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DVD LECTURE NOTES - QUESTIONS and ANSWERS

 
 
SKYWATCHING: 
SEEING AND UNDERSTANDING COSMIC WONDERS

Professor Alex Filippenko.


 
 
LECTURE ONE.  12 April 2016
Day and Night Skies Across all Distances. 

 Understanding question:
Suggest work these out and discuss at the next meeting.

1.1 If a star we call Peter is one hundred time brighter than a star called Paul, but is ten times the distance (from Earth) than is Paul, what is the relative perceived brightness between the two?

Answer: The apparent brightness is the same. With a good mental understanding of the maths you could do this in your head. You can wprk backwards on this through logfic. As luminosity is a factor of the square of distance, the luminosity of Paul is 10 (distance) squared = 100 times more brighter than Peter - which it is,- ipso facto it would show the same brightness from Earth. Mathematicaly we use the brightness formula:
. Enter calculation to expand.
 

1.2 In a model of the universe built here in Yarra, if the earth were the size of a golf ball (and thus a black hole, more on that later), where would you find our moon - could the model fit in a normal lounge room? And how would you represent the moon with a household object. 

Answer.  The moon is 384,000 km from Earth. The golf ball (earth) is 5cm. This is 5 / 1000 x 100  = 0.0005 km
Let's forget about the formal maths. We know that there are just over thirty 'earths' (Earth diameters) between the Earth and Moon, so the ratio of distance to size of Earth is 30. Thus, if a golf ball were the Earth, the moon would be 30 golf balls distant. Even in a small flat you could create a scaled model. But then how big would the model moon be? Could you even see it. 
The ratio of golf ball diameter to Earth is 0.0005 km to 12742 km. Thats a very small ratio. My calculator has had a hissy fit. But I make it around 9 mm, the size of a pea. 

1.3 What enables us to see a perfect eclipse if we are at the right place on earth, where the moon exactly 'covers' the sun?

Answer: By a remarkable conicidence - or is it the hand of God? The sun is 390 times larger than the Moon - and 390 times the distance of that of the Moon from Earth. Using trigonometry (draw a triangle) you can see why we can have a total eclipse when we all line up - the Sun, Moon and Earth that is. 



 
LECTURE TWO.  19April 2016
The Blue Skies, Clouds and Lightning

Understanding question:
Suggest answer these and discuss at the next meeting.

2.1 Apart from the fact that we would not be here, what would you see if you looked up from an Earth without an atmosphere during the day?

Answer: The universe is dark - pitch black. The sun gives us overall light due to its scattering through the atmosphere. No atmosphere means the sunlight would stream down in a narrow beam like a searchlight as there would be no scattering of light. And with no scattering, and with no clouds, we would see the stars against a black background. 

2.2 What does a photographer do to darken or enhance the sky? (No, using Photoshop is not the answer.)

Answer: The photographer would use a polaroid filter - actually two filters built into the one ring. Turn one ring and you will notice a change in the density and intensity (and 'colour'( of the light. Will a polarising filter work on a digital camera?

2.3 Why was I excited when I saw an uncinus and a floccus the other day?

Answer: They are cloud formations, the former having a hook like shape *like a comma), and the being small rounds 'puffs' of cloud.

2.4 In which level of the atmosphere do we find clouds ?

Answer: Mainly in the first level, the trophere, but there are other rarer formations in the stratosphere (called 'mother-of-pearl' clouds), and also as high up as the mesoshere - covered in a later chapter. 

2.5 Is there a similarity between clouds and fog ?

Answer: Yes, they are both formed from water droplets , in the case of fog, droplets condensed out of moist air and distinquished from clouds as they form close to the ground. 

2.6 What is a contrail and how is it formed? (I photographed one this morning.)

Answer: A contail over my home indicates there is a jet flight on its was to New Zealand. These 'artificial' clouds trail behind a jet aircraft and are formed when hot  moisture-laden exhaust gases from a jet engine are emiited into the atmospher, and thus cool and expand rapidly. When the overall temperature drops below the dew point, the water vapout in the gases condenses to droplets and even ice crystals. They are thus man-made clouds. Remeber the old school formula we had to remember in thermodynamics - P-one times V-one over T-one. That comes into play. 

2.7 What is the fear of lightning (and thunder) called ? 

Answer: No, it is not being s-scared. It is Astraphobia, also known as astrapophobia, brontophobia, keraunophobia, or tonitrophobia, is an abnormal fear of thunder and lightning, a type of specific phobia. It is a treatable phobia that both humans and animals can develop.
 



 
LECTURE THREE.  26 April 2016 
The Rainbow Family - Sunlight and Water

3.1 Suppose water droplets refracted all wavelengths of visible light exactly the same way - would rainbows still be seen. And what would they look like?

A. I need help on this. The rainbow is not just the result of the refraction of light into the spectrum. I think polarization would come onto it, so I would suggest that if there was no spectrum, there may still be a rainbow, but would be monochromatic, with bands of dark to light grey. Just a thought?

3.2 Would the appearance of a rainbow change with viewing through a polarizing filter?

A. Yes, it would change when rotated from no effect (except for a slight darkening due to the composition of the two glass lens), to dark as the various ‘colours’ are polarized. 

3.3 Can we touch a rainbow? 

A. Not really but you can come very close, a matter of centimeters in fact. But you will need to create your own ‘raindrops’. Maybe a light spray from a hose will do. 

3.4 What else can cause a rainbow to be readily seen. 

A. Any spray of water that conforms to the principles of the apparent rainbow, ie the angle of the sun, the position of the observer, the composition of the water droplets. Spray from a waterfall is a good example, or a garden hose.

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