PHYS1160

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University of New South Wales *

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1160

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Astronomy

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Oct 30, 2023

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PHYS1160 LESSON 1 - WHAT IS ASTRONOMY? AN INTRODUCTION INTO HISTORICAL AND MODERN ASTRONOMY. LEARNING CRITERIA 1. Summarise a basic history of astronomy, including the significance of astronomy in ancient cultures. 2. Compare the geocentric and heliocentric models of the Solar System. 3. Describe how Kepler’s laws explain key observable features in our Solar System. 4. Explain why we observe Venus to have phases. 5. Demonstrate an understanding of scale in the Universe and consequently, our place in it. 1.1 WHAT IS ASTRONOMY? Astronomy can be defined as the scientific study of celestial object like planets, stars, galaxies and the Universe. Tools of Astronomy: • Telescopes – Telescopes are used to observe celestial objects. To see more distant objects, you need to capture as much light from that object as possible. • Spacecraft – Spacecraft is used to go above the atmosphere and can be used to study space. • Computers – Computers are used to analyse the data and produce mathematical models which are then compares with observations. Often supercomputers are used due to how complicated the calculations are. • The Internet – Used to share and communicate data. • Laboratories – Laboratories are used to develop instruments for use of telescopes and spacecraft, and to measure fundamental properties needed to interpret astronomical data. 1.2 SCALE OF THE UNIVERSE A unit that is used to measure distances in astronomy is the light year. - The light year is the distance light travels in a year. - Speed of light is 300, 000 km/s - One light year is 9,460,700,000,000 km and about 63,241 astronomical units. 1.3 HUMAN HISTORY AND ASTRONOMY The study of astronomy dates back to the ancient civilisations of Mesopotamia (modern day Iraq). They studied the motions of celestial bodies and developed the sexagesimal system of numbers that we still use today for time and ageless.

In central Nigeria, the orientation of a waxing crescent Moon correlates with the average amount of rainfall at different times of the year. Some cultures built structures with astronomical alignments that still exist today, e.g., pointing to the rising and setting Sun on specific dates, because many cultures used the Sun to gauge the time of day and year. The modern clock can be traced back 4,000 years ago to ancient Egypt, where daytime and night-time were divided into 12 equal parts. By using the positions of particular stars in the sky, the Egyptians created star clocks to estimate the time of night. These were abandoned in 1500 BC for water clocks. 1.4 ANCIENT GREEK ASTRONOMY The Greeks developed ideas about the structure of the Universe which were influential for almost two thousand years. Pythagoras was a famous Greek astronomer. He created postulates that we know are true today including the fact that the Earth is spherical. GEOCENTRIC MODEL Greek philosophers developed the geocentric model which is a model of the universe where the Earth is in the centre.

An Early model proposed by Eudoxus, a pupil of Plato. His model, the Earth was at the centre and there were a series of concentric spheres carrying the Sun, Moon and the planets which an outer sphere carrying the stars. Armillary spheres feature the Earth at the centre with the planets and constellations on surrounding rings. Greek philosophers believed that celestial objects should move in circular paths, because, according to Plato, the circle was the most perfect figure. However through progression of research it was clear that planetary motion couldn’t be described by simple circular motion around the Earth. 1.5 THE PTOLEMAIC SYSTEM This system of explaining planetary motion was proposed by Claudius Ptolemy. He wrote the Almagest which is the only comprehensive work on ancient astronomy that is preserved today. This work includes: • Catalogue of stars believed to be based on the earlier work of Hipparchus. • His model for the planetary motion is based on the geocentric model with even more elaborate system of epicycles. • Tables predicting the positions of the Sun, Moon and Planets. An epicycle is a small circle that carried the planet, with the centre of the epicycle itself moving around the Earth on a larger circle called the deferent. This model was necessary to explain features of retrograde motion of the planet Mars. This is when at times Mars appears to move backwards to the stars compared with normal east to west motion. 1.6 THE COPERNICAN SYSTEM In 1543 a challenge to the Ptolemaic system was made by Nicolaus Copernicus, a Polish astronomer with his book De Revolutionibus Orbium Coelestium. This is the Copernican or the Heliocentric system where the sun is in the centre. This system provided a simpler explanation of the retrograde motion of Mars. It occurs due to Earth catching up and passing Mars as it has a faster orbital motion. Copernicus still used the platonic idea of circular motions, therefore he still needed systems of multiple circles to explain the actual motions of the planets.

