Origin of Modern Astronomy Early Ideas of the

Origin of Modern Astronomy

Early Ideas of the Heavens • Ancient Greek Astronomers – Through the use of models and observations, they were the first to use a careful and systematic manner to explain the workings of the heavens – Limited to naked-eye observations, their idea of using logic and mathematics as tools for investigating nature is still with us today – Their investigative methodology is in many ways as important as the discoveries themselves History of Astronomy 2

Early Ideas of the Heavens • The Shape of the Earth – Pythagoras taught as early as 500 B. C. that the Earth was round, based on the belief that the sphere is the perfect shape used by the gods – By 300 B. C. , Aristotle presented naked-eye observations for the Earth’s spherical shape: • Shape of Earth’s shadow on the Moon during an eclipse • A traveler moving south will see stars previously hidden by the southern horizon History of Astronomy 3

Early Ideas of the Heavens • The Size of the Earth – Eratosthenes (276 -195 B. C. ) made the first measurement of the Earth’s size – He obtained a value of 25, 000 miles for the circumference, a value very close to today’s value – His method entailed measuring the shadow length of a stick set vertically in the ground in the town of Alexandria on the summer solstice at noon, converting the shadow length to an angle of solar light incidence, and using the distance to Syene, a town where no shadow is cast at noon on the summer solstice History of Astronomy 4

Early Astronomy Ancient Greeks Astronomy is the science that studies the universe. It includes the observation and interpretation of celestial bodies and phenomena. The Greeks used philosophical arguments to explain natural phenomena. The Greeks also used some observational data.

Early Astronomy Ancient Greeks Geocentric Model = Ptolemy Greek Astronomer • In the ancient Greeks’ geocentric model, the moon, sun, and the known planets—Mercury, Venus, Mars, and Jupiter—orbit Earth. Heliocentric Model = Nicolaus Copernicus • In the heliocentric model, Earth and the other planets orbit the sun.

Early Astronomy Ancient Greeks Ptolemaic System • Ptolemy created a model of the universe that accounted for the movement of the planets. • Retrograde motion is the apparent westward motion of the planets with respect to the stars. March Sept. April May Aug. East Feb. Jan. Dec. June July Retrograde motion of Mars West

Retrograde Motion

99 Years of Astronomy

Early Astronomy The Birth of Modern Astronomy Nicolaus Copernicus • Copernicus concluded that Earth is a planet. He proposed a model of the solar system with the sun at the center. Heliocentric Model This model explained the retrograde motion of planets better than the geocentric model.

Early Astronomy The Birth of Modern Astronomy Tycho Brahe • Tycho Brahe designed and built instruments to measure the locations of the heavenly bodies. Brahe’s observations, especially of Mars, were far more precise than any made previously. Johannes Kepler • Kepler discovered three laws of planetary motion: 1. Orbits of the planets are elliptical. 2. Planets revolve around the sun at varying speed. 3. There is a proportional relationship between a planet’s orbital period and its distance to the sun.

Early Astronomy The Birth of Modern Astronomy German astronomer Johannes Kepler (1571 -1630) helped establish the era of modern astronomy by deriving three laws of planetary motion.

Johannes Kepler • 1599 – Kepler hired by Tycho Brahe – Work on the orbit of Mars • 1609 – Kepler’s 1 st and 2 nd Laws – Planets move on ellipses with the Sun at one focus – The radius vector sweeps out equal areas in equal times • 1618 – Kepler’s 3 rd Law – The square of a planet’s orbital period P is proportional to the cube of its semi-major axis R. R

Early Astronomy Johannes Kepler used Tycho Brahe’s data to develop three laws that explained the motions of the planets. Earth’s orbit June 15 th Equal areas (30 days) July 15 th January 15 th (30 days) Sun December 15 th KEPLER’S EQUAL AREA LAW states that a line connecting Earth to the sun will pass over equal areas of space in equal times. Because Earth’s orbit is elliptical, Earth moves faster when it is nearer to the sun.

Early Astronomy Equal areas law Faster Slower KEPLER’S EQUAL AREA LAW states that a line connecting Earth to the sun will pass over equal areas of space in equal times. Because Earth’s orbit is elliptical, Earth moves faster when it is nearer to the sun.

Johannes Kepler (1571 – 1630) • Used the precise observational tables of Tycho Brahe (1546 – 1601) to study planetary motion mathematically. • Found a consistent description by abandoning both 1. Circular motion and 2. Uniform motion. • Planets move around the sun on elliptical paths, with non-uniform velocities.

