共享题干题When you look up,how far back in time do you see?
Our senses are (91)in the past.There’s a flash of lightning,and then seconds Pass until we (92)the rumble of distant thunder.We hear the past.We are seeing into the past too.
(93)sound travels about a kilometer every three seconds,light travels 300,000 kilometers every second.When we see a flash of lighting three kilometers away,We are seeing something that happened a hundredth of a millisecond ago.That’s not exactly the distant past.
But as we look further afield,We Can Peer further back. (94)through a telescope,We Can look even further into the past.if you really want to look back in time,you need to look up.
The Moon is our nearest celestial neighbor-a world with valleys,mountains and craters.It’s also about 380,000km away,so it takes 1.3 (95)for light to (96)from the Moon to us.We see the Moon not as it is,but as it was l.3 seconds ago.
The Moon doesn’t change much from instant to instant,but this 1.3 second delay is (97)when mission control talks to astronauts on the Moon.Radio waves travel at the speed of light,so a message (98)mission control takes l.3 seconds to get to the Moon,and even the quickest of (99)takes another l.3 seconds to come back.
It’s not (100)to look beyond the Moon and further back in time.The Sun is about 150 million km away,so we see it as it was about 8 minutes ago.
Even our nearest planetary neighbors,Venus and Mars,are tens of millions of kilometers away, (101)we see them as they were minutes ago.When Mars is very (102)to Earth,We are seeing it as it was about three minutes ago,but at other times light takes more than 20 minutes to travel from Mars to Earth.
This (103)some problems if you’re on Earth controlling a Rover on Mars.If you’re driving the Rover at 1km per hour then the lag, (104)to the finite speed of light,means the Rover could be 200 meters ahead of (105)you see it, and it could travel another 200 meters after you command it to hit the brakes.
Not surprisingly,Martian Rovers aren’t breaking any speed records,travelling at 5cm Per second(0.18kph or O.11mph).On-board computers help with driving,to prevent rover wrecks with rovers following carefully (106)sequences and using on-board computers to (107)hazards and prevent punctures.
Let’s go a bit further out in space.At its closest to Earth,Saturn is still more than a billion kilometers away,so we see it as it was (108)than an hour ago.
When the world (109)into the Cassini spacecraft’s plunge into Saturn’s atmosphere in 2017,we were hearing echoes from a spacecraft that had already been destroyed more than an hour before.
So when you look up,remember you aren’t seeing things as they are (101)you’re seeing things as they were.
(选自The Conversation 2018年12月27日)