I know the light from the sun takes 8+ minutes to reach the earth. And when we see stars, we're actually seeing them where they actually were many (many!) years ago. My question: when we see a planet (e.g. Venus or Mars) are we seeing it where it actually is, or where it was a certain amount of time ago? And, if in the past, any idea how long ago?

Light travels at 186,000 miles per second.
So if an object is 186,000 miles away, you would be looking at it as it was one second ago.
If, say Mars was 60 million miles away (it varies greatly) then it would take about five minutes and twenty seconds for its reflected light to reach you.
When you look at the moon, you are seeing it the way it was 1.28 seconds ago.

Hi JosephEmerson, and welcome to BAUT ...

in each case, it will be as far back in time as the time required for the light to reach us - and because all of the planets are moving, those times vary ...

if I remember it correctly, it was something about the difference in the expected and observed positions of Jupiter that helped pin down the speed of light ...

as for the range of times - well, a matter of minutes for the inner planets, and some hours for the outer planets ... I imagine that the NASA website will have a table of distances/light travel times for at least the major bodies of the solar system ...

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An astronomical unit (average distance between Earth and Sun) is 8.32 light-minutes. At any given moment, multiply the distance between Earth and another planet by 8.32 and you get the number of light-minutes it is distant. At this moment the figures are:

Sun 1.016 AU = 8.45 LM
Mercury 1.236 AU = 10.28 LM
Venus 1.687 AU = 14.04 LM
Mars 2.227 AU = 18.53 LM
Jupiter 4.175 AU = 34.74 LM
Saturn 10.082 AU = 83.88 LM
Uranus 19.493 AU = 162.18 LM
Neptune 29.119 AU = 242.27 LM

__________________
Curt Renz - "Centaur"
For monthly astronomical calendar visit:
www.CurtRenz.com/astronomical.html

I'm looking at seeing my computer monitor the way it appeared 0.000000001 seconds ago.
Or pretty close to that.

That is nearly correct. In 1676 the Danish astronomer Ole Rømer determined a rough estimate of the speed of light by timing the eclipses of Jupiter’s satellite Io into the shadow of the planet when at different distances from Earth.

__________________
Curt Renz - "Centaur"
For monthly astronomical calendar visit:
www.CurtRenz.com/astronomical.html