Lecture Notes: An Introduction to Astronomy (A 111) - Cairo University

Astronomy, Space Science, & Meteorology Dept. General Prog.; 1st year students An Introduction to Astronomy A 111 Lecture 1: Basic Concepts Pro...

Astronomy, Space Science, & Meteorology Dept. General Prog.; 1st year students An Introduction to Astronomy A 111 Lecture 1: Basic Concepts Prof. Dr. Zainab Awad Some material is based on notes by Prof. Alaa Fouad Lecture 1 Basic Concepts 1  Course content Historical Bg and The solar Basic Concepts system Positional The Sun Astronomy Earth – Moon Stars System Observation tools Galaxies Lecture 1 Basic Concepts 2  In this lecture Astronomy in Old Civilization Models of the Universe Birth of Modern Astronomy Galileo Kepler Newton Our Place in the Universe Lecture 1 Basic Concepts 3  Important Remark Astronomy Astrology Space science Meteorology the study of the the predictions the study of the is the study of universe with of the future environment the changes in all of its knowing the surrounding the the Earth’s components of motion of Earth which atmosphere celestial planets, Moon will affect the (upper and objects, matter and Sun with artificial lower) and its and radiation. respect to the satellites influence on the the oldest of zodiac. orbiting us. weather and the natural Never consider Includes also climate change. sciences. as science. determining satellite orbits Lecture 1 Basic Concepts 4  Astronomy in Old Civilization  Ancient Egyptian ‫سقف معبد دندرة‬ ‫جدارية أحد املقابر‬ ‫الإةل نووت و رحةل الشمس‬ Lecture 1 Basic Concepts 5  Astronomy in Old Civilization  Arab and Islamic Era ‫املراصد الفلكية‬ ‫الهجزة الفلكية‬ ‫اسامء النجوم‬ ‫احلساابت الفلكية‬ Lecture 1 Basic Concepts 6  Models of the Universe Geocentric model Heliocentric model The Earth lays at the The Sun lay at the center center of the universe and of the universe and that that all other bodies move all other bodies move around it. around it. Lecture 1 Basic Concepts 7  Models of the Universe Geocentric model (Ptolemaic view) Two ideas supported it Ptolemy  The Sun appears to revolve around Earth once per day, same as for planets and other stars fixed in the bg.  Earth seems unmoving; it feels solid, stable, and stationary. Lecture 1 Basic Concepts Geocentric model 8  Models of the Universe Geocentric model (Ptolemaic view)  Good approximation to the orbits of the Sun and the Moon  could not account for  Variations in planetary brightness  Planetary retrograde (backward) motion  Suggest more complex model Lecture 1 Basic Concepts Complex Geocentric model 9  Models of the Universe Heliocentric model (Copernican revolution) Aristarchus was the first to mention Copernicus this idea, but didn’t succeed in spreading it. Copernicus rediscovered it in the 16th century. Succeeded in explaining the planetary retrograde (looping) motion. Lecture 1 Basic Concepts Heliocentric model10  Retrograde Motion Apparent motion (not real) Outer planets seem to move backward (i.e. from Earth cannot seen for Venus, but for Mars +) Due different orbital velocity of planets f e c d f d c e Lecture 1 Basic Concepts Retrograde motion 11  Birth of Modern Astronomy Galileo Italian mathematician and philosopher The 1st to use telescopes to achieve new insights into the universe. Remember: Galileo did not invent telescopes Lecture 1 Basic Concepts 12  Birth of Modern Astronomy: Galileo Revolutionary work Io Europa Ganymede Calisto Lunar craters Sunspots and Jupiter 4 moons solar rotation Lecture 1 (Galilean moons) Basic Concepts 13  Birth of Modern Astronomy Kepler  German mathematician and astronomer  Used real observations of Mars to describe the motion of planets  His work on the laws for planetary motion is based on the heliocentric model  He “empirically = no mathematical derivation” established 3 laws to describe the motion of the planets. Lecture 1 Basic Concepts 14  Kepler’s laws of planetary motion First law  deals with the shapes of the planetary orbits.  states that “the orbital paths of the planets are elliptical (not circular), with the Sun at one focus”. Lecture 1 Basic Concepts 15  Ellipse geometry longest Shortest distance distance Lecture 1 Basic Concepts 16  Changing eccentricity (e: 0-1) Circle Flatter Lecture 1 Basic Concepts 17  Kepler’s laws of planetary motion Second law  deals with the speed of the planet in its orbit.  