Newton and Herschel
In 1642, the year of Galileo's death, one of the greatest scientists in history, Isaac Newton, was born in England. Newton's interests and accomplishments were far ranging, but it was his studies of gravity that were of most importance to the advance of astronomy. Newton discovered that the same gravitational force that causes an apple to drop to the ground from an apple tree keeps the planets in orbit about the sun and holds the universe together.
Newton used his theory of gravity to calculate how the planets should move around the sun. The answers to his equations revealed that the planets should move in elliptical orbits, as Kepler had found from his analysis of Brahe's observational data.
Another scientist in England, William Herschel, became the preeminent telescope builder and astronomer of the late 1700's. Using reflecting telescopes (telescopes that gather and focus light with mirrors rather than lenses) up to 122 centimeters (48 inches) in diameter, Herschel observed the panorama of the heavens in even greater detail.
Much of Herschel's time was spent studying glowing patches of light called nebulae. Astronomers earlier in the 1700's had begun finding these unusual objects, many of which they noted were disk shaped. They assumed that the nebulae were part of the Milky Way, because the Milky Way was then thought to be the entire universe.
Other observations had suggested that the Milky Way itself was shaped like a disk. In a flash of insight, the German philosopher Immanuel Kant had proposed in 1755 that the disk shaped nebulae were other galaxies like the Milky Way but far removed from it. “Island universes” he called them.
Seeking to resolve this question, Herschel scanned the heavens for nebulae and discovered about 2,000 new ones. He determined that many were huge clouds of gas or clusters of stars in the Milky Way. The disklike nebulae, however, appeared only as hazy smudges to his gaze.
Observations by later astronomers revealed that many of the disk shaped nebulae had spiral shapes, but still no stars could be seen in them. So whether the nebulae lay within the Milky Way or were separate galaxies remained an open question for many years.