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Astronomy

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Astronomy
Measures of Astronomical Distances
• Let’s view the Universe in terms of kilometres – The size of the observable universe is roughly 130,000,000,000,000,000,000,000 km ! - Obviously measuring astronomical distances in km is inconvenient. - We need much BIGGER units of distance: • Some commonly used distance units: 1 ly (light year) = Distance traveled by light in 1 year = 9.46 trillion km 1 AU (astronomical unit) = average distance between the Sun & Earth = 150 million km

A unit of distance, not time. Often confused because of Suppose we define the unit: “a day’s drive”. Say we drive 100km/hr

The light year

and we are prepared to drive for 8 hours in one day. What distance can we travel in that day?

Measures of Astronomical Distances
The light year (ly) 1 ly = Distance light travels in 1 year Speed of light = 300,000 km/s In 1 second light travels 300,000 km In 1 minute light travels: 300,000 km x 60 = 18 million km In 1 hour light travels: 18 million km x 60 = 1.08 billion km In 1 day light travels 1.08 billion km x 24 = 25.92 billion km In 1 year light travels 25.92 billion km x 365 = 9.46 trillion km 1 ly = 9.46 trillion km=9,460,000,000,000 km

Measures of Astronomical Distances
The astronomical unit (AU) The earth’s orbit around the sun is elliptical. Hence the distance between the earth and sun varies at different points in its orbit. Minimum distance between earth & sun (Perihelion) = 148,000,000 km Maximum distance between earth & sun (Aphelion) = 152,000,000 km 1 au = Average distance between earth & sun = (Min. distance + Max. distance) /2 = (148000000km+152000000km)/2 = 150,000,000 km = 150 million km

Measures of Astronomical Distances
–The size of the observable universe is roughly 130,000,000,000,000,000,000,000 km ! What is the size of the observable universe in astronomical units ? 150,000,000 km = 1 au 130,000,000,000,000,000,000,000 km = (1au/150,000,000 km) x 130,000,000,000,000,000,000,000 km = =

Measures of Astronomical
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