discoveries of the XVIII century

1718 — Edmund Halley discovered the proper motion of the stars (Sirius, Aldebaran and Arcturus). Halley also drew attention to the "nebulous stars", discussed their possible structure and the causes of the glow. Halley compiled a catalog of them, later supplemented by Durham; the catalog included about two dozen nebulae.
1727 — J. Bradley discovered a one-year aberration (20.25"), and the fact of the Earth's movement received direct experimental confirmation. The first cosmogonic hypotheses began to appear. William Whiston suggested that the Earth was originally a comet that collided with another comet, after which the Earth began to rotate around an axis, and life appeared on it; Whiston's book "A New Theory of the Earth ..." (Eng. A New Theory of the Earth) received favorable reviews from Isaac Newton and John Locke. The great Georges Buffon also attracted a comet, but in his model (1749), the comet fell on the Sun and knocked out a jet of matter from which the planets were formed. Although the outraged church forced Buffon to renounce this hypothesis in writing, his treatise aroused great interest and was even republished in 1778. Catastrophic hypotheses appeared later (Fai, Chamberlin and Multon, Jeans and Jeffries). Interesting thoughts were contained in Ruger Boskovich's book "The theory of Natural Philosophy, reduced to a single law of forces existing in nature" (1758) — the structural infinity of the Universe, dynamic atomism, the possibility of compression or expansion of the Universe without changing physical processes in it, the existence of interpenetrating, but mutually unobservable worlds, etc.
1755 — philosopher Immanuel Kant publishes the first theory of natural cosmogonic evolution (without catastrophes). Stars and planets, according to Kant's hypothesis, are formed from clusters of diffuse matter: a star appears in the center, where there is more matter, and planets appear on the outskirts. The mathematical basis of the hypothesis was later developed by Laplace. English self-taught astronomer Thomas Wright was the first to suggest that the universe consists of separate "star islands". These islands, according to Wright's model, revolve around a certain "divine center" (he, however, admitted that there could be more than one center). Wright, as well as Swedenborg and later Kant, considered nebulae as distant star systems.
1757 — the first determination of the masses of planets without satellites (A. Clerault). Hollande creates the first achromatic (three-lens) lens, refuting Newton's skepticism in this regard.
1766 — Johann Titius discovers the hitherto unexplained law of planetary distances; the law became widely known after the works of Johann Bode (1772).
1771 — the expedition of Peter Simon Pallas discovers "Pallas iron" in Siberia.
1784 — J. Goodrake suggested that the variable brightness of Algol is caused by eclipses from another component of this binary star.

By the end of the XVIII century, astronomers received powerful research tools — both observational (improved reflectors) and theoretical (celestial mechanics, photometry, etc.). The development of methods of celestial mechanics continued. As the accuracy of observations increased, deviations of the planets' motion from Keplerian orbits were revealed. The theory of perturbation accounting for the many—body problem was created by the efforts of Euler, A. Clerault, Lagrange, but above all by Pierre Simon Laplace, who investigated the most complex cases, including the most obscure problem — the stability of the system. After Laplace's work, the last doubts disappeared that Newton's laws are sufficient to describe all celestial movements. Among other things, Laplace developed the first complete theory of the motion of Jupiter's moons, taking into account the mutual influence and disturbances from the Sun. This problem was very urgent, since it was the basis of the only known accurate method of determining longitude at sea, and the previously compiled tables of the positions of these satellites became obsolete very quickly.

William Herschel

An important role in the development of astronomy was played by the great English scientist of German origin William Herschel. He built reflectors unique for that time with a mirror diameter of up to 1.2 m and masterfully used them. Herschel discovered the seventh planet — Uranus (1781) and its satellites (1787), rotating "in the wrong direction" (1797), several satellites of Saturn, discovered seasonal changes in the polar caps of Mars, explained the stripes and spots on Jupiter as clouds, measured the rotation period of Saturn and its rings (1790). He discovered that the entire Solar system is moving towards the constellation Hercules (1783), while studying the spectrum of the Sun discovered infrared rays (1800), established the correlation of solar activity (by the number of spots) and terrestrial processes — for example, wheat harvest and prices for it. But his main occupation for all thirty years of observations was the study of stellar worlds. He has registered over 2,500 new nebulae. Among them were doubles and multiples; some were connected by jumpers, which Herschel interpreted as the formation of new star systems. However, at that time this discovery was ignored; interacting galaxies were rediscovered already in the XX century. Herschel was the first to systematically apply statistical methods in astronomy (introduced earlier by Michel), and with their help concluded that the Milky Way is an isolated stellar island that contains a finite number of stars and has a flattened shape. He estimated the distances to nebulae at millions of light-years. In 1784, Herschel noted that the world of nebulae has a large—scale structure - clusters and belts ("layers"); now the largest belt is considered as the equatorial zone of the Metagalaxy. He explained the variety of forms of clusters and nebulae by the fact that they are at different stages of development. Some round-shaped nebulae, sometimes with a star inside, he called planetary and considered clusters of diffuse matter in which a star and a planetary system are formed. In fact, almost all the nebulae he discovered were galaxies, but in essence Herschel was right — the process of star formation is happening today.