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1.7 KEPLERS LAWS The final solution to the problem of planetary motion was made by German astronomer Johannes Kepler. He used the best astronomical observations at the time to determine that orbits of the planets were ellipses and not circles. The elliptical orbit of a planet is shown above. The ellipticity of the orbits of the planets in our Solar System only depart from a circle by a very small amount. Kepler was able to outline three laws of planetary motion. Kepler's 1st law: The planets move in elliptical paths with the Sun at one focus. Kepler's 2nd law: (The law of areas) The line between the planet and the Sun sweeps out equal areas in equal times. This means that, in the figure above, for the areas to be equal, the distance that the planet travels around the arc (outline) of the ellipse much be larger for when the planet is closer.

Kepler's 3rd law: The square of the orbital period of the planet (in years) is proportional to the cube of the semi-major axis of the orbit (in astronomical units). Kepler's 2nd law has the implication that the planet moves fastest when near the Sun. Kepler's 3rd law implies that the planets furthest from the Sun move slower than those closer in. 1.8 GALILEO AND THE TELESCOPE The first use of the telescope to study the skies was done by Galileo in 1609. However he didn’t the telescope rather he heard about the invention and made one of his own. The observations Galileo made provided significant proof in favour of the heliocentric model developed by Copernicus. One of the observations Galileo made was of the moons of Jupiter of which he discovered four. These are known now as the Galilean satellites. As he watched the motion of these objects from night to night, he could see them moving around Jupiter. They orbit Jupiter with periods of between about 2 and 17 days. The existence of bodies orbiting Jupiter was inconsistent with the Ptolemaic idea that the Earth was the centre of all orbits. 1.9 THE PHASES OF VENUS Another discovery Galileo made was the phases of Venus. Seen through the telescope he could see that Venus went through a full cycle of phases, just like the phases of the Moon. It went from a thin crescent, to a fully illuminated phase, and it also changed in size being largest in its crescent phase, and smallest when fully illuminated.

The Phases of Venus can only be understood on the Copernican model as shown below. In the Ptolemaic model Venus never goes behind the Sun and so can never be fully illuminated. 1.10 NEWTON AND GRAVITATION Isaac Newton came up with his Universal Law of Gravitation. Newton showed that the elliptical orbits that Kepler had found were exactly what was expected if the planets were attracted to the Sun by a universal force of gravitation that acts between any two objects in the Universe and is dependent on their mass. He drew the picture on the left showing that if an object, such as a cannonball, were fired with sufficient speed from a high mountaintop, it would not fall to Earth because the curvature of the Earth would fall away at the same rate as the cannonball would fall, and thus it would complete an orbit of the Earth.

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1.11 STELLAR PARALLAX The stellar parallax states that a nearby star should change its position relative to the background of more distant stars as Earth orbits the Sun. The failure to observe such parallax was used as an argument against the Copernican theory. However, the reason it could not be measured was mostly because the stars are so distant, meaning that the parallax movement is extremely small. The first successful measurement of stellar parallax was made in 1838 by Friedrich Bessel. He measured the parallax of the star 61 Cygni and obtained a distance of 9.8 light years. Other measurements of stellar parallax rapidly followed, and Alpha Centauri was found to be the closest star system to our own Sun at a distance of about 4.3 light years. These measurements helped confirm that the Earth really does orbit around the Sun, as well as what had long been expected: that the stars were other suns. The distances measured were consistent with stars being objects as bright as the Sun but seen from much greater distances. 1.12 THE MILKY WAY GALAXY With the naked eye we can only see at best a few thousand stars, but even with Galileo's telescope revealed that the Milky Way was made up of many stars. As telescopes became more powerful, we realised that the number of stars in our Milky Way Galaxy was in the billions. In fact, it is now thought there are several hundred billion of stars in the Milky Way. In the early 20th century, astronomers began to realise that our Milky Way Galaxy was itself just one among many. They found that objects that had been called spiral nebulae, were not actually nebulae (clouds of gas), but were other galaxies comparable to our own. 1.13 BILLIONS OF GALAXIES With modern telescopes we can see galaxies out to distances of billions of light years, which means there are many billions of galaxies within our Universe. In the 2000 years or so since the time of the ancient Greeks, we have gone from a Universe that had the Earth at its centre to a universe in which the Earth is just one planet orbiting a fairly ordinary star, which itself is just one of hundreds of billions of stars in our Galaxy, with our Galaxy being just one of hundreds of billions within the Universe as a whole.