Starship Asterisk*

by **geckzilla** » Thu Dec 05, 2013 3:14 pm

Here's Abell 2744 itself! http://www.flickr.com/photos/geckzilla/11222343673/

by **rstevenson** » Thu Dec 05, 2013 2:58 am

geckzilla wrote:And so I spent a large proportion of the rest of the day doing just that...
Note if you download the original PNG file by right clicking the image at Flickr, you will be downloading a 12.3 megabyte file.
http://www.flickr.com/photos/geckzilla/11214379925/

Thank you, thank you, thank you!

Rob

by **geckzilla** » Thu Dec 05, 2013 2:16 am

And so I spent a large proportion of the rest of the day doing just that...
Note if you download the original PNG file by right clicking the image at Flickr, you will be downloading a 12.3 megabyte file.
http://www.flickr.com/photos/geckzilla/11214379925/

by **geckzilla** » Wed Dec 04, 2013 2:41 pm

With Frontier Fields, Hubble is going to do more ultra deep field observations so that the galaxy estimate can be more accurate. They're doing parallel observations of nearby sections of sky which should be pretty similar to the previous ultra deep fields while looking at lensed galaxies. They're doing six of these! The first one is already done and the data is available. I should go process it!

by **Mizar** » Wed Dec 04, 2013 7:04 am

rstevenson wrote:So the figure of 12.7 million is a bit off, but close enough for horseshoes.

From that, it would appear DavidLeodis was more correct than BDanielMayfield. But when I read STScI-2004-07 I noticed where BDanielMayfield must have seen his info:

Called the Hubble Ultra Deep Field (HUDF), the million-second-long exposure reveals the first galaxies to emerge from the so-called "dark ages"...

So indeed, it's a case of the experts having widely varying figures that affect what the exposure time was. Which I never precluded that possibility. It's a fairly common thing.

On that Ask an Astronomer page, interesting to note they got the number of galaxies in the observable universe wrong. They were off by two orders of magnitude, as from every source I've heard the estimate should be that

there are over 100 billion galaxies in the universe.

Extrapolation from number of stars

In any case, it's astounding there's 12.9 million times as much sky to examine as the 3.4 arc-minute square area of the HUDF, and that there's 10,000 galaxies in that small an area. My high power eyepiece on my Mak sees just 18.72 minutes across. The HUDF is only 18% of that width of view and only 3.3% the area, yet there's 10,000 galaxies there.

by **rstevenson** » Tue Dec 03, 2013 1:08 pm

Ignoring the discussion about who said what, here are some numbers, from the Ask an Astronomer site at Cornell University...

... the sphere of the sky is 41,253 square degrees. This corresponds to 148,510,800 square arcminutes.
According to the Space Telescope Science Institute, the Hubble Ultra Deep Field has an angular size of 11.5 square arcminutes. That means that it would take 12,913,983 Deep Field images to cover the entire sphere of the sky!

So the figure of 12.7 million is a bit off, but close enough for horseshoes.

Rob

by **Mizar** » Tue Dec 03, 2013 6:06 am

geckzilla wrote:
Mizar wrote:It reminds me of the no-longer-calling-Pluto-a-planet issue. Illogical.
The only thing illogical about it is to get emotional over the subject.

Who's getting emotional? Not me. But it did create a lot of emotional controversy for a long time. And no, there were a whole load of illogical aspects to it. But, that would be way digressing.

DavidLeodis wrote:In your post of Sep 03 2013 3:07 am Mizar you state "DavidLeodis wrote: The whole sky contains 12.7 million times more area than the Ultra Deep Field. To observe the entire sky would take almost 1 million years of uninterrupted observing.
That doesn't seem correct that the whole sky could be as much as 12.7 million times the HUDF area, but it's not like I calculated it. That means the HUDF did an exposure of the area for nearly 29 days straight! That's a lot of valuable Hubble observing time."

I got the information from the Hubble NewsCenter release STScI-2004-07. That will be found through clicking the second 'HUDF' link in the APOD of 2013 Aug 27. That brings up the APOD of 2004 Sept 29. Clicking the 'Hubble Ultra Deep Field' link in that APOD brings up STScI-2004-07, which has the information that I noted. If, as you suggest, the 12.7 million times is wrong then blame the Hubble people rather than your implication that I calculated it wrong.

