The explosion of the horizons

The recognition of the heliocentric motion of planets led to the geometrical determination, by Copernicus and Kepler, of the relative planetary distances. Completed by the determination of the true distance from the Earth to the Sun, by geometrical methods, this gave the total scale of distances in the planetary realm. In the years 1830–1840, the same geometrical methods could be applied to stars, the angular resolving power of telescopes having then greatly improved. This established the stellar world up to some hundreds of light-years, with well-determined distances. To go beyond, to the distant stars and to the galaxies, indirect methods were used, allowing a rather accurate gauge of the depths of the universe, up to some billions of light years. The penetration in depth leads naturally to the penetration back in time. The problem of timescales in the universe is one which is still debated, as the cosmologies presently available still contain many assumptions about the very notion of time. Neither the horizons nor the origins are yet within reach; we are merely extrapolating what is known; it is no longer pure astrophysics, but close to metaphysics.