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# If I am asked..."how far can you see?"...

22 replies to this topic

### #1 broca

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Posted 19 May 2013 - 12:09 PM

I mainly do AP but I am interested in presenting to my kids school. If asked the above question I'd like to answer it this way but I want to make sure I am getting this correctly...
A penny to an unaided eye has an angular size of 1" if 2.5 miles away. Using an FL scope of 1625mm at f/8 and an eyepiece FL 21mm with an apparent field of 68 degrees will that penny 2.5 mi away look 77x bigger?
Leaving atmospheric factors out of the equation let me know if I am on the right track .

### #2 StarStuff1

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Posted 19 May 2013 - 02:32 PM

This may be a little too much "mathy" for a lot of kids. My college level astro lab students would struggle with this. My suggestion would be to say simply that your scope with that eyepiece magnifies 77 times. So if the average distance to the Moon is about 238,000 miles away it would appear to be only a little over 3000 miles away in your scope. Other comparisons could be made for a distant lighthouse or other visible landmark as well as visible planets, etc.

But yes, the penny will look 77 times bigger.

Hope this helps.

### #3 Ira

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Posted 19 May 2013 - 04:43 PM

The farthest most people can see with their naked eye is the Andromeda Galaxy, at 2.5 million light years. With a telescope? Take the limiting magnitude of the scope and find the most distant galaxy that you can realistically make out with it. Not sure what that would be. That's how I would answer the question.

/Ira

### #4 omahaastro

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Posted 19 May 2013 - 07:23 PM

Yeah, I too usually talk about the Andromeda galaxy, as a naked eye object, the light year as a unit of distance ("186,000 miles... not per hour... 186,000 miles per -second-")... then share that I've seen the feeble glow of quasars over a billion light years away... 'how far I can see'.

### #5 broca

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Posted 20 May 2013 - 04:30 PM

Thanks to you all! Much appreciated.
Steve

### #6 wirenut

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Posted 20 May 2013 - 06:05 PM

When I'm asked that question I reply it's not how far but how bright that matters. then explain a streetlight can be seen farther away then a single xmas tree bulb or house light point out how the stars aren't all the same brightness. this also works for why I can show a galaxy but not pluto with a 8" scope

### #7 maroubra_boy

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Posted 20 May 2013 - 09:32 PM

When I'm asked that question I reply it's not how far but how bright that matters. then explain a streetlight can be seen farther away then a single xmas tree bulb or house light point out how the stars aren't all the same brightness. this also works for why I can show a galaxy but not pluto with a 8" scope

The "how far can you see" is quite a vexed one, and I too struggle with it. Most people who ask aren't upto speed with things astro, and to get technical with them can make one seem arrogant, a smartarse, or even too much of a rocket scientist. It's always been a challenge for me to come up with a reply that isn't over the top, and doesn't make assumptions of the understanding to astro novices.

Wirenut, I like your comparative reply. I might look at first mentioning how the perceive lights at night from an aircraft, a bright light doesn't mean it's closer - it could be a spot light in the far distance, or a feeble torch up close. Astro is the same. A galaxy is that massive spotlight, & Pluto the torch. Pluto's closer, but we can't see it. Then we can start to talk about distances.

I've got a viewing night coming up next month at a high school. I'm sure this question will come up. I'll see how it goes. I could just end up with egg on my face...

Alex.

### #8 Doc Willie

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Posted 21 May 2013 - 12:37 PM

There are a couple of quasars that can be seen with 4" and up of aperture. 2 billion light years or 10,000,000,000,000,000,000,000 miles away.

### #9 csrlice12

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Posted 21 May 2013 - 12:48 PM

To infinity....and beyond.

I see things that happened before the Earth even existed....

I don't see objects, I see the past.....

