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Introduction to Astronomy Part 2 — Pale Blue Dot Dr Adrian Jannetta FRAS Northumberland Astronomical Society Dr Adrian Jannetta FRAS Introduction to Astronomy Summary This presentation deals with history of astronomy from antiquity to the start of the modern age (the era of Newton’s discovery of the laws motion and gravity). Antiquity Before 500BC. Periodic motion of the Sun and Moon. Calendars and time keeping developed. Classical 500BC to 1400AD. Measurements of the sky. Mathematics, geometry and models of the universe. Renaissance 1400 - 1650. Collection of data, refinement of models. Invention of the telescope. Modern 1650 - present. Discovery of physical laws governing the universe. Dr Adrian Jannetta FRAS Introduction to Astronomy Astronomy in antiquity Early cultures identified celestial objects with supernatural gods and spirits. Prehistoric astronomy was concerned with: Daily motion of the Sun, moon and stars. Seasonal variations of constellations. Phases of the moon. Eclipses. It is likely that earliest astronomers were ‘priests’ and that their understanding of the world around them was linked with interpreting the movements in the heavens and therefore the intentions of the gods and spirits. Astronomy and astrology were synonymous. Dr Adrian Jannetta FRAS Introduction to Astronomy Early observatories Some ancient structures, such as Stonehenge, show a distinct north-south alignment. These ancient structures probably fulfilled dual astronomical and religious purposes. Dr Adrian Jannetta FRAS Introduction to Astronomy Devising a calendar Lunar and solar calendars Reliable calendars became an essential feature in many agricultural civilisations. Common calendar systems have included: Solar The year is divided into 365 or 366 days. This is the period repetition of the seasons, or between the Sun completing one complete cycle across the sky. Lunar The year is divided into 12 lunar months of 354 or 355 days, with a month being the period between new moons. This calendar slips by 11 or 12 days per year with respect to the seasons. Luni-solar A lunar calendar with some years containing 13 lunar months to stop the seasons getting out of sync. The Islamic Calendar is based on a lunar calendar. Each month begins with a sighting of the hilal (young crescent moon after new moon). Dr Adrian Jannetta FRAS Introduction to Astronomy Devising a calendar The thirteen stone towers at Chankillo, Peru were erected around 2,400 years ago. The first and last towers mark the positions of the sun at the summer and winter solstices. Dr Adrian Jannetta FRAS Introduction to Astronomy Devising a calendar Heliacal rising Many stars in the sky are visible for only part of the year. At other times they are too close to the Sun. A heliacal rising of a star is the day it becomes visible in the dawn sky, just before sunrise, after a period of invisibility. Dr Adrian Jannetta FRAS The heliacal rising of Sothis (now called Sirius) meant the annual flooding of the Nile was about to start. This event, although inaccurate, marked the start of the new year in the Egyptian calendar. Introduction to Astronomy Classical astronomy Greece Greek astronomers made observations of the sky. They devised models and made predictions. Logic and mathematics were tools; it was the beginning of the scientific method. Pythagoras (c500BC) believed the Earth was round because a sphere was deemed to be a perfect shape. Aristotle (c300BC) saw that the shadow of the Earth on the Moon during a total eclipse was always curved. This implied the Earth was spherical. Eratosthenes (c240BC) estimated to the circumference of the spherical Earth to within 2% of its modern value. Dr Adrian Jannetta FRAS Introduction to Astronomy Classical astronomy The size of the Sun and Moon Aristarchus of Samos attempted to calculate the sizes of the Sun and Moon their distances from the Earth. For example: At half-moon (first or last quarter) then a right-angle (90 degrees) is formed by the Sun-Moon-Earth line. Aristarchus estimated the angle ϕ to be 87 degrees. This implied the Sun to be nearly 20 times more distant than the Moon. The Sun and Moon are roughly