Science : organize knowledge, condense to testable theories, predict new phenomena that are (or can be)  verified.

Science does not concern itself with questions that cannot be tested (e.g. spiritual issues). However, at the frontiers of science there are plenty of questions in a very fuzzy state, and scientists keep harping at them to turn them into testable hypotheses (story about the drunk at the lamp post).
Why do we do it?  1) we are curious, 2) we find it useful (through this knowledge we can harness Nature and put it to use through numerous technological applications - but that is mainly the realm of Technology).

Scientific method: 1) question or puzzle, 2) hypothesis and prediction, 3) verification or nullification by experiment and measurement, 4) formulation of a theory or a law of nature that explains and interprets the facts as simply (and as beautifully) as possible and that makes further testable predictions.

In science measurement that is continuously refined plays an essential role. When new facts or measurements do not fit a theory, the theory is evolved or refined to a better theory, or sometimes completely overthrown. As an example, consider the evolution of the concepts and theories about the universe we live in:

• The universe as interpreted by myths and religions.
• Primitive observations: Flat earth to spherical earth (Aristoteles, Eratosthenes)
• How to measure the sizes of earth and moon (Greeks 200-300 BC). A more general setup (you need a .ps file viewer) or see this link.
• How to measure the size of the sun (and a project to do at home, see below).
• How to measure the distances between earth-moon and earth-sun (Greeks 200-300 BC)
• The motion of planets baffled the greeks. The earth centered geocentric universe emerged (Ptolemy 100-170 AD). Remarkable that it almost explains the motion of planets, but fundamentally wrong! (1400 years). Finally contradicted by more precise measurements, and discovery of more planets that did not fit the theory.
• Revolution: Sun centered heliocentric universe (Copernicus 1473-1543). But, still wrong (circular orbits), and incomplete.
• Kepler's universe - (1571-1630) ellipses. Kepler's laws of planetary motion (see also this).
• Galileo's (1564-1642) discoveries shattered  Ptolemy's universe. Birth of the scientific method.
• The nine planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto , and more


Relative sizes of the nine planets

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• The correct full explanation of planetary motion is due to Newton who proposed the law of universal gravity (to be explained later, see simulation1 ,  simulation2 and simulation3 ).
• But there is a lot more beyond the solar system, as discovered through large telescopes, in prticular by Hubble in the 1920's: The solar system is only a tiny part of the Milky Way galaxy, and there are billions of galaxies in the universe. The universe is in motion and in a state of expansion. We are far from being special or being at the center of the universe. Furthermore, as interpreted through Einstein's theory of General Relativity, it all was created in a Big Bang (a theory which makes predictions that are verified).

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