The Earth
Earth shimmers like a sapphire jewel. Taken by a geostationary weather satellite from 35800 kilometres distant.

Earth is the only planet with surface water, plate tectonics and an ozone layer. It is almost spherical (with the smallest oblateness of all the planets).


Water is a dominant feature, covering 71% of the earth’s surface. Most of it is in the oceans (97%), with 2% in ice and glaciers, and the rest in groundwater, rivers, lakes and the atmosphere. The water is thought to have come from comets and asteroids about four billion years ago. The theory holds that the ratio of the two isotopes of hydrogen in our oceans is the same as that of asteroids. Ordinary hydrogen (in water as H2O) has one proton in the nucleus; the other version is deuterium (as D2O in water) which has a proton and a neutron in its nucleus. The ratio is something like one deuterium to 6 400 hydrogens.

The Earth originally formed 4.6 billion years ago. In its first 40 million years a massive impact set fire to the planet, bringing iron and nickel in what theorists call “the Great Iron Catastrophe”. Having melted, these metals sank to the bottom, creating the solid core. About 10 million years after that Earth suffered another catastrophic collision with a trojan planet* the size of Mars. A trojan planet is one that shares the same orbit as the main body, holding a stable position at one of the Lagrangian points (either 60° behind or 60° in front of the main body). Theory holds that the powerful gravitational forces of Jupiter destabilised this “impactor planet” over time. It eventually broke loose and crashed into Earth. Their mantle and core merged, increasing the earth’s mass. The rest of the debris coalesced in orbit around the earth. The moon formed out of this at just 22 500 kilometres distant, when its gravitational pull was 200 times stronger than it is today. At the time the earth’s rotation was just eight hours. The moon dragged on the earth and slowed its rotation to its present 24 hours. Every 100 000 years the moon adds about two seconds to our day, and the moon itself is receding almost four centimetres a year. It is now at a mean nominal distance of 384 411 kilometres.

Jupiter has had a major influence on the formation of the planets, clearing orbital paths of debris and deflecting asteroids. In partnership with Saturn, it slung both Uranus and Neptune to their outermost orbits. After billions of years of mayhem things settled down and the solar system was left with eight planets and Pluto. Earth cooled and formed a crust of silicon dioxide. It is possible our water came from volcanic eruptions, condensing from steam to form the oceans. About four billion years ago the land started to coalesce and float on the water. Incidentally, though the oceans cover over 70% of the planet’s surface, the seawater, freshwater, rivers and glaciers only account for 0.06% of the Earth’s mass; the rest is rock.

  • *The ancient body was given the name Theia who was the Greek Titaness of Clear Sight. She gave gold its lustre and diamonds their sparkle. She is also the mother of Selene, the goddess of the moon.

Earth orbits at a speed of 107 270 kilometres per hour (29.8 kilometres per second) and ploughs through hundreds of meteors a day. Its orbit is almost circular with an eccentricity of just 0.0167. [Zero is exactly circular]. Neptune just pips the planet to the number one spot with an orbital eccentricity of 0.011.

All the planets move around the sun in the same direction. From Earth, they seem to move westwards across the night sky. Light from the sun takes about eight minutes to travel to Earth. But the planet Pluto is so far away that the sun’s light takes almost six hours to get there!

The current axial tilt of the earth is 23.5° and this causes the wide variation in our annual seasons. The earth is revolving on its axis at 1 675 kilometres per hour (at the equator), or 460 metres per second.


Earth is named after the Old English word, ‘eorpe’, from the German meaning ground [also, soil or land]; the only planet to have a generic name. The earth is silicon and oxygen-based with a solid iron core. We’ve already mentioned its surface is 71% covered in water. Maybe it should rather have been called Neptune, after the Roman god of fresh water, sea and horses!

The earth spins anti-clockwise as viewed from above the North Pole, as do most of the planets. The odd ones out are Venus (spins clockwise) and Uranus, whose axis is tilted at almost 90° (so it rolls along its orbit like a cannonball).

Earth is almost spherical with a polar diameter of 12 713 kilometres and an equatorial diameter of 12 756 kilometres. Oblateness is a measure of this flattening: 1:298.3 (or just 0.00335).

