Progress in medical science, better diet and improved hygiene have made the world a healthier place for many.
How long people are likely to live is called life expectancy. In 1950, the world average was just 40 years. Now it is over 63 years.
Life expectancy is usually high in richer countries. The Andorrans live on average for 83.5 years; the Japanese live for 80.8 years.
Life expectancy is much lower in poor countries. People in Zambia live just 37.3 years; people in Mozambique live 36.45 years.
Vaccination programs have reduced the effects of some major diseases. The terrible disease smallpox was thought to be wiped out in 1977.
Some diseases are on the increase in poorer parts of the world. AIDS (Acquired Immune Deficiency Syndrome) is now killing huge numbers of Africans.
In some parts of the world, disease, lack of food and water and poor healthcare mean that one child in every four dies before reaching the age of five in poor countries like Afghanistan and Sierra Leone. World geography
In the USA and Europe less than one child in a hundred dies before the age of five.
In wealthier countries such as Italy and Switzerland, there is on average one doctor for every 350 people.
In most poor African countries, there is just one doctor for every 50,000 people.
The first vaccination ever was given in 1796 by Edward Jenner. He used cowpox matter to vaccinate against smallpox.
Giraffes are the tallest mammals, growing to more than 5 m. Their height allows them to reach and eat the leaves, twigs and fruit at the tops of trees.
The legs of a giraffe are almost 2 m long.
A giraffe’s neck may be over 2 m long, but it only has seven bones — the same number as humans.
Giraffes live in Africa, south of the Sahara, in bush country.
The giraffe’s long tongue is so tough that it can wrap around the thorns of a thorn tree to grab twigs.
When drinking, a giraffe has to spread its forelegs wide or kneel down to reach the water. This position makes it very vulnerable to attack by lions.
When giraffes walk, they move the two legs on one side of their body, the other two on the other side. Their long legs mean that when it comes to running they can gallop along faster than the speediest racehorse.
A giraffe’s coat is patched in brown on cream, and each giraffe has its own unique pattern. The reticulated giraffes of East Africa have triangular patches, but the South African Cape giraffes have blotchy markings.
During the breeding season, rival male giraffes rub their necks together and swing them from side to side. This is called necking.
When it is first born, a baby giraffe is very wobbly on its legs and so cannot stand up for at least its first half an hour.
Crystals are particular kinds of solids that are made from a regular arrangement, or lattice, of atoms. Most rocks and metals are crystals, so are snowflakes and salt.
Most crystals have regular, geometrical shapes with smooth faces and sharp corners.
Most crystals grow in dense masses, as in metals. Some crystals grow separately, like grains of sugar.
Some crystals are shiny and clear to look at. Crystals got their name from the chunks of quartz that the ancient Greeks called krystallos. They believed the chunks were unmeltable ice.
Crystals form by a process called crystallization. As liquid evaporates or molten solids cool, the chemicals dissolved in them solidify.
Crystals grow gradually as more and more atoms attach themselves to the lattice, just as icicles grow as water freezes onto them.
The smallest crystals are microscopically small. Occasionally crystals of a mineral such as beryl may grow to the size of telegraph poles.
A liquid crystal is a crystal that can flow like a liquid but has a regular pattern of atoms.
A liquid crystal may change color or go dark when the alignment of its atoms is disrupted by electricity or heat. Liquid crystal displays (LCDs) use a tiny electric current to make crystals affect light.
X-ray crystallography uses x-rays to study the structure of atoms in a crystal. This is how we know the structure of many important life substances such as DNA.
Auroras are bright displays of shimmering light that appear at night over the North and South Poles.
The Aurora Borealis is the Northern Lights, the aurora that appears above the North Pole.
The Aurora Australis is the Southern Lights, the aurora that appears above the South Pole.
Auroras are caused by streams of charged particles from the Sun known as the solar wind crashing into the gases of the Earth’s atmosphere.
Oxygen gas glows yellow-green when it is hit low in the atmosphere, and orange higher up.
Nitrogen gas glows bright red when hit normally, and bright blue when ionized.
Auroras form a halo of light over the poles all the time, but they are usually too faint to see. They flare up brightly when extra bursts of energy reach the Earth’s atmosphere from the Sun.
Auroras appear at the poles and nowhere else in the world because there are deep cracks here in the Earth’s magnetic field (see magnetism).
Auroras are more spectacular when the solar wind is blowing strongly.
New York and Edinburgh get an average of ten aurora displays every year.
The Northern Lights above the Arctic Circle are among nature’s most beautiful sights. Shimmering, dancing curtains of colour — bright green rays flashing with red, and streamers of white — blaze into the darkness of the polar night.
The Voyagers are a pair of unmanned US space probes, launched to explore the outer planets.
Voyager 1 was launched on 5 September 1977. It flew past Jupiter in March 1979 and Saturn in November 1980, and then headed onwards on a curved path that will take it out of the Solar System altogether.
Voyager 2 travels more slowly. Although launched two weeks earlier than Voyager 1, it did not reach Jupiter until July 1979 and Saturn until August 1981.
The Voyagers used the ‘slingshot’ of Jupiter’s gravity to hurl them on towards Saturn.
While Voyager 1 headed out of the Solar System, Voyager 2 flew past Uranus in January 1986 and Neptune on 24 August 1989. It took the first close-up photographs of the two planets.
The Voyagers revealed volcanoes on Io, one of Jupiter’s Galilean moons.
Voyager 2 found ten unknown moons around Uranus.
Voyager 2 found six unknown moons and three rings around Neptune.
Voyager 2 discovered sulphur volcanoes on Jupiter, in 1979.
Voyager 2 reached Neptune in 1989, revealing a wealth of new information about this distant planet. Space travel Voyager 2 will beam back data until 2020 as it travels beyond the edges of the Solar System.
The space shuttle is a reusable spacecraft, made up of a 37.2-m-long orbiter, two big Solid Rocket Boosters (SRBs), three main engines and a tank.
The shuttle orbiter is launched into space upright on SRBs, which fall away to be collected for reuse. When the mission is over the orbiter lands like a glider.
The orbiter can go as high as a near-Earth orbit, some 300 km above the Earth.
The maximum crew is eight, and a basic mission is seven days, during which the crew work in shirtsleeves.
Orbiter toilets use flowing air to suck away waste.
The orbiter can carry a 25,000 kg-load in its cargo bay.
The first four orbiters were named after old sailing ships – Columbia, Challenger, Discovery and Atlantis.
The three main engines are used only for lift-off. In space, the small Orbital Maneuvering System (OMS) engines take over. The Reaction Control System (RCS) makes small adjustments to the orbiter’s position.
The shuttle program was brought to a temporary halt in 1986, when the Challenger exploded shortly after launch, killing its crew of seven.
In 1994 the crew of Discovery mended the Hubble space telescope in orbit.