Astronomy is the first science. It was developed in the West by the Egyptians, Babylonians, and Greeks, and in the East by the Chinese. This early history is dicussed in the books by Evans (1998) and Neugebauer (1969). The first book is about how the Babylonian and Greek astronomers carried out their observations. The second book describes the nature of Egyptian, Babylonian, and Greek astronomers to the time of Ptolemy.
The Western classics of astronomy are easily found. None of these books is easy to read. The books by Ptolemy and Copernicus are composed of long discussions of Euclidean geometry and the methods of calculating trigonometric function—specifically the numerical relationship between the cord of a circle and the angle subtending that cord. The works of Newton, which are translated into English from Latin, are difficult to read because Newton insisted on recasting his mathematics into the form employed by ancient Greek mathematicians.
Many of the groundbreaking works in astronomy in modern times are available through the SAO/NASA Astrophysics Data System (ADS). Some of the classic papers listed below are the early catalogs created by the Harvard College Observatory, particularly the Henry Draper Catalog, which is a suvey of the spectra of 225,300 stars.
Evans, James. The History and Practice of Ancient Astronomy. New York: Oxford University Press, 1998.
Neugebauer, O. The Exact Sciences in Antiquity. New York: Dover Publications, 1969.
Copernicus, Nicolaus. On the Revolution of Heavenly Spheres. Translated by Charles Glenn Wallis. Amherst, New York: Prometheus Books, 1995.
Kepler Johannes. Epitome of Copernican Astronomy and Harmonies of the World. Translated by Charles Glenn Wallis. Amherst, New York: Prometheus Books, 1995.
Newton, Isaac. The Principia. Translated by Andrew Motte. Amherst, New York: Prometheus Books, 1995.
Ptolemy, Claudius. Almagest. Translated by G.J. Toomer. Princeton, New Jersey: Princeton University Press, 1998.
Pickering, Edward C. “The Draper Catalogue of Stellar Spectra.” Annals of the Astronomical Observatory of Harvard College 27 (1890): 1–388.
Cannon, Annie J., and Pickering, Edward C. “The Henry Draper Catalogue” Annals of the Astronomical Observatory of Harvard College 91 (1918): 1–290; 92 (1918): 1–309; 93 (1919): 1–271; 94 (1919): 1–299; 95 (1920): 1–315; 96 (1921): 1–235; 97 (1922): 1–261; 98 (1923): 1–259; 99 (1924): 1–272; 100 (1936): 1–226.