4. In astronomy, a light year ( ly) is de ned to be the distance that light woul

Question

4. In astronomy, a light year ( ly) is dened to be the distance that light would travel in one year. How many meters are there in a ly? 5. The Earth moves in a very nearly circular path around the Sun over the course of one year. Express the speed of the Earth in astronomical units per year and in kms. 6. In astronomy the so-called Hubble constant is usually expressed in units of kms-1Mpc-1. What are the 'dimensions' of the Hubble constant? It is commonly stated that the age of the Universe is approximately given by the inverse of the Hubble constant. Is this statement dimensionally consistent? If it is, estimate the age of the Universe in years given that H=  70:8kms-1Mpc-1.

Explanation / Answer

4. 1ly=299792458*365*24*3600m=9.45*1015m

5.1 au=distance between earth and sun.

Speed=2/yr AU=6.28AU/yr

=9.4*1011km/yr=29806km/hr

6.Dimension=LT-1/L-1=T-1

yes the statement is dimensionally consistent since inverse of hubble constant has dimension of time.

Age=1/70.8*3.08568025 × 1022/1000=4.36*1017s=1.382*1010years

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