Kepler’s Laws of Planetary Motion 1. The orbits of the planets are ellipses with the sun at one focus. c Eccentricity e = c/a

Eccentricities of Ellipses 1) 2) e = 0. 02 3) e = 0. 1 e = 0. 2 5) 4) e = 0. 4 e = 0. 6

Eccentricities of Planetary Orbits of planets are virtually indistinguishable from circles: Earth: e = 0. 0167 Most extreme example: Pluto: e = 0. 248

Planetary Orbits (2) • A line from a planet to the sun sweeps over equal areas in equal intervals of time. • A planet’s orbital period (P) squared is proportional to its average distance from the sun (a) cubed: Py 2 = a. AU 3 (Py = period in years; a. AU = distance in AU)

Early Astronomy Galileo Galilei Italian scientist Galileo Galilei (1564— 1642) used a new invention, the telescope, to observe the Sun, Moon, and planets in more detail than ever before.

Early Astronomy The Birth of Modern Astronomy Galileo Galilei • Galileo’s most important contributions were his descriptions of the behavior of moving objects. • He developed his own telescope and made important discoveries: 1. Four satellites, or moons, orbit Jupiter. 2. Planets are circular disks, not just points of light. 3. Venus has phases just like the moon. 4. The moon’s surface is not smooth. 5. The sun has sunspots, or dark regions.

Galileo Galilei (1564 -1642) • Galileo was one of the first to use a telescope to study astronomical objects, starting in about 1609. • His observations of the moons of Jupiter and the phases of Venus provided strong support for the heliocentric model.

Jupiter’s Moons • The 4 objects circled Jupiter, and not the Earth!

Jupiter’s Moons • You can watch Jupiter’s moons move from one side of Jupiter to the other in a few days.

Jupiter’s Moons • Not all bodies go around the Earth!

Venus • Venus, the brightest planet, is never far from the Sun: it sets at most a few hours after sunset, or rises at most a few hours before sunrise.

Venus • Venus, the brightest planet, is never far from the Sun: it sets at most a few hours after sunset, or rises at most a few hours before sunrise. • It is never out in the middle of the night.

Venus • Galileo discovered that Venus had phases, just like the Moon.

Venus • Galileo discovered that Venus had phases, just like the Moon. • Furthermore, the crescent Venus was always larger than the full Venus.

Venus • Galileo discovered that Venus had phases, just like the Moon. • Furthermore, the crescent Venus was always larger than the full Venus. • Conclusion: Venus shines by reflected sunlight, and it is closer to Earth when it is a crescent.

Venus in the Geocentric View • Venus is always close to the Sun on the sky, so its epicycle restricts its position. • In this view, Venus always appears as a crescent.

Venus in the Heliocentric View • In the heliocentric view, Venus orbits the Sun closer than the Earth does. • We on Earth can see a fully lit Venus when it is on the far side of its orbit.

Venus in the Heliocentric View • The correlation between the phases and the size is accounted for in the heliocentric view.

• Galileo’s observations of Jupiter and Venus strongly favored the heliocentric view of the Universe.

• Galileo’s observations of Jupiter and Venus strongly favored the heliocentric view of the Universe. • Galileo was put before the Inquisition and forced to recant his views.

• Galileo’s observations of Jupiter and Venus strongly favored the heliocentric view of the Universe. • Galileo was put before the Inquisition and forced to recant his views. • Pope John Paul II admitted in 1992 that the Church was wrong to denounce Galileo.

Isaac Newton (1642 -1727) http: //www-history. mcs. st-andrews. ac. uk/history/Pict. Display/Newton. html

Early Astronomy Sir Isaac Newton English scientist Sir Isaac Newton (1642— 1727) explained gravity as the force that holds planets in orbit around the Sun.

Early Astronomy The Birth of Modern Astronomy Sir Isaac Newton • Although others had theorized the existence of gravitational force, Newton was the first to formulate and test the law of universal gravitation. The universal law of gravitation, helped explain the motions of planets in the solar system. Universal Gravitation • Gravitational force decreases with distance. • The greater the mass of an object, the greater is its gravitational force.

Gravity’s Influence on Orbits

Newton’s Laws of Motion • 1 st Law – A body at rest, or in uniform motion, will remain so unless acted upon by an unbalanced force • 2 nd Law – The change in motion (acceleration) acceleration is proportional to the unbalanced force • 3 rd Law – For every action there is an equal and opposite reaction

Gravity • Gravity is the force that – holds us to the Earth – causes a rock to fall towards the ground – causes the Earth to go around the Sun – causes the Sun to be pulled towards the center of the Milky Way galaxy • Gravity acts between any two objects even if they are far apart. – “action at a distance” distance

Isaac Newton (1642 -1727) • Isaac Newton was born the year Galileo died.

Isaac Newton (1642 -1727) • Isaac Newton was born the year Galileo died. • He was professor of mathematics at Cambridge University in England. (Steven Hawking currently hold’s Newton’s Chair at Cambridge).