the same size in the sky, the implication was also that the Sun was 20 times the diameter of the Moon and also much larger than the Earth. Such measurements gave credence to the Sun-centred (heliocentric) model of the solar system. Dr Adrian Jannetta FRAS Introduction to Astronomy Classical astronomy Ptolemy’s Almagest A significant amount of astronomical knowledge from Greek astronomy was embodied in the work of Ptolemy’s Almagest. Ptolemy advocated the geocentric system and an unmoving Earth. The Almagest was a hugely influential collection of books. It was adopted later by Islamic astronomers and by the Catholic Church. Claudius Ptolemy (c85 — c165) Dr Adrian Jannetta FRAS Scribes stopped copying other works; knowledge of earlier astronomers was lost. Introduction to Astronomy Classical astronomy Geocentric model The geocentric model of the solar system flourished because it successfully accounted for the motion of the Sun, Moon and planets. Stars were embedded in a distant, outer celestial sphere. It became increasingly complicated as it attempted to more accurately match the real positions of the planets. Dr Adrian Jannetta FRAS Introduction to Astronomy Classical astronomy Retrograde motion of the planets The planets generally travel eastwards among the constellations (prograde motion). The planets also stop to perform a loop in the sky, temporarily moving west (retrograde) before continuing eastwards again. Retrograde motion is particularly noticeable for Mars and Venus, but all planets display it. Any model of the solar system had to account for this motion. Dr Adrian Jannetta FRAS Introduction to Astronomy Click to start Courtesy of UNL Astronomy Education Classical astronomy Islamic astronomy Islamic astronomers salvaged many of the manuscripts from earlier eras (8th — 15th centuries). These were later passed back to western cultures with the spread of Islam. Great advances in mathematics and science. Many astronomical terms and star names and terms come from this era (Altair, Aldebaran, azimuth, etc). Persian astronomer Ulugh Beg built an impressive observatory at Samarkland for observing the heavens. Ulugh Beg (1394 — 1449) Dr Adrian Jannetta FRAS Introduction to Astronomy Renaissance astronomy Rebirth of a sun-centred system Copernicus was a Polish monk (and a diplomat, mathematician, economist, artist and physician). He revived the notion of a sun-centred solar system (heliocentrism) with planets in circular orbits. Heliocentric system was simpler and accounted for the observed motion of the planets fairly well. Nicolaus Copernicus (1473 — 1543) Dr Adrian Jannetta FRAS Heliocentric system was not a perfect fit with observed positions; discrepancies remained because planetary orbits weren’t actually circular. Introduction to Astronomy Renaissance astronomy Heliocentric model Click to start Courtesy of UNL Astronomy Education Renaissance astronomy Religion v Science A sun-centred solar system conflicted with the scriptures. For example: Then spake Joshua to the LORD in the day when the LORD delivered up the Amorites before the children of Israel, and he said in the sight of Israel, Sun, stand thou still upon Gibeon; and thou, Moon, in the valley of Ajalon. Joshua 10:12 Copernicus delayed publication of his life’s work until he was on his deathbed. The first printed editions of his book contained a preface, not authored by Copernicus, which attempted to deflect controversy. It stated that the work probably wasn’t true; it merely provided a practical method for calculating the positions of the planets. As the ‘Copernican’ system spread it received a hostile reception from religious authorities. Dr Adrian Jannetta FRAS Introduction to Astronomy Renaissance astronomy Tycho Brahe was a Danish nobleman and astronomer. An man of eccentric character! A fervent supporter of Ptolemy’s geocentric system. Tycho built an observatory (Uraniborg) on the island of Hven. Tycho Brahe (1546 — 1601) Dr Adrian Jannetta FRAS He was a prolific observer of the planets - perhaps the greatest observer prior to the invention of the telescope. Tycho extensively observed a supernova (an exploding star) in 1572. Introduction to Astronomy Renaissance