Earth Explained Simply

Three journeys at the same time

You feel none of it for the same reason you do not feel a smooth aeroplane at cruising speed: your body notices changes in speed, not steady speed. The orbit is also the reason Earth is not "floating". Floating suggests nothing is pulling. In fact, the Sun's gravity pulls constantly and Earth's sideways speed makes it keep missing. For the full explanation, see Gravity and Orbits.

What really causes the seasons?

Earth's axis points at the same spot in space all year, so for half the orbit the Southern Hemisphere leans toward the Sun and for the other half the Northern Hemisphere does. Leaning toward the Sun does two things at once: the Sun climbs higher in the sky, so sunlight hits more directly and warms more strongly, and the days get longer, so there are more hours of heating. Try it yourself, then switch the tilt off to see what the world would be like without it.

If you live in the Southern Hemisphere, you already know the surprise built into this: Christmas happens in summer, when Earth is near its closest point to the Sun, and that proximity does make the Southern summer very slightly stronger, though the tilt remains the main driver. Near the equator the tilt matters little, so day length barely changes and many places have wet and dry seasons instead of hot and cold ones.

Oceans: the reason you can see Earth from space

About 71% of Earth's surface is ocean, and roughly 97% of all the planet's water is salt water. Only about 3% is fresh, and most of that is locked in ice caps and glaciers, so the water in lakes and rivers is a tiny fraction of the total.

Two things have to be right for oceans to exist at all. First, the temperature has to sit in the narrow band where water stays liquid. Earth's distance, its atmosphere and its reflectivity put the average surface temperature at around 15 °C. Without the natural greenhouse effect of its atmosphere, the average would be close to −18 °C and the oceans would freeze. Second, the planet needs enough gravity to hold an atmosphere, otherwise water would evaporate away into space, as on small, airless worlds.

Where the water came from is still debated. Earth formed hot, and scientists think a lot of its water arrived with icy asteroids and comets, with some already locked in the rocks that built the planet. Chemical clues in ancient crystals suggest liquid water was present surprisingly early, within roughly the first few hundred million years.

A hot core, a moving crust and a thin sky

Oxygen was not always there. For roughly the first two billion years, the air had almost none. Microbes that use sunlight to make food slowly released oxygen, and around 2.4 billion years ago it began to build up in what scientists call the Great Oxidation Event. Life did not just appear on Earth. It changed Earth.

Earth is a careful balance, not a lucky target

It is tempting to say Earth is in the "Goldilocks zone", but the story is subtler. Venus is nearly Earth's twin in size and has a thick carbon dioxide atmosphere that trapped heat until the surface reached around 465 °C. Mars is smaller, lost most of its atmosphere, and now has air pressure under 1% of Earth's. All three began with the same raw materials. Their different outcomes came from size, atmosphere, geology and the presence or absence of oceans and life. Studying these neighbours helps scientists understand how fragile Earth's climate really is.

• 4.54 billion years ago: Earth forms with the rest of the Solar System.
• About 4.5 billion years ago: A giant impact likely creates the Moon (see The Moon).
• At least 3.5 billion years ago: Life is established in the oceans.
• About 2.4 billion years ago: Oxygen starts to accumulate in the air.
• About 540 million years ago: Animals with hard shells and complex bodies appear in large numbers.
• About 300,000 years ago: The earliest known Homo sapiens.

If you could dig down, here is what you would pass through

Humans have never come close to the mantle. The deepest hole ever drilled, the Kola Superdeep Borehole in Russia, reached about 12 kilometres, which is less than 0.2% of the way to the centre. Everything we know about the deep interior comes from studying earthquake waves, which bend and reflect as they pass through different layers.

A natural thermostat, and how it is changing

The air contains small amounts of gases such as carbon dioxide and water vapour that trap some of the heat Earth radiates. This natural greenhouse effect is why the average surface temperature is a comfortable 15 °C instead of about −18 °C. Over millions of years, volcanoes release carbon dioxide while rock weathering and the oceans remove it, and that slow balance has kept Earth's climate within a range that supports liquid water.

Measurements show that atmospheric carbon dioxide has risen from roughly 280 parts per million before the Industrial Revolution to more than 420 parts per million today, mainly through burning fossil fuels. Scientific agencies including NASA and NOAA track this rise, and it is the basis of current research on climate change. The thermostat analogy is useful but imperfect: it works over very long timescales, and humans are now changing the inputs far faster than the natural processes can respond.

Magnetic field, auroras and a compass that has flipped before

When charged particles from the Sun reach Earth, the magnetic field steers most of them around the planet. Near the poles, some are funnelled down into the upper atmosphere, where they strike oxygen and nitrogen and make them glow. These are the auroras, called the northern lights and southern lights, and the green light comes from oxygen about 100 km up.

The field is not permanent. Rocks record that it has reversed many times, so that compasses would have pointed south. The last full reversal was around 780,000 years ago. The magnetic poles also drift now, and scientists track them so navigation systems stay accurate.

The same water has been going round for billions of years

Earth's water is not used up. Sunlight evaporates ocean water, winds carry the vapour over land, it condenses into clouds and falls as rain or snow, and rivers return it to the sea. The glass of water on your table may contain molecules that have been through clouds, glaciers, rivers and perhaps the bodies of animals long before you. Total water on the planet changes very little. What changes is where it sits.