Isaac Newton (1642 -1727) • Isaac Newton was born the year Galileo died. • He was professor of mathematics at Cambridge University in England. (Steven Hawking currently hold’s Newton’s Chair at Cambridge). • He was later the Master of the Mint in London, where first proposed the use of grooved edges on coins to prevent shaving.

Isaac Newton (1642 -1727) • Newton was perhaps the greatest scientist of all time, making substantial contributions to physics, mathematics (he invented calculus as a college student), optics, and chemistry.

Isaac Newton (1642 -1727) • Newton was perhaps the greatest scientist of all time, making substantial contributions to physics, mathematics (he invented calculus as a college student), optics, and chemistry. • His laws of motion and of gravity could explain Kepler’s Laws of planetary motion.

Newton’s Laws of Motion

Newton’s Laws of Motion 1. 2. 3. A body in motion tends to stay in motion in a straight line unless acted upon by an external force. The force on an object is the mass times the acceleration (F=ma). For every action, there is an equal and opposite reaction. (For example, a rocket is propelled by expelling hot gas from its thrusters).

What is Gravity?

What is Gravity? • Gravity is a force between all matter in the Universe.

What is Gravity? • Gravity is a force between all matter in the Universe. • It is difficult to say what gravity is. However, we can describe how it works.

What is Gravity? • Gravity is a force between all matter in the Universe. • It is difficult to say what gravity is. However, we can describe how it works.

What is Gravity? • The gravitational force between larger bodies is greater than it is between smaller bodies, for a fixed distance.

What is Gravity? • As two bodies move further apart, the gravitational force decreases. The range of the force is infinite, although it is very small at very large distances.

Newton’s Laws • Using Newton’s Laws, we can…

Newton’s Laws • Using Newton’s Laws, we can… § Derive Kepler’s Three Laws.

Newton’s Laws • Using Newton’s Laws, we can… § Derive Kepler’s Three Laws. § Measure the mass of the Sun, the Moon, and the Planets.

Summary • Kepler’s and Galileo’s Laws provided Newton with important clues that helped him formulate his laws of motion • Newton arrived at 3 laws that govern the motion of objects – The law of inertia – The law of force – The law of action and reaction • Newton also arrived at a law of gravity – But it seemed to require action at a distance!

Isaac Newton & Birth of Astrophysics • Isaac Newton (1642 -1727) was born the year Galileo died • He made major advances in mathematics, physics, and astronomy • He pioneered the modern studies of motion, optics, and gravity and discovered the mathematical methods of calculus • It was not until the 20 th century that Newton’s laws of motion and gravity were modified by theories of relativity History of Astronomy 63

The Growth of Astrophysics • New Discoveries – In 1781, Sir William Herschel discovered Uranus; he also discovered that stars can have companions – Irregularities in Uranus’s orbit together with law of gravity leads to discovery of Neptune • New Technologies – Improved optics lead to bigger telescopes and the discovery of nebulas and galaxies – Photography allowed the detection of very faint objects History of Astronomy 64

The Growth of Astrophysics • The Nature of Matter and Heat – The ancient Greeks introduced the idea of the atom (Greek for “uncuttable”), which today has been modified to include a nucleus and a surrounding cloud of electrons – Heating (transfer of energy) and the motion of atoms was an important topic in the 1700 s and 1800 s • The Kelvin Temperature Scale – An object’s temperature is directly related to its energy content and to the speed of molecular motion – As a body is cooled to zero Kelvin, molecular motion within it slows to a virtual halt and its energy approaches zero Þ no negative temperatures – Fahrenheit and Celsius are two other temperature scales that are easily converted to Kelvin History of Astronomy 65

Earth Science Light and Astronomical Observations

Important Astronomical Measurements • An ellipse is an oval-shaped path. An astronomical unit (AU) is the average distance between Earth and the sun; it is about 150 million kilometers. Light-year The distance that light travels in one year, about 9. 5 trillion kilometers. Parsec: A unit of measurement used to describe distances between celestial objects, equal to 3. 258 lightyears.

The study of light Electromagnetic radiation • Visible light is only one small part of an array of energy • Electromagnetic radiation includes • Gamma rays • X-rays • Ultraviolet light • Visible light • Infrared light • Radio waves *Energy radiated in the form of a wave, resulting from the motion of electric charges and the magnetic fields they produce.