Okay, thanks for the source. But, instead of all that nested browsing, don't you have a direct link? I didn't imply you calculated it wrong, I merely noted the contradiction between what you and BDanielMayfield were saying, and hence that one of you (or both) had to not have it right (the info). I said nothing of your calculations, nor that you were the original source of the info. Where did you get any of that from?

by **DavidLeodis** » Tue Sep 03, 2013 12:54 pm

In your post of Sep 03 2013 3:07 am Mizar you state "DavidLeodis wrote: The whole sky contains 12.7 million times more area than the Ultra Deep Field. To observe the entire sky would take almost 1 million years of uninterrupted observing.
That doesn't seem correct that the whole sky could be as much as 12.7 million times the HUDF area, but it's not like I calculated it. That means the HUDF did an exposure of the area for nearly 29 days straight! That's a lot of valuable Hubble observing time."

I got the information from the Hubble NewsCenter release STScI-2004-07. That will be found through clicking the second 'HUDF' link in the APOD of 2013 Aug 27. That brings up the APOD of 2004 Sept 29. Clicking the 'Hubble Ultra Deep Field' link in that APOD brings up STScI-2004-07, which has the information that I noted. If, as you suggest, the 12.7 million times is wrong then blame the Hubble people rather than your implication that I calculated it wrong.

by **geckzilla** » Tue Sep 03, 2013 2:18 am

Mizar wrote:It reminds me of the no-longer-calling-Pluto-a-planet issue. Illogical.

The only thing illogical about it is to get emotional over the subject.

by **Mizar** » Tue Sep 03, 2013 2:07 am

Ann wrote:I once built a super-simple model of the inner solar system. I used cotton balls, 0.02 meters in diameter, for the Earth, Venus and Mars. I used yellow peas for Mercury and the Moon, and a round table-cloth, 2 meters in diameter, for the Sun. I then placed the "planets" at the appropriate distances from the Sun. The Earth was about 200 meters from the Sun.

When I was a kid I did an even cruder scale model, really it was just marking the distances of the planets from the Sun across the floor of the house. My sister harshly criticized me saying something like it was stupid. In retrospect, that was more of a self-reflection.

DavidLeodis wrote:The whole sky contains 12.7 million times more area than the Ultra Deep Field. To observe the entire sky would take almost 1 million years of uninterrupted observing.

That doesn't seem correct that the whole sky could be as much as 12.7 million times the HUDF area, but it's not like I calculated it. That means the HUDF did an exposure of the area for nearly 29 days straight! That's a lot of valuable Hubble observing time.

BDanielMayfield wrote:Great comment David. So the image this simulation was made from covered only 1/12,700,000th of the sky. There was absolutely nothing special about this location in space. This site was simply just an extremely empty looking patch of sky, with no foreground stars or galaxies in the way. In even large land based telescopes this must look like an untold number of other boring looking dark, empty places with maybe only a few, barely discernable faint fuzzies, if anything at all. But keep Hubble focused on this place for nearly a million seconds and voila, 10,000 galaxies appear! (To make the video Dr. Summer’s team estimated the distances of about 5,000 of these.)

A million seconds would be only about 11.5 days for the exposure time. One of you (or both) doesn't have this right.

BDanielMayfield wrote:So this random sampling probes a volume of space of between about 18.4 and 30 billion light-years out and found about 10,000 galaxies. Across the entire celestial sphere this would indicate the presence of 10,000 x 12.7 million = 127 billion galaxies, not counting any less than 18.4 B ly nor any more than 30 B ly away. If the density of galaxies inside the 18.4 billion light-year radius is about the same as that inside the volume probed by the HUDF survey we would have another 38 billion galaxies, so this survey suggests the presence of at least around 165 billion galaxies in the observable universe.

And you're sure that the HUDF didn't image any galaxies closer than 18.4 billion light years?

BDanielMayfield wrote:
DOUGLAS L. MARTIN wrote:HOW CAN SOMETHING TRAVEL THROUGH 30 BILLION YEARS IN A UNIVERSE ONLY 13.7 BILLION YEARS OLD?
We can see objects that are NOW further than 13.7 billion light-years, but no light has been traveling longer of course. But as light travels through space, space itself is expanding, carrying source and observer apart. The estimated distance of 30 billion light-years is the co-moving distance, accounting for the separation that has occurred during the time the light has taken to get to us.

