Selected Topics in History of Science Some history of astronomy and calendar Lecture 2 7 March 2024 Astronomy – highly sophisticated an allowing mathematization ● Subject matter: Sun, Moon, planets, stars ● Stable and simple – compared to physics, biology, chemistry ● Early observations: Bronze Age – images of the Sun and the Moon, perhaps also some stars ● Evidence: Stonehenge – principal axis in the direction of sunrise at summer solstice (winter s.; vernal / spring and autumn equinox) ● (churches in Europe: altar facing East) Astronomy in Babylonia ● Pattern of celestial omens (bad signs): harvest, epidemic, … ● By 2000 BCE („before Christian era“) - established (pre-astrology) ● Social function of astronomy in Babylonia – stability in taking and keeping the records (clay tablets – copied, when broken) ● Observations by temple astronomers – visible to the naked eye – „re-emergence“ of Venus (not too close to the Sun) – „retrograde“ motion of Jupiter ● Sophisticated prediction of planetary motions: 300 BCE Astronomy in Ancient Greece ● Recorded in literature, e.g. Homer ● Constellations: Orion, the Great Bear (Velký vůz), Pleiades ● Lunar eclipse: observation that the Earth's shadow is round, hence Earth is a sphere (Aristotle, On the Heavens, 350 BCE) ● Measuring the circumference of the Earth: 72 thousand kilometers, but given in stadia (now: 40 thousand km), Eudoxos of Cnidus ● Eratosthenes: measuring the the circumference of the Earth at summer solstice (Aswan / Syene and Alexandria): 45 thousand km https://www.nagwa.com/en/videos/287108723870/ Ancient observations: studying the planets Planets known from Ancient times: ● Mercury ● Venus ● Mars ● Jupiter ● Saturn discovered since the 18th century: Uranus: 1781 Neptune: 1846 Pluto: 1930 (until 2006) Claudius Ptolemaeus, Almagest (2nd c. BCE) ● geocentric model: the Sun, the Moon, and all planets orbit a stationary Earth ● heavenly bodies must move in the most perfect possible fashion – circles ● retain such motion and still explain the erratic apparent paths of the bodies: shift the centre of each body's orbit (deferent) from Earth—accounting for the body's apogee and perigee—and add a second orbital motion (epicycle) to explain retrograde motion. Regiomontanus, 1496, Epitome ● Johannes Müller von Königsberg (1436-1476) ● Cardinal Bessarion asked R.'s teacher Peuerbach to remedy the problems in the translation of Almagest by George of Trebizond (1450) ● Regiomontanus finished the work 1462 (print: 1496) ● An alternative to Ptolemy's model of the orbits of Mercury and Venus – the „geometric key“ for Nicolaus Copernicus (1473-1543) to reorient planetary motions around the Sun (not the Earth) Calendar and its reforms ● Solar calendar: in step with the seasons ● Lunar calendar: movable religious feasts (Easter etc.) ● Roman republican calendar: probably only 10 months / 304 days ● Martius, Aprilis, Maius, Juniiue, Quintilis, Sextilis, September, October, November, December (M, A, M, J, 5, 6, 7, 8, 9, 10) ● Adding January and February (7th c. BCE, changed 5th c. BCE) ● Continuing confusion; Julian calendar: 46 BCE (Julius Caesar) ● Tropical year: 365 and ¼ of a day – nowadays: leap year Further calendar reforms ● Julian calender: 365.25 days, but correct value: 365.242199 ● error 11 minutes 14 seconds per year → ¾ of a day in 100 years, and by 1545, the vernal equinox (20 or 21 March) was 10 days later ● 1572: Pope Gregory XIII agreed to the proposal of Christopher Clavius (1537-1612) ● 24 February 1582: bull issued determining that 5 October became 15 October that year; accepting 365.2422 days ● Introduction of a leap year: every 4 years, but not in the years divisible by 100, unless divisible by 400 (1900 not, 2000 leap) Adoption of Gregorian calendar ● Draft of the bull: an astronomer and scientist, Christopher Clavius, who also introduced decimal comma ● Accepted by Kepler and Brahe (science) ● Accepted by Catholics – issued by Pope ● Not accepted by Protestants; only gradually ● Jacob Bernoulli: born late December 1654, or early January 1655 ● All of Europe eventually accepted, Russia only in 1918 – (Great October Revolution – 7 November 1917) Johannes Kepler (1571-1630) astronomer Laws of planetary motions: ● Planets move in elliptic orbits ● „area law“ ● „harmonic law“ made his living as an astrologer ● Snowflakes ● Tycho Brahe Mysterium Cosmographicum Five regular solids: ● Tetrahedron (4 triangles) ● Cube (6 squares) ● Octahedron (8 triangles) ● Dodecahedron (12 pentagons) ● Icosahedron (20 triangles) What do we want to achieve? Harmony – easy model - four or five elements; everything in small numbers - … Predictions - regularities occuring again predicting eclipses (lunar / solar) Another take on cosmology - Spheres „cloud nine“ Heaven beyond these spheres Going beyond the sphere