In the brief descriptions of the planets I have given them their mass as a ratio of Earth = 1. The actual mass of Earth is 5987.43 x 109 tonnes.

Earth’s core is predominantly iron metal (85%), with significant amounts of nickel (about 5%); and the balance are lighter elements, thought to be oxygen, with traces of silicon, hydrogen and carbon. The central solid core is about 2 440 kilometres in diameter, where the temperature is about 4 700°C. Crushing pressure keeps it solid despite the extreme heat. It is surrounded by a liquid shell of about 2 280 kilometres thick, also primarily consisting of iron, with nickel and smaller quantities of other metals. The interaction of the rotating earth and this liquid metal, and the convection of the materials within provide us with our powerful—and protective—magnetic field. Above this lies the mantle, about 2 800 kilometres thick and containing rocks rich in magnesium and iron. Finally, we have the earth’s crust, which is thinner under the oceans than on land. It comprises many different types of rocks and minerals, predominantly silicates.

Structure of EarthDepthThicknessComposition
Crust0 - 35 kms35 kmsRock
Upper Mantle35 - 660 kms660 kmsSilicates
Lower Mantle660 - 2 890 kms2 230 kmsSilicates
Upper Core2 890 - 5 150 kms2 280 kmsLiquid iron-nickel
Lower Core5 150 - 6 360 kms1 220 kmsSolid iron-nickel

There are five million tonnes of atmosphere on earth. It originally formed from gases spewed out by ancient volcanoes. However, the oxygen came from an entirely different source: plants. The atmosphere is densest at the surface because of the effects of gravity. It rapidly thins out towards space. For example, at 30 kilometres of altitude, the pressure is just 1% of that on the surface. Lower down is the troposphere where all our weather systems are generated. The earth absorbs more of the sun’s heat at the equator and less at the poles, which creates an imbalance in air pressure. Winds flow from high to low-pressure areas, and from regions of different temperature. The earth’s rotation also influences wind direction and pressure. The wind acts on the ocean currents, collects moisture into the atmosphere and brings rain.

You may not always like the weather it brings, but you have a lot to thank the atmosphere for.

Earth’s magnetic field protects us from the solar wind – a super-hot stream of charged particles from the sun. Planetary magnetic fields are formed by the interaction between the convection of interior conducting material (molten rock and metal) and the planet’s own rotation. Earth has a moderately strong magnetic field caused by the iron core. As the earth spins the hot liquid metal in the outer core revolves around the solid inner core and causes a magnetic field to outflow from the north and south poles. Good job too. This deflects solar winds arriving at 1.6 million kilometres an hour, which would otherwise blast our atmosphere and water out into space. As it is the earth is still losing about 50 000 tonnes of mass net a year, mostly from the loss of hydrogen and a small amount of helium flying out into space from the top of our atmosphere. That is after accounting for the arrival of an estimated 40 000 tonnes of dust and meteors each year. Don’t worry, this is very small in the scheme of things.

Not all planets have magnetic fields: that of Venus and Mars are almost negligible. The four gas giants have extremely strong magnetic fields. Mercury has an extremely weak field because it rotates so slowly. Venus doesn’t have an appreciable field because there appears to be little convection in its molten interior. Mars doesn’t have an appreciable field – although it did in the past – because its interior has now solidified.

The atmosphere also protects us from meteors—well, most of the time. It is quite common for meteors to hit the earth’s atmosphere. Ten tonnes of rock travelling at fifty times the speed of sound (around 18 kilometres a second) once a month is typical. From atmospheric friction they can reach temperatures of as high as 6 000°C, and finally explode and split into millions of bits, falling harmlessly to the ground.

Out in space, we also have a guardian: the biggest baseball bat in the galaxy. Jupiter protects Earth from comets and meteors by harvesting them in its powerful gravitational wake.

Our third defence is the ozone layer—ironically a health hazard for us at ground level. But at high altitude it protects is from harmful ultra-violet rays. We are also protected, again ironically, by carbon dioxide in the atmosphere. Without the greenhouse gases the earth would be too cold to sustain life.

