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B Light Travel Time

  1. Mar 11, 2017 #1
    Hey guy,
    I was browsing YouTube and watched a speed comparison video and it was just different races timed. I noticed that light speed took 12 minutes to reach Mars from earth. So I thought this

    If we look at Mars through a telescope or what ever, Does that mean we are actually looking back in time? We are not actually there but does it mean we are seeing past events (assuming there was an event lmao)
    Excited to see the responses. This is my first ever science post cuz I'm 14
  2. jcsd
  3. Mar 11, 2017 #2


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    Yes indeed, we are always looking at how things were in the past. When you look at the moon you are seeing it as it was 1 second ago. When you see the Sun you are seeing it as it appeared about 8 minutes ago. The nearest major galaxy (Andromeda Galaxy) to our own is over 2 million light-years away, so if you ever see it through a telescope you are seeing it as it appeared more than 2 millions years ago.

    Also, I've edited the title of this thread to something more appropriate for the topic.
  4. Mar 11, 2017 #3
    Yes light travels at known speed, so when you see a distant object. what you see is what it was like in the past.
    For far away galaxies it can be billions of years
    Last edited: Mar 11, 2017
  5. Mar 11, 2017 #4
    If you want to learn even more about the "oddities" of time (aka the basis of Special Relativity :oldbiggrin:), I suggest searching "Time Dilation" and the "Einstein Clock" on google.
  6. Mar 11, 2017 #5
  7. Mar 12, 2017 #6


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    That 12 min light travel time to Mars is about the average. Since the actual distance between Earth and Mars changes due to their different orbits, this time can vary from a bit over 4 min to almost 21 min.
    Since this delay also applies to radio signals, this is why we can't control the rovers on Mars in "real time". If we send a command to the rover to perform an action it will take from 8 to 41 min before we will see the result of that command on the rover. The same is true during landing operations. By the time we could find out that there was a problem and tried to send a correction, the landing will have already either succeeded or not.
  8. Mar 12, 2017 #7


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    Indeed. Also, the video of the JPL team during the landing sequence was wonderful (landing occurs about an hour into the video). For the OP, if you watch the video, you may notice that they don't send any commands to the spacecraft during the landing sequence. All of it is pre-programmed in.
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