# Speed of light AU's/h?

#### Struggling

hi all, having problem with 1 question.

speed of light = 2.998 x 10^8m/s
AU(average distance from earth to sun) = 1.50 x 10^8km

what i tried doing was converting speed of light into km/h then dividing the two and vice versa. i keep getting 7183. something along those lines.

but the answer is simply 7.20 AU's(using significant figures) am i getting the right answer and dont know how to write it? or am i not working it out correctly?

thanks

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#### dextercioby

Homework Helper
Nope.It takes approximately 8 minutes for light to travel through (approximately) empty space between the surfaces of the 2 celestial bodies.

So 60/8=7.5 It's the same order of magnitude at least and one accurate digit.

Daniel.

#### HallsofIvy

Homework Helper
2.998 x 10^8m/s= (2.998x10^8 m/s)/(1000 m/km)= 2.998 x 10^5 km/s= (2.998x10^5 km/s)*(3600 s/h)= 10792.8 x 10^5 km/h= 1.07928 x 10^9 km/h= (1.07928 x10^9 km/h)(1/(1.50 x 10^8 km/Au)= 0.71952 x10^1 Au/h= 7.1952 Au/h which rounds to 7.20 Au/h at 3 significant figures (the accuracy of the least accurate data "1.50 x10^8 km/Au").

Dextercioby's method is lovely!

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