The three cycles: the eccentricity of the earth’s elliptical orbit around the sun changes over a cycle of about 100 000 years; its axial tilt changes over a cycle of about 42 000 years; and the earth wobbles like a spinning top. This is caused by the tugging gravity of the sun and the moon. This wobble, called “precession”, alters the direction in which Earth’s axis points over a cycle of about 25 800 years. The upshot of these three interwoven cycles is the effect it has on our long-term climatic variations, and is thought to be one important factor in, for example, heralding the ice age.

Plate tectonics

German explorer Alfred Wegener (1880-1930) coined the phrase “continental displacement” to explain how one gigantic supercontinent of 200 million years ago came to separate into the continents we know today. He called it Pangaea, meaning “All-earth”, and it consisted of all of Earth’s land masses. Pangaea existed from the Permian through Jurassic periods. It began breaking up during the Jurassic period, forming continents Gondwanaland and Laurasia, separated by the Tethys Sea.

It was not until around 1950 that the concept of continental drift, as it became termed, was generally accepted. According to modern theory the earth’s solid outer layer—the “lithosphere”—floats on a bed of molten magma. The lithosphere is fragmented into about a dozen rigid blocks, or “plates”. Their thickness ranges from a couple of kilometres to 110 kilometres. The plates are dragged along by convection currents deep within the mantle, generated by radioactive heating of the interior. These plates move anything between one to ten centimetres a year, either towards one another, crushing into ridges, then mountains (destructive plate margin), or pulling apart (divergent plate margin) to release an upsurge of hot materials through volcanic action. Another situation occurs at a convergent or tensional plate margin. Here an oceanic plate is forced under the lighter continental plate (subduction). Friction may cause melting of the oceanic plate and may trigger earthquakes. Magma can squeeze up through the cracks onto the surface.

Of course the process is extremely slow. Some plates, instead of battering each other head on, actually slide past each other (conservative plate boundary). The San Andreas fault is one such tectonic boundary in California where the slip along the fault ranges from 2 to 3.5 centimetres a year. Over time the pressure build-up becomes extreme. The two plates finally overcome friction and slip past each other in a sudden movement, resulting in severe earthquakes.

What is a year?

Sidereal year (from Latin Sidus meaning star) is the time it takes the earth to orbit the sun once with respect to the fixed stars, that is, to get back to its starting point. On the other hand, the Solar year (also termed the ‘tropical year’) is the time it takes the sun to return to the same place in our sky, as for example, from vernal equinox to vernal equinox. The equinox is when the centre of the sun is directly above the earth’s equator. This happens twice a year: the vernal equinox in spring and the autumnal equinox in autumn.

We use the Gregorian Calendar, which developed out of the Roman Lunar Calendar. It counts the year (orbital period of the earth), as 365 days (so it is almost six hours short each year). This error is adjusted every four years by adding the 29th February in a “leap year” to catch up the six-hour bits. However, the true orbital period (the sidereal year) is actually 365.256 days; that is 365 days 6 hrs 9 minutes and 9.54 seconds. To fix this discrepancy the Gregorian calendar adds more leap years: one for every one hundred years and one for every four hundred years respectively. There is an exception! Where a century year is not divisible by 400, then a leap year adjustment is not made.

However, the mean solar year is 365.2422 days, as against the Gregorian calendar’s 365.2425 days. And further, the Solar year (vernal equinox to vernal equinox), which corresponds to the seasons, is about 20 minutes shorter than sidereal, which is 365.256 days.

A year on EarthSiderealSolarGregorian
Days in decimal365.256365.2422365.2425
Extra hours in decimal6.144 hr5.808 hr5.82 hr
6 hrs 9.62 mins5 hrs 48.48 mins5 hrs 49.2 mins
Actual time6 hrs 9 mins 9.54 secs5hrs 48 mins 45.1 secs5hrs 49 mins 12secs

To summarise:

  • Sidereal year –365.256 days as measured by the stars (same place in orbit).
  • Solar year – 365.2422 days as measured by the position of the sun (same place in sky).
  • Gregorian calendar — something written down on a piece of paper.

But let’s not lose any sleep over it!
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By Nigel Benetton, science fiction author of Red Moon and The Sands of Rotar.

Last updated: Wednesday, 5th February 2020