Solar Eclipses Umbra – region of total shadow Penumbra – region of partial shadow Totality lasts...

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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