This is a good explanation of how it's supposed to work. The only thing I'd add is the age of the universe has been slightly upgraded to 13.8 billion years.

geckzilla wrote:If a guy throws a baseball at you while he is 13.7 meters away from you and then you both immediately run away from the ball after it's released, by the time the ball hits you in the back of the head the guy and yourself are already 30 meters apart from one another. The distance between yourself and the guy at the moment the ball was released and the distance between the both of you the moment it hit you in the head are two different measurements.

That's a good analogy, except for the 13.7 meters corresponds to the current time (universe is 13.7 billion years old) and the 30 meters corresponds to the far distant age of the universe. Therefore, the analogy is centered on an explanation for 16.3 billion years from now.

Anthony Barreiro wrote:Isn't the observable universe estimated to be 42 billion light years across? So if the universe suddenly stopped expanding, light would take 42 billion years to travel across the universe, right?

Yes, but the observable universe is actually estimated at 93 billion light-years. http://en.wikipedia.org/wiki/Observable_universe

I understand why astronomers are calling the observable universe this large. Essentially it's due to the expansion of space, in a nutshell. But I have to say I object to this definition. You're not actually observing anything up to 46–47 billion light-years away (minus the time/distance for the universe to become transparent). This is only observing something that far away as it is at the current time. Looking out in space is looking back in time. So we are not observing anything any further away than 13.8 billion light-years, since we're observing it as it was in the distant past, and at the distance it was then. We're only assuming that what we're looking at now is currently up to 47 billion light-years away, but we don't know that nor do we receive any data from it. For all we know, some mysterious force has halted its recession from us and is really just out of view at 13.9 billion LY, at the current time. For this reason, I don't think astronomers have any business saying the observable universe is any larger than 27.6 billion LY across.

It reminds me of the no-longer-calling-Pluto-a-planet issue. Illogical.

by **Zesago** » Thu Aug 29, 2013 11:33 am

firstmagnitude wrote:Where's Carl Sagan and his dandelion spaceship from Cosmos when you need him with a little Vangelis music in the background?

I heard the Cosmos music in my head when I was watching this. Cheers!

by **Anthony Barreiro** » Wed Aug 28, 2013 10:56 pm

geckzilla wrote:
Anthony Barreiro wrote:On the other hand, the relatively slow speed of light allows cosmic distances to be understood within human terms -- light takes eight minutes to get from the Sun to the Earth, a second and a half to get from the Moon to the Earth, four hours to get from the Sun to Neptune, four years to get from the nearest star to Earth, tens or hundreds of years to get from most of the visible stars to Earth, 100,000 years to get from one edge of the Milky Way to the other, 2.5 million years to get from the Andromeda Galaxy to the Milky Way, 50 million years to get from the big Virgo cluster galaxies to our local group ... . And if spacetime would just stop expanding, light could get from one edge of the observable universe to the other in 42 billion years ... . Okay, 42 billion years is incomprehensible in human terms, but all the earlier steps are comprehensible ... .
Comprehensible to some, at least. I think it's kind of like visiting the Pyramids of Egypt. Until you are standing right next to one, its mass just isn't tangible. They exist in one's mind as a series of adjectives and it is known that they are enormous but to see even a very good photo is much different from standing near one. It's easy to see our solar system as an illustration in a book and have everything feel comfortably close together and a visit to one object or another something like a trip across an ocean. An ocean simply doesn't suffice as a metaphor for space. The word "lightyear" is used so much and most people understand it by definition but for it to really sink in I think it takes more than a verbal description.

I find it more helpful to stand outside under a dark night sky with somebody who knows what they're talking about than to look at illustrations in a book or on a computer screen. Or rather, observing the actual sky is a necessary complement to book learning, perhaps like visiting the pyramids makes your preparatory study come alive.

Ann wrote:
Anthony Barreiro wrote:
Okay, 42 billion years is incomprehensible in human terms, but all the earlier steps are comprehensible ... .
Anthony, you mean 42 billion years as in 42 billing light-years, don't you?

Isn't the observable universe estimated to be 42 billion light years across? So if the universe suddenly stopped expanding, light would take 42 billion years to travel across the universe, right?

Ann wrote:I once built a super-simple model of the inner solar system. I used cotton balls, 0.02 meters in diameter, for the Earth, Venus and Mars. I used yellow peas for Mercury and the Moon, and a round table-cloth, 2 meters in diameter, for the Sun. I then placed the "planets" at the appropriate distances from the Sun. The Earth was about 200 meters from the Sun.

