In 1572, Tycho Brahe saw a new star in the sky, a 'nova' as we say now. This was the first evidence against the ancient view that the heavens were unchangeable.
In 1610, Galileo Galilei saw four 'new stars', actually moons of Jupiter, further evidence that the skies were not, as orthodox rulers claimed, unchangeable.
Stars have an absolute magnitude that is always the same, no matter where they are. They also have a relative magnitude depending on where they are seen from.
Stars have an apparent brightness from a particular viewpoint, with distant objects appearing dimmer than similar objects that are nearer to us.
Relative magnitude follows the inverse square law. Of two stars of identical absolute magnitude, the one twice as far off has a quarter the relative magnitude.
In 1826, Heinrich Olbers posed his paradox: if the universe is infinite, he asked, why is the sky so dark at night? The answer involves the red-shift.
Armand Fizeau was the first to point out that a star, moving away from us, should show an observable Doppler effect red-shift in the spectral absorption lines.
Distant objects have a larger redshift, which is taken by astronomers to indicate that they are travelling away from us faster than nearby objects.
The Hubble constant relates distance to redshift, and tells us how fast the universe is expanding, from which we can deduce that the universe will not collapse.
In 1869, William Huggins used red-shift data to estimate that the star Sirius is moving away from the Earth at about 30 kilometres (20 miles) a second.
Barnard's star is the fastest moving star as it shifts position in the star field: it takes 180 years to move across half a degree of sky, the moon's width.
Cepheid variables are stars that have a frequency of variation which relates to their absolute magnitude, which means we can use them as 'standard candles'.
In 1912, Henrietta Leavitt discovered the period to luminosity relationship for Cepheid variable stars from studies of the Large and Small Magellanic Clouds.
In 1914, Ejnar Hertzsprung estimated distance to the Small Magellanic Cloud using Cepheid variables, as 3000 light years, a lot lower than the current 210,000.
In 1918, Harlow Shapley set out to use Cepheid variables and offered a model for the shape of the Milky Way. He was unaware there are two types of Cepheids.
In 1924, Edwin Hubble used Cepheid variables in a nebula, Messier 31 in Andromeda to show the nebula was some 750,000 light years away, outside our own galaxy.
Spectrum analysis of the absorption and transmission in light from distant objects can reveal a great deal about them and the elements that they are made of.
Astronomy includes the study of cosmology, the branch of science that tries to explain how the universe and everything in it began, using the laws of physics.
In 1782, John Goodricke noticed that the variations in brightness of Algol are periodic and proposes that it is partially eclipsed by a body moving around it.
In 1803, William Herschel found, as John Michell had suggested, that binary stars existed. Michell felt this was more likely than stars being near each other.
In 1863, Richard Carrington discovered that sunspots rotate at different speeds at different latitudes on the sun, revealing that the Sun was in some way fluid.
In 1975, Gerald Smith, Frederick Landauer, and James Janesick use a charge-coupled device to observe Uranus, the first astronomical CCD observation.
Quasars got their name from being quasi-stellar objects. Each is believed to be powered by a black hole, a very dense object, from which light cannot escape.