Solar Eclipses Umbra – region of total shadow Penumbra – region of partial shadow Totality lasts only a few minutes! Why isn’t there a solar eclipse every month?

Solar Corona

Annular Eclipse 1 3 2

Lunar Eclipses Moon moves into earth’s shadow… …and out of it (takes hours!)

Question Why isn’t there an eclipse every month ?

The Scientific Method Systematized by Francis Bacon, Descartes and Galileo in the 17 th century Not the only way of knowing, but a very successful one A method to yield conclusions that are independent of the individual Conclusions are based on observation

Francis Bacon ( ) Contemporary of Shakespeare, Elizabeth I, Kepler and Galilei Rejects Aristotelianism and Scholasticism Major Work: Novum Organum (1620) (“New Tools”, the old Organum was Aristotle’s)

Francis Bacon ( ) Novum Organum Title: Allegory on the daring mind: A Ship passes through the “pillars of Hercules”, beyond old knowledge.

Rene Descartes – The Rationalist Described the method to do science, known for his mind-body dualism Major Works: Discourse (1637) [full title: Discours de la méthode pour bien conduire sa raison et chercher la vérité dans les sciences] Meditations on first Philosophy (1641) [6 Meditations: Of the Things that we may doubt; Of the Nature of the Human Mind; Of God: that He exists; Of Truth and Error; Of the Essence of Material Things; Of the Existence of Material Things; Of the Real Distinction between the Mind and the Body of Man] Rene Descartes ( )

Rene Descartes’ Discourse Describes the method to do science in a straightforward way (see below) Major points: Science must be based on correct reasoning (logic) Science must be formulated in mathematical language Starting line: “Good sense is the most evenly distributed thing in the world, for all people suppose themselves so well provided with it that even those who are the most difficult to satisfy in every other respect never seem to desire more than they have.”

A Classical Example Aristotle observes that during lunar eclipses the Earth’s shadow on the moon is curved He assumes it will be curved for all eclipses A hypothesis that explains this: the earth is round A prediction of this theory is that the location of the stars in the sky should be different for observers at different latitudes This is confirmed by additional observations –E.g. Canopus is visible in Egypt but not further north

Scientific Literacy An important skill to have in an increasingly complex society As a voting citizen you should know what you are voting on As a consumer you should know (or be able to figure out) what you buy Makes life more interesting Nobody knows everything, but smart people can figure out fast if something is obviously wrong

Electromagnetic Radiation

“Light” – From gamma-rays to radio waves The vast majority of information we have about astronomical objects comes from light they either emit or reflect Here, “light” stands for all sorts of electromagnetic radiation A type of wave, electromagnetic in origin Understanding the properties of light allows us to use it to determine the –temperature –chemical composition –(radial) velocity of distant objects

Waves Light is a type of wave Other common examples: ocean waves, sound A disturbance in a medium (water, air, etc.) that propagates Typically the medium itself does not move much

Wave Characteristics Wave frequency: how often a crest washes over you Wave speed = wavelength ( )  frequency (f)

Electromagnetic Waves Medium = electric and magnetic field Speed = 3  10 5 km/sec

Electromagnetic Spectrum Energy: low  medium  high

Electromagnetic Radiation: Quick Facts There are different types of EM radiation, visible light is just one of them EM waves can travel in vacuum, no medium needed The speed of EM radiation “c” is the same for all types and very high (  light travels to the moon in 1 sec.) The higher the frequency, the smaller the wavelength ( f = c) The higher the frequency, the higher the energy of EM radiation (E= h f, where h is a constant)

Visible Light Color of light determined by its wavelength White light is a mixture of all colors Can separate individual colors with a prism

Three Things Light Tells Us Temperature –from black body spectrum Chemical composition –from spectral lines Radial velocity –from Doppler shift

Temperature Scales FahrenheitCentigradeKelvin Absolute zero  459 ºF  273 ºC 0 K Ice melts32 ºF0 ºC273 K Human body temperature 98.6 ºF37 ºC310 K Water boils212 ºF100 ºC373 K

Black Body Spectrum Objects emit radiation of all frequencies, but with different intensities Higher Temp. Lower Temp. I peak f peak <f peak <f peak

Cool, invisible galactic gas (60 K, f peak in low radio frequencies) Dim, young star (600K, f peak in infrared) The Sun’s surface (6000K, f peak in visible) Hot stars in Omega Centauri (60,000K, f peak in ultraviolet) The higher the temperature of an object, the higher its I peak and f peak

Wien’s Law The peak of the intensity curve will move with temperature, this is Wien’s law: Temperature / frequency = constant So: the higher the temperature T, the smaller the frequency f, i.e. the higher the energy of the electromagnetic wave

Measuring Temperatures Find maximal intensity  Temperature (Wien’s law) Identify spectral lines of ionized elements  Temperature

Color of a radiating blackbody as a function of temperature Think of heating an iron bar in the fire: red glowing to white to bluish glowing