I couldn't believe how tiny cotton ball Earth looked when it was placed 200 meters from table-cloth Sun! And I couldn't believe how vast the depth of those 200 meters looked in comparison.

I had "scaled down" the size of the inner solar system some 700,000,000 times. I was able to picture in my mind, more or less anyway, how big the Earth actually is. But when I tried to picture the actual, physical depth of those 200 X 700,000,000 meters in my mind, trying to sum it up by adding kilometer after kilometer until I got to 140,000,000,000 kilometers - okay, it should have been 150,000,000,000 kilometers instead, big deal - I found that I couldn't do it.

I'm sure that there are people out there who are much better at imagining the stark reality of 8 light-minutes in their minds. I'm sure there are those who can do it.

But if people tell me they can actually imagine a light-day, 24 light-hours, I'm not sure I believe them any more.

And imagining - actually imagining the physical reality of a light-year, I'm quite convinced it is humanly impossible.

So, Anthony, if you can actually comprehend all the steps leading up to 42 billion light-years, then you are 42 billion light-years and 8 light-minutes ahead of me.

Me, I can juggle the figures a bit. I can easily understand the difference between 21 billion light-years and 42 billion light-years, for example. Sure I can do that sort of thing.

But to comprehend the actual reality behind those figures... I can find no word to explain how impossible that is to me. Impossible, impossibler, impossiblest, mega super duper gobsmackedly impossiblest. And then some.

And you know what? I like it. The universe doesn't have to stop expanding as far as I'm concerned!

Ann

Scale models are pretty cool. Guy Ottewell's 1000 yard model of the solar system (or, the Earth as a peppercorn) is a lot of fun to experience. I once had the opportunity to walk along such a model on Ocean Beach here in San Francisco. At this scale, Alpha Centauri would be a 20 cm wide beach ball in Lima Peru. That is a vast scale, but comprehensible.

by **BDanielMayfield** » Wed Aug 28, 2013 10:11 pm

It works for this dummy. However, I must admit slight aversion due to the metaphor’s implication of rampant cowardice.

by **geckzilla** » Wed Aug 28, 2013 6:36 pm

BDanielMayfield wrote:
DOUGLAS L. MARTIN wrote:HOW CAN SOMETHING TRAVEL THROUGH 30 BILLION YEARS IN A UNIVERSE ONLY 13.7 BILLION YEARS OLD?
I’m going to boldly go where I haven’t dared to go before and attempt to answer your question Douglas, since it is such a commonly occurring, logical one. Hopefully my crude, simple answer will be corrected if needed and elaborated on by others more informed than I.

We can see objects that are NOW further than 13.7 billion light-years, but no light has been traveling longer of course. But as light travels through space, space itself is expanding, carrying source and observer apart. The estimated distance of 30 billion light-years is the co-moving distance, accounting for the separation that has occurred during the time the light has taken to get to us.

If my explanation is wrong or can be improved upon I welcome comment.

If a guy throws a baseball at you while he is 13.7 meters away from you and then you both immediately run away from the ball after it's released, by the time the ball hits you in the back of the head the guy and yourself are already 30 meters apart from one another. The distance between yourself and the guy at the moment the ball was released and the distance between the both of you the moment it hit you in the head are two different measurements.

That's my metaphor-for-dummies explanation for this one. Made by a dummy for dummies.

by **BDanielMayfield** » Wed Aug 28, 2013 6:00 pm

DOUGLAS L. MARTIN wrote:HOW CAN SOMETHING TRAVEL THROUGH 30 BILLION YEARS IN A UNIVERSE ONLY 13.7 BILLION YEARS OLD?

I’m going to boldly go where I haven’t dared to go before and attempt to answer your question Douglas, since it is such a commonly occurring, logical one. Hopefully my crude, simple answer will be corrected if needed and elaborated on by others more informed than I.

We can see objects that are NOW further than 13.7 billion light-years, but no light has been traveling longer of course. But as light travels through space, space itself is expanding, carrying source and observer apart. The estimated distance of 30 billion light-years is the co-moving distance, accounting for the separation that has occurred during the time the light has taken to get to us.

If my explanation is wrong or can be improved upon I welcome comment.

by **BDanielMayfield** » Wed Aug 28, 2013 5:28 pm

So many insightful comments, lightly seasoned with humor. What a great thread this is!

