 23 degrees, and the South Pole is tilted East 23 degrees. In this way, the Earth spins on its side.

What this means is that, depending where on Earth you stand, you will experience a different amount of sunlight each day, and the amount of sunlight you experience will change each day as our distance from the Sun changes.

The Earth (and all planets) are conceptually split into two halves – the Northern and Southern hemispheres. A hemisphere is half a planet. The line which splits the hemispheres is called the equator, and is an imaginary line that runs East/West around the ‘thickest’ part of the Earth, exactly half way between the North and South poles, which are at the North and South ‘tips’.

When we are at the Western tip of our elliptical orbit around the Sun, the Northern hemisphere is pointing more away from the Sun than the Southern hemisphere, because the Earth is tilted 23 degrees West, causing the Northern hemisphere to tilt West away from the Sun and the Southern hemisphere to tilt East towards the Sun. This means the Northern hemisphere is not getting much heat or sunlight (it’s facing away from the Sun), but the Southern hemisphere is getting lots of heat snd sunlight (it’s facing towards the Sun). This position in our orbit is the height of winter in the Northern hemisphere, and the height of summer in the Southern hemisphere.

Conversely, when we are at the Eastern tip of the orbit, the Northern hemisphere is still tilted West, but now it is facing more of the Sun, so it is summertime. The Southern hemisphere is still tilted East, but now it is pointing more away from the Sun, so it is wintertime.

You should now also be able to see why the North and South poles have 24-hour daylight in summer and 24-hour darkness in winter, and why the equator has a roughly even amount of daylight (12 hours) all year round. The poles do not move very much because they are close to the Earth’s axis of rotation, so when it is the height of summer at one pole, the pole is pointing at the Sun almost all the time and so it is always light outside. The other pole (which is in winter) is pointing away from the Sun almost all the time and so it is always dark. At the equator, however, the revolution of the Earth on its axis causes much more rotation, because this is where the Earth is ‘widest’ and points on the equator are therefore furthest away from the axis of rotation. So, regardless of our distance from the Sun, points on the equator are pointing towards or away from the Sun for about an equal amount of time every day, minimising seasonal variations.

Read more details about how seasons work at The Seasons.

Inside The Earth: Geology, Magnetism and Tectonic Activity

What’s the Earth made of? We’ve never been to the centre of the Earth, so we can’t say for sure, but by examination of the world around us we’ve come up with a pretty good idea. It’s not important to remember the numbers below, I’ve just given them so you have an idea how big and hot each section is!

• Crust (Crust @ Wikipedia) – This is the top-most layer, the surface we walk on and the soil plants grow in. It is about 20-70km deep (depending on where you are on the Earth) and is compromised mainly of basalt (a type of rock).
• Mantle (Mantle @ Wikipedia) – From about 30-2,890km deep is an area called the mantle. This is composed of a wide variety of rocks and substances; it is mostly liquid at the top (in the form of lava) and solid at the bottom, with temperatures ranging from about 1,000C at the top to 4,000C at the bottom.
• Outer core – From 2,890-5,150km deep the outer core can be found. It is liquid and made mostly out of the metals iron and nickel, with some sulphur and oxygen. Temperature between 4,000C and 5,000C.
• Inner core – From 5,150km and down to the centre (at about 6,370km) is the inner core, which is a solid ball of iron, nickel and a few other trace elements. Temperature between 5,000C and 6,000C.

Because the outer core is liquid and the inner core is solid, the outer core ‘flows’ around the inner core as the Earth revolves. This is what is responsible for generating the Earth’s magnetic field and polarising it. Read more about the Earth’s core at Thinkquest – The Core.

The crust and upper layer of the mantle form something called the lithosphere, which is split up into a number of large pieces across the Earth. These pieces are called tectonic plates. Because the mantle is liquid and molten, it moves around in a slow, sticky kind of way, like melted wax or treacle (thick liquids that move around slowly like this are said to be ‘viscous’). The continents sit upon the tectonic plates, so over time, their positions on the Earth change slowly, and they move closer together or further apart. When tectonic plates collide or shear against each other, huge pressure occurs within the mantle. If two plates are pushing against each other, there may be no way for the lava to go except up – through the crust and out onto the surface of the Earth. This is how volcano eruptions occur. Volcanos are typically found on so-called “fault lines” (the lines where the edges of tectonic plates pass close to each other). Earthquakes also occur most often at these lines.

