• 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 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 light years? 9,460,000,000,000 km = 1 ly 130,000,000,000,000,000,000,000 km = (1ly/ 9460,000,000,000km) x 130,000,000,000,000,000,000,000 km = =

Scientific Notation

Scientists use a convenient shorthand for expressing very large or very small numbers: Every number can be expressed as a multiple of powers of 10: Example: 4812 = 4.812 x 1000 = 4.812 x 103 Example: 0.0000312 = 3.12 x 1/100000 = 3.12 x 10-5 Rules for expressing any number in scientific notation: • Place the decimal after the first nonzero digit • Count the number of places that the decimal point moved. This gives the power of 10. • If the decimal moves to the left, the power is positive. If the decimal moves to the right the power is negative. Read Appendix C in the textbook

Scientific Notation

How many years would a spaceship traveling at 10,000,000 km/hr take to travel a distance of 2.0 ly ? Recall that Speed = Distance/Time, so Time = Distance/Speed Using scientific notation, 1 ly = 9.46 x 1012 km 2 ly = 1.992 x 1013 km Time = (1.992 x 1013 km) / (1.0 x 107 km/hr) = 1.99 x 106 hrs Converting to years: 1.89 x 106 hrs x 1day/24 hrs x 1yr/365days =

Measures of Astronomical Distances

You should be able to convert between different distance, time and speed units. Read Appendix C in the textbook.

How old is the Universe?

• Let’s build a scale model The Cosmic Calendar Say we speed up our clocks by a factor of 13.6 billion. – Then the entire age of the Universe would be one calendar year. – one month would be approximately 1 billion real years

How old is the Universe?

• On the cosmic calendar, Earth was formed in mid September. How old is the earth ? On the Cosmic Calendar the Earth is 3.5 months old. 1 yr on the cosmic calendar = 13.6 billion actual years Thus 3.5 months on the cosmic calendar = (1.36 x 1010 yrs / 1 year) x (3.5 mths x 1...