$($a$)$ How long in seconds does it take a radio signal to travel $150$ km from a transmitter to a receiving antenna? $($b$)$ We see a full Moon by reflected sunlight. How much earlier did the light that enters our eye leave the Sun? The Earth $-$ Moon and Earth $-$ Sun distances are $3\mathrm{.}8{10}^{5}km$ and $1\mathrm{.}5{10}^{8}km,$ respectively. $($c$)$ What is the round$-$trip travel time in seconds for light between Earth and a spaceship at a $4\mathrm{.}4{10}^{9}km$ distance from Earth? $($d$)$ Suppose astronomers observe a supernova about $7000$ light$-$years $($ly$)$ distant. How long ago in years did the explosion actually occur?

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