Read more about the composition of the Earth at Earth @ Wikipedia.

The Moon

How does the Moon fit into our picture of the Earth? Some people think the Moon is a planet of its own but this is not the case – a planet by definition must orbit a star. The Moon orbits the Earth, and so it is said to be a ‘satellite’ of the Earth (just like a TV or weather satellite orbits the Earth, but the Moon is much further away!). Some planets have lots of moons or satellites; some don’t have any. The Earth happens to have just one.

The Moon is about a quarter of a million miles (250,000 miles) away from Earth and rotates on its axis once every 27.3 days (that is to say, one day on the Moon is about 650 hours long). It has a radius of about 1730km, making it about a quarter of the size of the Earth. Gravity is also correspondingly less – about 1/6th that of the Earth’s gravity. This means you can jump really high on the Moon with no extra effort!

Moon Phases

The Moon orbits the Earth once every 27 days, and this motion is what is responsible for the so-called “phases of the Moon”, where the Moon is visible as a crescent, full moon, or not at all depending on the position of its orbit around the Earth.

The Moon does not generate its own light – it reflects starlight from the Sun. As with the Earth, at any given time, half of the Moon is lit by the Sun and half is in darkness. Similarly, only half of the Moon is visible to us at any given time – the half that is facing us. As the Moon orbits the Earth, its angle relative to the Sun and Earth changes. The half of the Moon that is lit by the Sun is not always the same as the half that we can see, and so most of the time we see the Moon from an angle that is partially lit and partially in darkness – giving rise to crescents, half-moons and so on.

When the Sun and Moon are in the same direction relative to the Earth, the Sun’s light is shining precisely on the half of the Moon that is facing away from us, so we see no moon at all in the night sky. When the Sun and Moon are opposite each other relative to the Earth, the Sun’s light is shining precisely on the half of the Moon that is facing towards us, and we see a full moon in the night sky. The only exception is when the Earth sits directly between the Sun and Moon, in which case there will be a lunar eclipse.

Read more about moon phases at Keith’s Moon Page – Moon Phases.

Lunar Tides

You may have heard that our ocean tides are caused by the Moon. This is true – and without the Moon to do this, there would be no life on Earth.

Tides occur because of gravity trying pulling the Earth and Moon together. The Moon tugs at the Earth, but is not as big as Earth, so it fails to drag Earth towards the Moon – except for the water (the rest of the Earth is solid). A bulge is created in the oceans on the sides of the Earth facing directly towards and away from the Moon (the side towards the Moon because the Moon is pulling the water on that side away from Earth, and on the side away from the Moon because the Moon is pulling the Earth away from the water on that side). The points of ocean at the peak of the bulges are said to be at high tide (the water level is highest here), and the points of the ocean furthest away from the bulges are said to be at low tide (the water level is lowest here because this part of Earth is feeling the least gravitational pull from the Moon).

Due to the Earth’s revolution on its own axis, and the Moon’s orbit around the Earth, the Moon appears to orbit in our sky once every 25 hours. As this apparent orbit occurs, the points of high tide – the bulges – follow the Moon’s apparent orbit round as its direction of gravitational pull changes. In this way, the high and low tides move around the Earth in line with the Moon’s apparent orbit. Since any two points at opposite ends of the Earth are at high tide and low tide at any given time, one complete orbit of the Moon will cause two high tides and two low tides at any given point on Earth. Therefore one cycle from high tide to low tide and back to high tide takes half the time of one complete apparent orbit, or 12.5 hours.

Read more about moon tides at Keith’s Moon Page – Moon Tides.

In the next chapter, I’ll talk about the other planets in our neighbourhood and the general structure of the Universe.

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