SOLAR SYSTEM
The solar system consists of the Sun and an array of objects that orbit it. These objects include the nine known planets, their 64 known moons, asteroids, comets, meteoroids and huge amounts of gas and dust. The Sun’s great size relative to the other objects in the Solar System gives it the gravitational pull to keep them permanently in orbit around it.
The planets orbit the Sun in the same direction (anticlockwise in this illustration) and in elliptical (oval-shaped) paths. Pluto’s orbit is the most elliptical of all the planets. For part of its journey around the Sun, its orbit actually lies inside that of Neptune. All the planets, and most of their moons, travel on approximately the same plane, with the exception of Mercury and, once again, Pluto, both of which have tilted orbits.
Constantly streaming away from the Sun in all directions is the solar wind, made up of electrically-charged particles (parts of atoms).Travelling at more than 400 kilometres per second, it produces electric currents inside a giant magnetic “bubble” called the heliosphere. The heliosphere protects the Solar System from cosmic rays arriving from space. Its edge, some 18 billion kilometres from the Sun, marks the true boundary of the Solar System.
EARLY ASTRONOMERS
Thousands of years ago, in the time of the ancient civilizations of Egypt and China, people thought that the Sun and Moon were gods, the Earth was flat and the sky was a great dome suspended above it.
In later years, astronomers from ancient Greece proved that the Earth was round. Many believed that the stars were fixed to a great sphere that rotated around the Earth each day. One Greek astronomer, Aristarchus, proposed that the planets, including Earth, orbited the Sun, a star, but most astronomers of this time thought that the Sun, Moon and planets all travelled in circular paths around Earth, the centre of the Universe. Ptolemy, who lived in the 2nd century AD, observed that, while the stars moved across the night sky along regular paths, the planets appeared to “wander” from theirs. He proposed that they each moved in their own small circles, called epicycles, as they orbited Earth.
The Polish priest and astronomer, Nicolaus Copernicus, challenged Ptolemy’s view of the Solar System, declaring that the Sun lay at the centre of a system of orbiting planets. Only the Moon orbited the Earth. Copernicus wrongly believed that the planets’ orbits were perfect circles and that they moved in epicycles. It was left to the German astronomer Johannes Kepler (1571-1630), who showed that the planets moved in elliptical, rather than perfectly circular, orbits. The shapes of their orbits also explained the “wandering” that so perplexed earlier observers, thus disproving the idea that the planets moved in epicycles.
The Italian astronomer Galileo (1564-1642) was the first to use a telescope. From his observations of the moons of Jupiter in orbit around that planet, and the changing shape of Venus as it orbited the Sun, he concluded that Copernicus had been correct: the planets do orbit the Sun.
Picture Credit : Google