Preamble: This is just me thinking out loud and hopefully starting an interesting discussion. I apologize if any of this is inaccurate. Please correct me if so. The numbers I'm using are approximations, of course. Just something I was thinking about last night when looking at Mars with my naked eye.
So, a lot of us are excited about Mars opposition coming up. Earth will be in between the Sun and Mars, and Mars will be very close to the Earth. At this point, it takes light around 4 minutes to reach us from Mars - half the time it takes for light to reach us directly from the Sun. But we know that the light from Mars did not originate from Mars. It was reflected off of Mars when sunlight reached the planet.
So in actuality when you think about it more deeply, the light that we are seeing from Mars is not 4 minutes old. It's around 16 minutes old. But the photons that hit our eyes appear as Mars because Mars is what reflected that light.
It's cool to think that while, to us, the light we see reflected off of Mars is very "close", in actuality, that light left the Sun, passed us on Earth (at a fair distance, of course), traveled to Mars, reflected off of Mars and eventually hit our eye.
So while the light reflected from Mars during opposition may only be traveling 62 million km to reach our eyes, the total distance that light actually travels before we can see it is around 274 million km. That's basically how far light takes to reach Earth-Sun distance plus twice the distance Earth will be from Mars (a round trip).
It gets really interesting if you think about what would happen if the Sun just hypothetically...disappeared. If this happened, the photons that left the Sun right after the Sun vanished would take 8 minutes to get to us, so we wouldn't know that the Sun disappeared until 8 minutes later. At that point, the sky would be dark and we could look at Mars and still see it for quite some time afterwards, because the light that left the Sun right after it disappeared took considerably longer to reach Mars and reflect back to Earth, so we would then see Mars disappear as well some time later. Same goes for the other planets. One by one, they would fade away. The farther away they are from us, the longer it would take.
One other thing I was thinking about - when you look at Mars and someone else is standing (physically distanced, of course) next to you, the view of Mars may be the same, but your image of Mars is different than theirs because the photons of light that are entering your eye cannot be the same photons that are entering their eye. Your view of Mars is, technically, your very own. And those photons that left the Sun, reflected off Mars and came back to Earth only to arrive at your eyes are yours and yours alone. They started at the Sun and ended at you.
Light is fascinating.
Edited by Fiz79, 30 September 2020 - 09:22 AM.