# When you look up, how far back in time do you see?

Our senses are stuck in the past. There's a flash of lightning, and then seconds pass until we hear the rumble of distant thunder. We hear the past.

We are seeing into the past too.

While sound travels about a kilometre every three seconds, light travels 300,000 kilometres every second. When we see a flash of lighting three kilometres 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. We can see seconds, minutes, hours and years into the past with our own eyes. Looking through a telescope, we can look even further into the past.

A second back in time

If you really want to look back in time, you need to look up.

The moon is our nearest celestial neighbour—a world with valleys, mountains and craters.

It's also about 380,000km away, so it takes 1.3 seconds for light to travel from the moon to us. We see the moon not as it is, but as it was 1.3 seconds ago.

The moon doesn't change much from instant to instant, but this 1.3-second delay is perceptible when mission control talks to astronauts on the moon. Radio waves travel at the speed of light, so a message from mission control takes 1.3 seconds to get to the moon, and even the quickest of replies takes another 1.3 seconds to come back.

Minutes and hours

It's not hard 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 neighbours, Venus and Mars, are tens of millions of kilometres away, so we see them as they were minutes ago. When Mars is very close 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 presents 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, due to the finite , means the rover could be 200 metres ahead of where you see it, and it could travel another 200 metres 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 0.11mph), and on-board computers help with driving, to prevent rover wrecks.

Not surprisingly, Martian Rovers aren't breaking any speed records, travelling at 5cm per second (0.18kph or 0.11mph), with rovers following carefully programmed sequences and using on-board computers to avoid 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 kilometres away, so we see it as it was more than an hour ago.

When the world tuned into the Cassini spacecraft's plunge into Saturn's atmosphere in 2017, we were hearing echos from a spacecraft that had already been destroyed more than an hour before.

Years

The night sky is full of stars, and those stars are incredibly distant. The distances are measured in , which corresponds to the distance travelled by light in one year. That's about 9 trillion km.

Alpha Centauri, the nearest star visible to the unaided eye, is at a distance 270,000 times the distance between Earth and the Sun. That's 4 light years, so we see Alpha Centauri as it was 4 years ago.

Some are much more distant still. Betelgeuse, in the constellation Orion, is about 640 light years away. If Betelgeuse exploded tomorrow (and it will explode one day), we wouldn't know about it for centuries.

Even without a telescope we can see much much further. The Andromeda galaxy and the Magellanic Clouds are relatively nearby galaxies that are bright enough to be seen with the unaided eye.

The Large Magellanic cloud is a mere 160,000 light years away, while Andromeda is 2.5 million light years away. For comparison, modern humans have only walked the Earth for about 300,000 years.

Billions

With the unaided eye you can look millions of years into the past, but how about billions? Well, you can do that at the eyepiece of an amateur telescope.

Quasar 3C 273 is an incredibly luminous object, which is brighter than individual galaxies, and powered by a huge black hole.

But it's 1,000 times fainter than what the unaided eye can see because it's 2.5 billion light years away. That said, you can spot it with a 20cm aperture telescope.

A bigger telescope allows you to peer even further into space, and I once had the pleasure of using an eyepiece on a 1.5-metre diameter telescope. Quasar APM 08279+5255 was just a faint dot, which isn't surprising as it's 12 billion years away.

Earth is just 4.5 billion years old, and even the universe itself is 13.8 billion years old. Relatively few people have seen APM 08279+5255 with their own eyes, and in doing so they (and I) have looked back across almost the entire history of our universe.

So when you look up, remember you aren't seeing things as they are now; you're seeing things as they were.

Without really trying, you can see years into the past. And with the aid of a telescope you can see millions or even billions of years into the past with your very own eyes.

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Explainer: Light-years and units for the stars

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Dec 28, 2018
"Astrogation" is like skeet shooting. You can't waste time aiming at where the skeet disk came from, but rather in the direction you think it is traveling.

Dec 29, 2018
What is the oldest light that can be seen with the naked eye? The fuzzing blur of the Andromeda Galaxy? A million years? or so? That would make the surface of the moon the oldest thing you can see with the naked eye? Parts of it 4billion years.

Dec 29, 2018
The furthest object I've been able to see with my naked eyes is the Andromeda galaxy. That's about 2.3 million light years, which is diddly compared to what the Hubble can see.

Dec 31, 2018
if the speed of light is slowing down like measurements are showing, wouldnt it be tens of thousands of years instead of millions?

Dec 31, 2018
if the speed of light is slowing down like measurements are showing, wouldnt it be tens of thousands of years instead of millions?

What measurements?

Dec 31, 2018
As for the farthest/oldest thing I've ever seen through a telescope, I couldn't tell you. I've seen Markarian's Chain, Omega Centauri, and examined giant molecular clouds in the Triangulum galaxy. I've seen most of the Messier objects one time or another. The most incredible thing I've ever seen was Saturn (which is pretty close) on a night of incredible seeing, hanging like a Christmas tree ornament striped with Day-glo green and yellow paint in my objective. Though giant molecular clouds in Triangulum came close.

Dec 31, 2018
if the speed of light is slowing down like measurements are showing, wouldnt it be tens of thousands of years instead of millions?
Only if the Andromeda galaxy were hundreds of times bigger than ours. And its gravity tells us it cannot be.

Jan 01, 2019
@Eikka

C appears to be decreasing by approximately 0.02 m/s per year. (2 cm/s per year).

https://arxiv.org...908.0249

Jan 01, 2019
@Anonym216579

No, it cuts the age by less than half at 14 billion light years.

Jan 01, 2019
Trolls always repeat their lies like they'd never been repudiated by evidence.

Not to mention you previously claimed it was 2cm/year.

https://phys.org/...rsy.html

So now you admit you were lying?

Jan 01, 2019
@Schneib

I wasn't addressing you. I was answering two questions. Go away. Post your opinion like civil grownup and leave it at that.

the article claims 2 cm/s per year. since light is a speed and cm is a distance, 2cm/year makes no sense with respect to a decrease in a speed. So, maybe you found a typo if that's what I wrote. I have to admit I don't have your desire to scour old posts, so I haven't actually looked.

Jan 01, 2019
The problem being that your reference seems to ignore LIGO results, @Jax.

Jan 01, 2019
@Schneib
I wasn't addressing you. I was answering two questions. Go away. Post your opinion like civil grownup and leave it at that.

the article claims 2 cm/s per year. since light is a speed and cm is a distance, 2cm/year makes no sense with respect to a decrease in a speed. So, maybe you found a typo if that's what I wrote. I have to admit I don't have your desire to scour old posts, so I haven't actually looked.

> jax...Da Schneibo is a Physorg Moderator, and as such he has access to their own website Search Engine that you NEVER have access to. Therefore he can in just a couple of minutes find everyone who ever made a peculiar statement about something & have the Comment at his fingertips to quote right back at you.

Everyone one who posts in this chatroom had better get used to the idea that the discussions here are in some part being rigged by the three administrators of Science x, they are using their embedded moderators to do it, one being Da Schneibo.

Jan 01, 2019
In case anyone had questions: LOSER: Tired light

https://www.forbe...80578ec3