### #10 darthwyll

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Posted 22 May 2013 - 10:24 AM

I usually tell people I've seen back in time about 50 million years. Typically, the faint fuzzy galaxies sit in a range between 5 and 80 million light years away. Of course it depends on what you're looking at and where it is. I missed the chance to check out a quasar at Hodges Gardens in Louisiana. Maybe next time!

### #11 DaemonGPF

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Posted 22 May 2013 - 12:38 PM

I put a powerpoint together which seems to work for kids of all ages - I've used it in my daughter's 3rd grade class, and just 2 weeks ago I used it with a Highschool Physics class. It gives some perspective in terms they can universally understand, and shows how far "we humans" can see with all of our technology while demonstrating scale at the same time. You're welcome to peruse it and see if something in this context might help. Let me know and I can PM you the link.

### #12 maroubra_boy

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Posted 25 May 2013 - 06:12 PM

Here in the Southern sky, we have the galaxy group "The Grus Quartet", aka The Grus Trio (something like the Leo Trio). These four galaxies are some 65 million light years away, roughly how long ago the dinosaurs were wiped out. I tell folks that's how long the light of those galaxies has been travelling & has only just got to us!

I used to be able to see this group in an 8" scope from home, but light pollution in the last 2years has made them a dark sky option now for visual. My little video rig will now make this group a centre piece again,

Great thread this one! Has stirred up some good discussion & provoked some new outreach ideas with targets.

Thanks Steve!

### #13 Skylook123

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Posted 26 May 2013 - 12:41 AM

I tend to avoid the specific answer unless asked "how far away is that?" when discussing a naked eye or eyepiece object.

When asked "How far can that telescope see?" or the companion question, "How high a power can that telescope go?", I turn it into a bit of a telescope technology discussion by saying "Technically, all telescopes see to infinity and to infinite power. WHAT you can see is dependent on, first of all, the aperture which is how much information can come in. Then atmospherics, telescope technology, and other criteria play into the answer." Then specific examples follow, like ideal power per inch versus actual under real conditions, how even the Hubble can't get down to the resolution needed to see man's traces on the moon, maximum power capable on any scope at sea level, etc., always ending up with aperture and reality, and why their 10X50 binoculars can't see the flag on the moon, and why their 60mm refractor can't do 750X or spot every moon of Saturn or the entire Virgo cluster. But why portability can still bring the gorgeous star fields in context as great fields to sweep, and double stars, selected planetary nebulae, and even galaxies under the right circumstances are achievable with low to moderate cost (compare the cost of an 8" f/6 Newtonian on a dob mount with, say, same tube on an Atlas EQ-G). I usually have a crowd at the scope, so if one person asks, all are thinking the same thing so it's work the pause for a reality check versus the dime store scope boxes or Hubble pictures.

### #14 cpsTN

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Posted 26 May 2013 - 02:01 PM

I don't think this is the question most people mean when the ask "how far can I see" because the answer is meaningless to most people, even me. For example, due to a disability, I can lift only about 60 pounds. So, saying that an object weighs either 400 pounds or 10,000 pounds doesn't really mean anything really because both are so far beyond what I can negociate that, to me, one might as well be the other. Its the same with distance. I can imagine something that is, say, 500 miles away because I have experience with that, but if I say something is 53 million away (very close astromonically), it is still TOO far for people to visualize. Even the penny (cent) seen from so far is not usable, since people seeing things on Earth doesn't equate to looking into the sky.

### #15 broca

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Posted 26 May 2013 - 02:52 PM

I guess what I am trying to do is give examples that we are all familiar with. We all (kids and adults) have held a penny and we can all relate to how far 2 miles is, so I was trying to relate if one could resolve 2 pennies from 2.5 miles away using the above telescope. I like maroubra_boy's explanation using dinosaurs, that light seen from a distant galaxy left that galaxy when a long extinct species roamed the earth.

