Ancient cultures were fascinated with the heavens, and astronomy developed into one of the earliest sciences as these cultures formalized their studies of the night skies. Much of the work of these early astronomers focused on observations and predictions of the motions of objects visible to the naked eye, and some cultures erected large monuments that likely have astronomical significance. Early Jewish, Chinese, and other cultures established calendars based on observations and calculations of the Moon cycles, and these calendars became essential for determining seasons and knowing when to plant crops. By the year 1000 b.c.e. Chinese astronomers had calculated Earth's obliquity to the ecliptic, or the tilt of the planet's axis relative to the orbital plane about the sun. Early astronomers included the study of astrology, celestial navigation, and time calculations such as making calendars in their field, but modern professional astronomy is equivalent to astrophysics, with branches of observational and theoretical astronomy.
A revolution in astronomy and science was marked by Polish astronomer Nicolaus Copernicus's (1473-1543) proposal in his book De revolutioni-bus orbium coelestium (On the revolutions of the heavenly spheres), where he proposed that the Earth is not the center of the universe as most previous scientists believed, but that the Earth and other planets orbit around the sun. Astronomy changed from its classical period to its modern period with the invention of the telescope in the late 16th century. Some of the early Islamic scholars described the optics of lenses required for telescopes, but the first surviving instruments are from the Netherlands, invented by eye spectacle makers Hans Lippershey and Zacharias Janssen of Middleberg, and Jacob Matius of Alkaamar. In 1602 Galileo Galilei, a physicist from Tuscany, improved on these designs and produced a telescope that earned him the nickname father of modern observational astronomy. These designs were further improved by the English physicist Isaac Newton in 1668.
The German astronomer and natural philosopher Johannes Kepler (1571-1630) further described and refined the laws of planetary motion. Newton further explained these laws in his law of universal gravitation, still used as a general approximation for most gravity-driven processes. Albert Einstein's general theory of relativity more accurately describes gravity, but Newton's laws work for most applications. Further significant advances in astronomy came with the inventions of new technologies, including photography and the spectroscope. Spectroscopic observations of the Sun by Bavarian optician Joseph von Fraunhofer (1787-1826) showed about 600 spectral bands present, which were correlated with different elements by the German physicist Gustav Kirchhoff in 1859. Spectral observations of other stars revealed similar spectral bands, and hence similar compositions to the Sun.
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