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uniqueuponhim
2005-Nov-18, 11:33 PM
A while ago, the evening Mars was in opposition with the earth, I noticed that Mars was exceptionally bright and large, and posted on here about it. I had thought I was able to resolve it as a disk, however, I was told that that is impossible.

Tonight, Venus is up, and quite bright and large, and there's a fairly dim star right next to it to compare it to. Now, when I squint, all of the glare disappears, and the dim star next to Venus is definately a point. Venus, however, seems immense by comparison - I would say it is definitely a disk and not a point. It's likely that I am wrong, however, as I'm sure one of you will point out.

So my question is, why is it that in these cases, I think I can resolve these objects as disks, and they look for all the world to me as if they are disks, yet it is impossible for that to happen?

Also, on a somewhat unrelated note, while the big dipper is usually pretty high up in the sky when I see it, tonight when I was out, it was almost right on the horizon (in fact, it was covered up by trees most of the time) and it appeared to be a lot bigger than I remember seeing it before. Is this a result of the same sort of optical illusion which supposedly makes the moon look bigger when it is near the horizon?

grant hutchison
2005-Nov-18, 11:53 PM
So my question is, why is it that in these cases, I think I can resolve these objects as disks, and they look for all the world to me as if they are disks, yet it is impossible for that to happen?You are definitely not resolving Venus, since Venus is in crescent phase just now, and should not appear as a disc if you are seeing it clearly. I think you're just seeing the common illusion that bright point sources of light appear bigger than dim point sources of light: I've no idea of the physiological mechanism for this, but it's certainly a common perception.
(Some people are able to make out Venus as a crescent, BTW: at maximum diameter it's quite close to the resolution limit of the human eye.


Is this a result of the same sort of optical illusion which supposedly makes the moon look bigger when it is near the horizon?Yes, the same "Moon illusion" has been described for constellations.

Grant Hutchison

The Mangler
2005-Nov-23, 04:37 AM
Also, on a somewhat unrelated note, while the big dipper is usually pretty high up in the sky when I see it, tonight when I was out, it was almost right on the horizon (in fact, it was covered up by trees most of the time) and it appeared to be a lot bigger than I remember seeing it before. Is this a result of the same sort of optical illusion which supposedly makes the moon look bigger when it is near the horizon?

Are you sure it wasn't Pegasus? Pegasus looks similar to the big dipper, but a lot bigger.

redshifter
2005-Nov-23, 07:14 AM
Pegasus is pretty high up in the evenings these days, while the big dipper is pretty low this time of year, certainly in the early evening when Venus is visible.