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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.