Theme 4 – Galileo

ASTR 101Prof. Dave Hanes

The First Real Experimentalist

Explored dynamics – forces and motions

Studied balls rolling on slopes and planes

Recognized the importance of friction and air resistance

All falling objects behave in the same way. (Tower of Pisa?)

http://www.youtube.com/watch?v=5C5_dOEyAfk

All Motion is RelativeThe Galilean relativity principle

Is the weight falling straight down, or along a curved path?

It depends: are you on shore, or on board?

Lesson: The behaviour of stones tossed in the air can’t tell us if the Earth is moving or not (an ancient argument).

http://www.astro.queensu.ca/~hanes/ASTR101-Fall2015/ANIMS/GalRel.mp4

Inertia!

His reasoning:

His misunderstanding:

The Human EyeIts Function and Limitations

Light Refracts (Changes Direction) as it Moves from One Medium to Another

- so lenses can focus light

The Basic Steps

Light enters the pupil is refracted and focussed by the lens to form an image on the retina.

The energy of the light has a chemical effect on the pigments (enzymes) in

the receptor cells that make up the retina, and an electrical signal is sent along the optic nerve to

the brain.

The brain does the rest!

Some Important Concepts

1. Dynamic Range (day vs night)How it’s achieved (in part): the pupil expands!

in dim light vs in bright light

Much more important: the replenishment of the pigments – hence, we get dark-adapted.

2. Accommodation [focussing our eyes]

We lose this ability as we age – hence, reading glasses.

3. Persistence of Vision

Allows us to watch movies, TV, etc.

http://www.astro.queensu.ca/~hanes/ASTR101-Fall2015/ANIMS/Persist.mp4

4. Colour Vision

To discriminate colours, we need at least two different kinds of receptors with different enzymes, having different sensitivities.

Our retinas contain rods and cones.

The Difference

Rods are most sensitive, but respond merely to the presence or absence of light. With them, we see only shades of black/grey/white.

Cones are less sensitive, but respond to colour. There are three kinds, with different enzymes, broadly sensitive to red, green and blue.

One Implication, and One Puzzle

1. The implication: When it’s dark, there is too little light to stimulate the Cones, so everything looks shades of black and white at night. With rare (bright) exceptions, the stars look colourless! (But have you ever noticed the subtle ‘redness’ of Betelgeuse, Antares, and Mars?)

2. The puzzle: If our Cones are sensitive to red, green and blue, why do some things look yellow, orange or purple?

The Puzzle AnsweredTogether, red light and green light give us the sensation of yellow! Similarly, green and blue yield teal; red and blue produce magenta. (All three mixed together produce white.)

The light from a yellow object stimulates both the red and green receptors to some extent – and we see yellow.

http://www.astro.queensu.ca/~hanes/ASTR101-Fall2015/ANIMS/RGB-Col.mp4

That is how old colour televisions work: electronsstimulate various tiny spots on the screen to glow red, green or blue – in combination, this generates all colours.

http://www.youtube.com/watch?v=xzmXrC-Yzfc&feature=related

5. ResolutionHow much detail can you discern?

Finer Grain yields Better Resolution

How many pixels, or “picture elements,” are there? My camera has 12 megapixels.

In your eye: 125 million rods and cones!(But they are clumped in groups, so not really quite that many in effect.)

The Size of the Image Also Matters

If two photons (‘lumps’of light) arrive at well-separated spots on the retina, landing on different ‘pixels,’ they can be seen to be coming from two separate objects.

But two photons that arrive closely side by side can’t be distinguished (the details are “unresolved”).

Example: To the Unaided Eye,

Saturn is Just a Point of LightIts image formed on the retina is tiny – just a

dot!

To see details, either: Move closer to the target so that it looks bigger (that is, the image is spread over a larger area on the retina) Or use a telescope to magnify the image.

Galileo’s Telescope

Not invented by Galileo, but he saw its potential for astronomy.

Four Important Discoveries –Interesting but Not Definitive

1. Features on the moon2. Sunspots and their motions3. ‘Extensions’ on Saturn4. Stars in the Milky Way

1. The Moon

Galileo’s Interpretations

1. Maria! (seas, like the Mediterranean)

2. Mountains!

Imperfection!

Galileo’s Calculationspeaks of mountains lit by the setting Sun

2. Sunspots that Movethey go once around every ~25 days or so

The sun can clearly spin without flying to pieces!

So the Earth can too…

http://www.astro.queensu.ca/~hanes/ASTR101-Fall2015/ANIMS/SSpots.mp4

3. Saturn’s ‘Extensions’with a modern picture at the bottom

The Vanishing Ringsthey are very thin!

4. The Milky Way Resolved: Stars! we may be far from unique

Plus Two Critical Discoveries

Jupiter has moons that orbit it

Venus displays a full range of phases (from new to crescent to full and back)

The Moving Moons of Jupiteras sketched by Galileo

They orbit Jupiterand ‘keep up’ with it as it moves

Presumably our Moon can keep up with us!

Modern Imagesthey are comparable to our Moon

2. The Phases of Venus

A prediction fromPtolemy’s model:

Since Venus is always between us and the Sun, we should never see the face of Venus fully lit up (just crescents)

But If Copernicus is Right

Venus goes aroundto the far side of theSun. So, from the Earth, Venus should appear full (and small!) at those times.

Monitor it for ~18 Months

Why not use your Starry Night-sky simulation software to find Venus and monitor its appearance as the months pass?

That’s shown here: http://www.astro.queensu.ca/~hanes/ASTR101-Fall2015/ANIMS/VPhases.mp4

and in this seriesof still photos:

Copernicus was right!

Full Venus

Guilty of Heresy, Sentenced to House Arrest

”Wine is light held together by moisture.” – G Galileo

One Bizarre Relic