states that “An imaginary line connecting the Sun to any planet sweeps out equal areas of the ellipse in equal intervals of time”. Lecture 1 Basic Concepts 18  Kepler’s laws of planetary motion Third law  Relates the size of a planet's orbit (a) to its orbital period (P)  P is the time needed for the planet to complete one circle around the Sun.  States that “The squares of a planet's orbital period (p) is proportional to the cube of its semi-major axis (a)”. In Earth’s In AU Lecture 1 Basic Concepts years 19  Kepler’s laws of planetary motion Third law Larger (a)  longer orbital period (p) P2 (Years) = a3 (AU) Lecture 1 Basic Concepts 20  Birth of Modern Astronomy Newton  Neither Copernicus understood why his heliocentric model of the solar system worked nor Galileo explained why the planets orbit the Sun. The British mathematician Isaac Newton developed a deeper understanding of the way all objects move and interact with one another.  Establishment of the Newtonian mechanics; the three laws of motion and the law of gravity. Lecture 1 Basic Concepts 21  Newton’s laws of motion first law  States that “An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless some external force changes its direction of motion” First law of motion Lecture 1 Basic Concepts 22  Newton’s laws of motion first law  The Force is the action on an object that causes its momentum to change.  The rate at which the momentum changes is numerically equal to the force.  The inertia is the tendency of an object to continue in motion at the same speed and in the same direction, unless external force acted upon it.  An objects mass is a good indicator on its inertia. The greater the object's mass, the more inertia it has, and the greater is the force needed to change its state of motion. Lecture 1 Basic Concepts 23  Newton’s laws of motion second law  states that “the acceleration of an object is directly proportional to the applied force and inversely proportional to its mass” Units: Newton = kg x (m/s2)  The acceleration is the rate of change of the velocity of an object. Lecture 1 Basic Concepts 24  Newton’s laws of motion third law  States that “forces cannot occur in isolation; if body A exerts a force on body B, then body B necessarily exerts a force on body A that is equal in magnitude, but oppositely directed.” Lecture 1 Basic Concepts 25  Newton's law of gravity  The gravity is the attractive effect that any massive object has on all other massive objects.  The gravity is  A force that acts at a distance  Decreases as the distance increases  Obeys an inverse-square law F α r-2 Lecture 1 Basic Concepts 26  Newton's law of gravity  Every particle attracts every other particle in the universe with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers. At balance: where Lecture 1 Basic Concepts 27  Kepler’s 3 rd law in light of Newton's gravity  Introducing the gravity modified Kepler’s 3rd law by introducing the effect of the mass of the system as follows 4p2 P2 (Years) = a3 (AU) G*Msystem where: Note: sometimes the mass of the system is reduced to be the mass of the large body e.g. Jupiter and one of its small moons Lecture 1 Basic Concepts 28  Our Place in the Universe Supercluster (Vergo) Local group The Milky Way Orion small arm Solar system Lecture 1 Basic Concepts 29  Distances in the Universe Parsec (pc) The distance to an object of parallax 1” 1 pc ~ 206265 AU = 3.26 Ly Light Year (Ly) The distance light travels in a year Astronomical unit (AU) Average distance between Earth and the Sun 1 AU = 93M miles ~ 149.5M km Lecture 1 Basic Concepts 30  Parallax The parallax is the apparent displacement or the difference in apparent direction of an object as seen from two different points not on a straight line with the object. D (pc) = 1 / p(“) Lecture 1 Basic Concepts 31  Parallax The parallax is the apparent displacement or the difference in apparent direction of an object as seen from two different points not on a straight line with the object. Lecture 1 Basic Concepts 32 Lecture 1 Basic Concepts 33