astronomy Tycho observed the stars and planets. He hoped his observations would disprove the heretical ideas of Copernicus. Tycho was assisted at Uraniborg by his German assistant, Johannes Kepler. Renaissance astronomy Inconstant stars Tycho observed the appearance of a new star in Cassiopeia in 1572. It became as bright as Venus before slowly fading from view. Tycho referred to the star as ‘Nova Stella’. David Fabricius noticed a new star whilst observing Mercury in 1596. The star brightened and then faded from view. Unlike Tycho’s star, it eventually reappeared months later. It was named Mira, meaning ‘wonderful’ or ‘astounding’. Another ‘nova’ was seen in 1604. These discoveries may have motivated astronomers to make accurate star catalogues more systematic observations. Dr Adrian Jannetta FRAS Introduction to Astronomy Renaissance astronomy Fall of the geocentric system The geocentric system was discarded because of the contributions of Kepler and Galileo in the early 17th century. Johannes Kepler (1571 — 1630) Dr Adrian Jannetta FRAS Galileo Galilei (1564 — 1642) Introduction to Astronomy Renaissance astronomy Kepler’s laws of planetary motion Kepler used Tycho’s observations of the planets as evidence of the heliocentric system of Copernicus. His attempts to fit the data to circular orbits failed. Elliptical orbits fit perfectly. Kepler formulated three laws of planetary motion. The orbits of the planets are ellipses. The Sun is at one focus of the ellipse; the other is empty. The radius vector of the planet sweeps out equal areas in equal time periods. The cube of the radius vector r of the planet is proportional to the square of the orbital period T . So that r3 ∝ T 2 . The need for epicycles and deferents was abandoned in his simplified version of the solar system. Renaissance astronomy Kepler’s laws of planetary motion Click to start Courtesy of UNL Astronomy Education Renaissance astronomy Galileo and the telescope Galileo used a telescope to make astronomical observations. He discovered: The moons of Jupiter Mountains and craters on the Moon The Milky Way is comprised of countless stars The phases of Venus Sunspots on the Sun Many of his observations were contradictory to the geocentric system. Galileo published his findings as support for the heliocentric solar system but was later forced to recant his testimony by Church and spent the last part of his life under house arrest. Renaissance astronomy Galileo’s observations Records of Galileo’s observations of Jupiter from Sidereus Nuncius (‘Starry Messenger’) in 1610. The four largest moons are still referred to as ‘Galilean moons’. Modern astronomy Isaac Newton and gravity Isaac Newton developed new mathematics (calculus) to understand problems in physics connected with motion. He identified the rule governing the behaviour of gravity and demonstrated that it accounted for Kepler’s laws. Newton studied light and optics - paving the way for the new discoveries based on the analysis of light. Isaac Newton (1642 — 1727) Dr Adrian Jannetta FRAS Newton’s work heralded a new age of explanation, prediction and discovery. Introduction to Astronomy Pale Blue Dot A portrait of the planets of the solar system composed from images taken by the Voyager 1 spacecraft in 1990. At that time Voyager 1 was 4 billion miles from Earth and 32 degrees above the plane of the ecliptic. American astronomer Carl Sagan described the Earth as a pale blue dot. This image highlights a significant reduction in our perception of the size of the Earth in comparison to earlier eras. Alternatively, it can be taken a step in a growing realisation that the universe is much larger than earlier astronomers could possibly comprehend! Dr Adrian Jannetta FRAS Introduction to Astronomy Further reading P Aughton. The Story of Astronomy. Quercus, 2011. A Aveni. People and the Sky: Our Ancestors and the Cosmos. Thames & Hundson, 2008. H Couper and N Henbest. The History of Astronomy. Cassell Illustrated, 2007. M Hoskin, editor. The Cambridge Illustrated History of Astronomy. Cambridge University Press, 1997. P Moore. Patrick Moore’s History of Astronomy. Harpercollins, 1984. Dr Adrian Jannetta FRAS Introduction to Astronomy