### #16 cpsTN

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Posted 26 May 2013 - 03:21 PM

I suppose that would do because it is would give them the time it takes light. Although I understand it, I don't put a lot of stock in what science says about this or that as far as how great the distances are or their time tables. I DO understand that things are an unbelievable distance away.

### #17 skyguy88

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Posted 26 May 2013 - 06:07 PM

I like to use a ladder approach starting with the solar system, then various objects in the MW and its size, then Andromeda, M82, M51, M 104, M 84. The ladder adds more credibility as you move to greater distances and helps folks to get their heads around extreme astro distances.

I use a 20 inch model of the MW to help with the scaling of things. I can point to the fact that Andromeda is twenty something MW diameters distant...and that at the scale of the MW model, the SS diameter out to Neptune's orbit is 1/2 of a millionth of an inch.

Long winded, but it gets to a lot of the themes that I like to address in public programs. Using a video display allows me to address a group and display some of the objects as the discussion progresses.

Bill

### #18 Doug Reilly

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Posted 16 July 2013 - 09:10 AM

I approach this in different ways. I have never been asked that exact question, but one of the common themes I try to structure my outreach program around is our place in the universe, and trying to provide a mental sense of depth to the night sky. I like to start up close and move outward, and I like to get them to make the transition from thinking in terms of distance only to both distance and time early on. By the time we get to deep sky objects, it makes more sense to me to talk in terms of time/distance and not just distance. With solar system objects, I like to calculate driving time if one were to go in a car...that helps bring it home. I also do a "mental model" of the milky way using a cannister of salt and a black cloth that gives people a good sense of the enormity of the Milky Way.

I will, however, try to figure out the farthest thing I have observed with a given telescope. What are the quasers that can be seen in smaller scopes?

### #19 brianb11213

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Posted 16 July 2013 - 10:03 AM

What are the quasers that can be seen in smaller scopes?

The obvious one is 3C273 which is usually about mag. 12.5 (visible with 3" aperture under good conditions) and is approximately 2.4 x 10^9 light years distant ... that's approximately 1.4 x 10^22 miles. This number is somewhat hard to grasp but if you think of one mile for every grain of sand on all the beaches on Planet Earth you're somewhere in the right ballpark. Or, think of it another way: if there was a whacking great mirror on 3C273 and you had a hugely powerful telescope, if you looked in the mirror you'd still have to wait another 300 million years or so to see the reflection of the solar system forming.

### #20 skyguy88

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Posted 16 July 2013 - 06:50 PM

I use a 20 inch model of the Milky Way as a yardstick. Andromeda is 25 MW diameters away, M81/82 are 120 diameters distant, and M 51 is 230 diameters. 3C273 would be about 20,000 diameters. All of these numbers are at least fathomable.

The model is also good for putting the solar system in perspective, being a fraction of a micro inch.

It also helps to show visitors where the scope is pointed.

Bill

### #21 schluterdude

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Posted 06 August 2013 - 03:58 PM

Most people can't even fathom the distance from the Earth to the Moon, which is understandable, it's "close" in our terms, but still far away by mortal standards!

I tend to steer it into a resolution question. Ideally, you have a really sweet termination line on the moon, and then I start to talk about how when you drop a pebble into water, how you see the little "blip" in the middle from the water rebounding. I tell them that rock behaves the same during impact, and you can see the frozen remnants in the moon.

The first response is always "no way!", then I let them have a peek works every time! On very nice nights, Saturn works great too!

Sorry, just noticed the last comment was nearly a month ago, sorry to bring up an old topic!

### #22 csrlice12

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Posted 24 August 2013 - 10:50 AM

Take a look.....

### #23 haywool

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Posted 24 August 2013 - 06:01 PM

When asked something like "how far can you see with that thing?" I usually respond with something like, "Since the telescope is a light gathering tool, then if the light can reach the telescope, NO MATTER HOW FAR AWAY IT STARTED FROM, then THAT's how far we can see with it."

Oversimplified, yes. But it works.

Rich (haywool)

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