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Can we distinguish stars and planets based on twinkling?

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#26 vtornado

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Posted 25 June 2021 - 11:03 PM

Especially for those of us with lots of trees and buildings blocking the horizon - I still have not had the opportunity to put a scope on it.

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#27 Redbetter

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Posted 26 June 2021 - 02:10 AM

Uranus can be seen without optical aid around opposition from a dark site.

From a dark site in moonless sky it is visible any time that it isn't close to the horizon/Sun.  It is brighter than 6th magnitude year around because its distance relative to the Earth doesn't change that much even when we are on the other side of our orbit.  

 

 

It's worth nothing that none of the ancient astronomers ever noticed Uranus, despite the fact that they cataloged hundreds of fainter stars. Stars of that magnitude tend to flicker in and out with averted vision anyway, so it would take a super-human memory to notice that one of them is out of place.

The ancient catalogs/charts were quite incomplete at the brightness level of Uranus.  They tended to tally around 1,000 identified stars or less.  The final pre-telescope effort, Bayer's Uranometria, based primarily on Brahe's measurements, only listed ~1,200 stars (maybe more...seem to be various numbers floating about on that.)  There are roughly twice that many stars down to 5.5 mag alone. 

 

The lack of completeness can be surprising looking at something common and well placed for northern observers, such as Hercules.  In looking at an image of Bayer's chart I don't see the ~5.0 mag star 51 Her midway between Beta and Delta Hercules.   Ditto for ~5.3 53 Her between Zeta and Epsilon.  I notice a number of other stars missing around 4.9 to 5.1 but others shown that are far dimmer.  So it is pretty hit or miss by the 5 mag range.  Of course M13 is not shown as a star or something else either.

 

Uranus actually was noticed, in the sense that it was recorded multiple times before it was discovered to be a planet.  Perhaps the earliest potential recording of it was by Hipparchos who seems to have cataloged it as a star in Virgo.  At that time it would have been near its brightest at about 5.4 mag.   Many centuries later, Flamsteed assigned it the designation 34 Tauri.  The difference of course is that people did not recognize it for what it was, because it is dim and moves relatively slowly.  

 

It isn't that Uranus wasn't seen or recorded, it was that it is so faint as to be superfluous in star fields.  Had it been 4th magnitude or at least brighter than 5.0 magnitude its movement would have been more noticeable over time since it would have disrupted constellations more.  But there is a substantial gap between it and the other naked eye planets (when the latter are positioned to be visible.)

 

Ironically, the slow movement is what makes it relatively straightforward to identify naked eye in dark sky each year--or at least as it has been with it well away from Milky Way star fields.  Knowing where it was a prior year, or a month or two before, gets me in the area.  I just start looking near the ecliptic in the region for a star dimmer than 5th magnitude that is not on my chart.  There are enough stars visible in the 5.5+ mag range that identifying it often takes a bit of checking, and sometimes a few tries to narrow in on if there are several near.

 

Other times, the first candidate turns out to be Uranus...

 

Sure, but it's pretty dim. One isn't going to just look up and say "hey, there it is."

...so that when already familiar with the dim field stars in a constellation I sometimes do just look up and say, "hey, there it is."  I simply point to it via Telrad, and go right to modest power (e.g. ~150x) in the scope to see its disc and confirm.  

 

M13 is somewhat more prominent and many don't see it either.  In dark sky it is very obvious.  



#28 Dave Mitsky

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Posted 26 June 2021 - 12:52 PM

From a dark site in moonless sky it is visible any time that it isn't close to the horizon/Sun.  It is brighter than 6th magnitude year around because its distance relative to the Earth doesn't change that much even when we are on the other side of our orbit.

Yes, I should have said that it is easier to see Uranus, which varies in apparent magnitude from 5.4 to 6.0 and has an average brightness of magnitude 5.7, around the time that the planet is at opposition.  



#29 Ulmer Spatz

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Posted 26 June 2021 - 02:13 PM

deleted, wrong thread.


Edited by Ulmer Spatz, 26 June 2021 - 02:46 PM.



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