The study of light v. Electromagnetic radiation • All forms of radiation travel at 300, 000 kilometers (186, 000 miles) per second v. Light (electromagnetic radiation) can be described in two ways A continuum depicting the range of v. Wave model v. Wavelengths of radiation vary electromagnetic radiation, with the longest wavelength at one end and the shortest at the other. v. Radio waves measure up to several kilometers long v. Gamma ray waves are less than a billionth of a centimeter long v. White light consists of several wavelengths corresponding to the colors of the rainbow

v. Light (electromagnetic radiation) can be described in two ways • Particle model • Particles called photons • Exert a pressure, called radiation pressure, on matter • Shorter wavelengths correspond to more energetic photons

The study of light v. Spectroscopy • The study of the properties of light that depend on wavelength • The light pattern produced by passing light through a prism, which spreads out the various wavelengths, is called a spectrum (plural: spectra)

The study of light A spectrum is produced when white light passes through a prism

The study of light v. Spectroscopy • Types of spectra • Continuous spectrum: A spectrum that contains all colors or wavelengths. • Produced by an incandescent solid, liquid, or high pressure gas • Uninterrupted band of color • Dark-line (absorption) spectrum • Produced when white light is passed through a comparatively cool, low pressure gas • Appears as a continuous spectrum but with dark lines running through it

Formation of the three types of spectra

Emission spectrum of hydrogen Emission Spectrum Absorption Spectrum A spectrum consisting of individual lines at characteristic wavelengths produced when light passes through an incandescent gas; a bright-line spectrum. A continuous spectrum crossed by dark lines produced when light passes through a nonincandescent gas. Absorption Spectrum of Hydrogen

The study of light v. Doppler effect • The apparent change in wavelength of radiation caused by the relative motions of the source and observer • Used to determine • Direction of motion • Increasing distance – wavelength is longer ("stretches") • Decreasing distance – makes wavelength shorter ("compresses") • Velocity – larger Doppler shifts indicate higher velocities

The Doppler effect Originally discovered by the Austrian mathematician and physicist, Christian Doppler (1803 -53), this change in pitch results from a shift in the frequency of the sound waves.

The Doppler effect The electromagnetic radiation emitted by a moving object also exhibits the Doppler effect. • Redshift, a phenomenon of electromagnetic waves such as light in which spectral lines are shifted to the red end of the spectrum.

The Doppler effect Blueshift: This spectrum shows hydrogen shifted to the blue end of the spectrum. This star is moving toward Earth. The radiation emitted by an object moving toward an observer is squeezed; its frequency appears to increase and is therefore said to be blueshifted. In contrast, the radiation emitted by an object moving away is stretched or redshifted. Blueshifts and redshifts exhibited by stars, galaxies and gas clouds also indicate their motions with respect to the observer. Redshift: This spectrum shows hydrogen shifted to the red end of the spectrum. This star is moving away from Earth.

Astronomical tools v. Optical (visible light) telescopes • Two basic types (1) Refracting telescope • Uses a lens (called the objective) to bend (refract) the light to produce an image • Light converges at an area called the focus • Distance between the lens and the focus is called the focal length • The eyepiece is a second lens used to examine the image directly • Have an optical defect called chromatic aberration (color distortion)

A simple refracting telescope

Astronomical tools v. Optical (visible light) telescopes • Two basic types (2) Reflecting telescope • Uses a concave mirror to gather the light • No color distortion • Nearly all large telescopes are of this type

A prime focus reflecting telescope

Cassegrain focus reflecting telescope

Newtonian focus reflecting telescope

The 200" (5 m) Hale Reflector of Palomar Observatory is shown above. Until recently it was the world's largest optical/infrared telescope.

Astronomical tools v. Optical (visible light) telescopes • Properties of optical telescopes • Light-gathering power • Larger lens (or mirror) intercepts more light • Determines the brightness • Resolving power • The ability to separate close objects • Allows for a sharper image and finer detail

Astronomical tools v. Optical (visible light) telescopes • Properties of optical telescopes • Magnifying power • The ability to make an image larger • Calculated by dividing the focal length of the objective by the focal length of the eyepiece • Can be changed by changing the eyepiece • Limited by atmospheric conditions and the resolving power of the telescope • Even with the largest telescopes, stars (other than the Sun) appear only as points of light

Astronomical tools v. Detecting invisible radiation • Radio radiation • Gathered by "big dishes" called radio telescopes • Large because radio waves are about 100, 000 times longer than visible radiation • Often made of a wire mesh • Have rather poor resolution • Can be wired together into a network called a radio interferometer

Radio Telescope A steerable radio telescope at Green Bank, West Virginia

Astronomical tools v. Detecting invisible radiation • Radio radiation • Gathered by "big dishes" called radio telescopes • Advantages over optical telescopes • Less affected by weather • Less expensive • Can be used 24 hours a day • Detects material that does not emit visible radiation • Can "see" through interstellar dust clouds

Radio Telescope The 300 -meter radio telescope at Arecibo, Puerto Rico