Anthony, I’m right with you in your thinking that we sort of have a mental handle on astronomical distances, but then Geckzilla and Ann had to go and pull the rug out from under us didn’t they. There’s nothing quite like actually building a scale model to put things in proper perspective, and Ann was just modeling the solar system, the distances of which pale into insignificance when compared even just with the next rung up the ladder; interstellar distance.

DavidLeodis wrote:The Hubble Ultra Deep Field view is fascinating. Mind-boggling, but fascinating!

A couple of things in the HUDF release (STScI-2004-07, released on March 9 2004) help give some sense of scale:-

7. If astronomers made the Hubble Ultra Deep Field observation over the entire sky, how long would it take?

The whole sky contains 12.7 million times more area than the Ultra Deep Field. To observe the entire sky would take almost 1 million years of uninterrupted observing.

8. How wide is the Ultra Deep Field's slice of the heavens?

The Hubble Ultra Deep Field is called a "pencil beam" survey because the observations encompass a narrow, yet "deep" piece of sky. Astronomers compare the Ultra Deep Field view to looking through an eight-foot-long soda straw.

The Ultra Deep Field's patch of sky is so tiny it would fit inside the largest impact basin that makes up the face on the Moon. Astronomers would need about 50 Ultra Deep Fields to cover the entire Moon.

Great comment David. So the image this simulation was made from covered only 1/12,700,000th of the sky. There was absolutely nothing special about this location in space. This site was simply just an extremely empty looking patch of sky, with no foreground stars or galaxies in the way. In even large land based telescopes this must look like an untold number of other boring looking dark, empty places with maybe only a few, barely discernable faint fuzzies, if anything at all. But keep Hubble focused on this place for nearly a million seconds and voila, 10,000 galaxies appear! (To make the video Dr. Summer’s team estimated the distances of about 5,000 of these.)

So this random sampling probes a volume of space of between about 18.4 and 30 billion light-years out and found about 10,000 galaxies. Across the entire celestial sphere this would indicate the presence of 10,000 x 12.7 million = 127 billion galaxies, not counting any less than 18.4 B ly nor any more than 30 B ly away. If the density of galaxies inside the 18.4 billion light-year radius is about the same as that inside the volume probed by the HUDF survey we would have another 38 billion galaxies, so this survey suggests the presence of at least around 165 billion galaxies in the observable universe.

I suspect that 165 billion could still be a major undercount, due to the fact that the further out we look the brighter the galaxy must be to be seen at all, and so many dim and or small galaxies could be overlooked, even by Hubble. Therefore the density of galaxies could be underestimated. Galactic formation and destruction via mergers complicate the accounting, of course.

by **neufer** » Wed Aug 28, 2013 3:36 pm

DOUGLAS L. MARTIN wrote:
HOW CAN SOMETHING TRAVEL THROUGH 30 BILLION YEARS IN A UNIVERSE ONLY 13.7 BILLION YEARS OLD?

The same way that photons from the Big Bang did it

by **DOUGLAS L. MARTIN** » Wed Aug 28, 2013 2:42 pm

HOW CAN SOMETHING TRAVEL THROUGH 30 BILLION YEARS IN A UNIVERSE ONLY 13.7 BILLION YEARS OLD?

by **Psnarf** » Wed Aug 28, 2013 2:11 pm

Sulu, reverse engines....Slowly!

by **DavidLeodis** » Wed Aug 28, 2013 11:48 am

I'm confused by the image used as the APOD as I do not seem to be able to spot where that view is in the video, though as it is full of objects it would seem to be near the start. I wonder if it is a very greatly enlarged part of the video? The various colours in that image are intriguing.

Edit added around 15:20 on August 28 2013. On looking at the video again a few times it now seems to me that the APOD is an enlargement of a scene at about 28/29 seconds in the video.

by **DavidLeodis** » Wed Aug 28, 2013 11:38 am

The Hubble Ultra Deep Field view is fascinating. Mind-boggling, but fascinating!

A couple of things in the HUDF release (STScI-2004-07, released on March 9 2004) help give some sense of scale:-

7. If astronomers made the Hubble Ultra Deep Field observation over the entire sky, how long would it take?

The whole sky contains 12.7 million times more area than the Ultra Deep Field. To observe the entire sky would take almost 1 million years of uninterrupted observing.

