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 Eclipse1
3
2
Lunar Eclipses Moon moves into
earth’s shadow…
…and out of it
(takes hours!)
Not an Eclipse !
Towards Totality
Almost total…
Totality
Totality
Question
Why isn’t there an eclipse every month ?
The Scientific Method• Systematized by Francis
Bacon, Descartes and Galileo in the 17th 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 (1561-1626)
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 (1561-1626)
Novum Organum Title:
Allegory on the daring mind: A Ship passes
through the “pillars of
Hercules”, beyond old
knowledge.
Rene Descartes – The RationalistDescribed 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 (1596-1650)
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
Mars
“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 = 3 105 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
Fahrenheit Centigrade Kelvin
Absolute zero 459 ºF 273 ºC 0 K
Ice melts 32 ºF 0 ºC 273 K
Human body temperature
98.6 ºF 37 ºC 310 K
Water boils 212 ºF 100 ºC 373 K
Black Body Spectrum• Objects emit radiation of all frequencies,
but with different intensities
Higher Temp.
Lower Temp.Ipeak
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