# Astronomy in the 18th century

After Newton's time, astronomy branched out in various directions. With the law of universal gravitation, the old problem of planetary movement was re-studied as celestial mechanics. The refinement of the telescope allowed the exploration of planets' surfaces, the discovery of many weak stars and the measurement of stellar distances.

The most appropriate measurement system was triangulation or parallax, which consists of making two observations of the same object in different places and at the same time. The observed object will appear to move with respect to the starry background according to its distance. By calculating the angle of displacement and knowing the distance that separates the two observation points you can find the distance to the object.

The realization of the parallax required the use of precise time measurement systems, as well as accurate measurement of geographical distances, this was only achieved when mainly shipping needs led to the development of more accurate chronometers and the science of cartography.

In 1718 the English astronomer Edmund Halley (who had already calculated the elliptical orbit of "his" comet, in 1682), discovered that three of the brightest stars - Sirius, Proción and Arthur - were not in the position recorded by astronomers Greeks Halley concluded that the stars were not fixed in the sky, but were moving independently. The movement is very slow and so imperceptible that, until the telescope could be used, they seemed to be fixed.

In 1785, Herschel suggested that the stars were arranged lenticularly in the sky. If we look at the Milky Way, we see a huge number of stars; but when we look at the sky at right angles to this wheel, we see relatively less number of them. Herschel deduced from this that the celestial bodies formed a flattened system, with the longitudinal axis in the direction of the Milky Way. Today we know that, within certain limits, this idea is correct, and we call our star system Galaxy, another term used to designate the Milky Way (galaxy, in Greek, means "milk").

Herschel tried to assess the size of the Galaxy. The count of star samples at different points of the Milky Way allowed Herschel to estimate that there should be about 100 million stars in the entire Galaxy. And by the values ​​of its brightness he decided that the diameter of the Galaxy was about 850 times the distance to the bright Sirian star, while its thickness corresponded to 155 times that distance.

For his part, the French mathematician and astronomer Joseph Louis Lagrange directs the commission for the establishment of a new system of weights and measures, the Decimal Metric System). In 1788 he published "Analytical Mechanics", which will serve as the basis for future astronomical research. Among his research in astronomy, the calculations of the moon's liberation and the movements of the planets also stand out.

Also during this century, Charles Messier publishes the valuable catalog of celestial objects with a foggy appearance that he collected from 1758 to 1784. Kant attributes in 1755 the genesis of the solar system to a mechanical process. Lagrange studies in 1788 the well-known problem of the three bodies and some special cases with solution. Laplace publishes in 1799 his Celestial Mechanics and discovers the invariability of the major axis of the planetary orbits.

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