8. How wide is the Ultra Deep Field's slice of the heavens?

The Hubble Ultra Deep Field is called a "pencil beam" survey because the observations encompass a narrow, yet "deep" piece of sky. Astronomers compare the Ultra Deep Field view to looking through an eight-foot-long soda straw.

The Ultra Deep Field's patch of sky is so tiny it would fit inside the largest impact basin that makes up the face on the Moon. Astronomers would need about 50 Ultra Deep Fields to cover the entire Moon.

by **Ann** » Wed Aug 28, 2013 5:04 am

Anthony Barreiro wrote:
Okay, 42 billion years is incomprehensible in human terms, but all the earlier steps are comprehensible ... .

Anthony, you mean 42 billion years as in 42 billing light-years, don't you?

I once built a super-simple model of the inner solar system. I used cotton balls, 0.02 meters in diameter, for the Earth, Venus and Mars. I used yellow peas for Mercury and the Moon, and a round table-cloth, 2 meters in diameter, for the Sun. I then placed the "planets" at the appropriate distances from the Sun. The Earth was about 200 meters from the Sun.

I couldn't believe how tiny cotton ball Earth looked when it was placed 200 meters from table-cloth Sun! And I couldn't believe how vast the depth of those 200 meters looked in comparison.

I had "scaled down" the size of the inner solar system some 700,000,000 times. I was able to picture in my mind, more or less anyway, how big the Earth actually is. But when I tried to picture the actual, physical depth of those 200 X 700,000,000 meters in my mind, trying to sum it up by adding kilometer after kilometer until I got to 140,000,000,000 kilometers - okay, it should have been 150,000,000,000 kilometers instead, big deal - I found that I couldn't do it.

I'm sure that there are people out there who are much better at imagining the stark reality of 8 light-minutes in their minds. I'm sure there are those who can do it.

But if people tell me they can actually imagine a light-day, 24 light-hours, I'm not sure I believe them any more.

And imagining - actually imagining the physical reality of a light-year, I'm quite convinced it is humanly impossible.

So, Anthony, if you can actually comprehend all the steps leading up to 42 billion light-years, then you are 42 billion light-years and 8 light-minutes ahead of me.

Me, I can juggle the figures a bit. I can easily understand the difference between 21 billion light-years and 42 billion light-years, for example. Sure I can do that sort of thing.

But to comprehend the actual reality behind those figures... I can find no word to explain how impossible that is to me. Impossible, impossibler, impossiblest, mega super duper gobsmackedly impossiblest. And then some.

And you know what? I like it.

The universe doesn't have to stop expanding as far as I'm concerned!

Ann

by **Beyond** » Wed Aug 28, 2013 3:06 am

by **neufer** » Wed Aug 28, 2013 2:28 am

ta152h0 wrote:
This is similar to diving in a lake near shore and watching all the beings and things float by you as you "paddle away "

One difference is there is a dock waiting for you, maybe !

Maybe

by **geckzilla** » Wed Aug 28, 2013 12:12 am

Anthony Barreiro wrote:On the other hand, the relatively slow speed of light allows cosmic distances to be understood within human terms -- light takes eight minutes to get from the Sun to the Earth, a second and a half to get from the Moon to the Earth, four hours to get from the Sun to Neptune, four years to get from the nearest star to Earth, tens or hundreds of years to get from most of the visible stars to Earth, 100,000 years to get from one edge of the Milky Way to the other, 2.5 million years to get from the Andromeda Galaxy to the Milky Way, 50 million years to get from the big Virgo cluster galaxies to our local group ... . And if spacetime would just stop expanding, light could get from one edge of the observable universe to the other in 42 billion years ... . Okay, 42 billion years is incomprehensible in human terms, but all the earlier steps are comprehensible ... .

Comprehensible to some, at least. I think it's kind of like visiting the Pyramids of Egypt. Until you are standing right next to one, its mass just isn't tangible. They exist in one's mind as a series of adjectives and it is known that they are enormous but to see even a very good photo is much different from standing near one. It's easy to see our solar system as an illustration in a book and have everything feel comfortably close together and a visit to one object or another something like a trip across an ocean. An ocean simply doesn't suffice as a metaphor for space. The word "lightyear" is used so much and most people understand it by definition but for it to really sink in I think it takes more than a verbal description.

Starship Asterisk*

APOD: A Flight through the Hubble Ultra... (2013 Aug 27)

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