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True horizon Vs visible?

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#1 unn

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Posted 12 July 2020 - 10:41 PM

I am in Dallas, Texas. I went out to the lake today to possibly view the Neowise comet. The tree line on the other side of the lake is quite far and there are no high rises in the direction I was looking. I have seen the sunset from there many times and the time the sun actually disappears behind the tree line is very close to the published sunset time. I was using my phone app to get the landmarks. On the phone the horizon line appeared much below the viewed tree line. The position of the comet at the time was shown in the online planetarium to be about 10 degrees altitude. Which means in my effort to view it, the comet may already be near or below the tree line. Please comment whatever useful info on this issue. Thanks.


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#2 barbarosa

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Posted 12 July 2020 - 11:53 PM

Does your planetarium program allow for refraction?



#3 stoest

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Posted 13 July 2020 - 12:11 AM

I've always used the width of my hand at arms length to approximate 5 degrees, so two hands width should be about 10. It seems unlikely that if the trees are quite far that they're that high. Depending on which lake you went to, I'm in the metro area as well, there might have been a lot of haze which is pretty common this time of year and I've heard that it can be a threshold object if the haze is bad. I'm waiting till it's an evening object instead of a morning object so I'll see then.



#4 TOMDEY

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Posted 13 July 2020 - 01:56 AM

The definition of ~Horizon~ (like most things technical) gets rather arcane among those steeped in geodesy, astrometry, surveying, inertial guidance, orbital mechanics, sailing, chronometry, GPS, etc. In order to start with an indisputable universal geometric that is independent of local viewing conditions, horizontal is defined as orthogonal to the vector running to the center of gravity of the earth. For our purposes, this is almost exactly the same as referenced to the local gravity field. Think of it as the zero-altitude vacuum horizon. Local conditionals then modify what we see: Altitude above local ground depresses the seen vacuum horizon by an easily derived amount. Atmospheric refraction raises extra-atmospheric targets, depending on Elevation Angle, atm pressure, humidity (but not temperature!... bone of contention)... exceeding half a degree at the horizon [but only 6 arc-min for this comet when ten degrees up].

 

Practitioners almost always use the vacuum constructs, and then correct for earth's oblateness, altitude, atm refraction, etc. which will affect sailors' sightings relative to the plumb or seen horizons. These corrections generally also apply for us amateur astronomers, when we are trying to assess "how high" a celestial object will appear above ~the seen horizon~ aka whether, and for how long... we can see it!    Tom


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#5 barbarosa

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Posted 13 July 2020 - 03:41 AM

Practitioners? Of the dark arts?

 

I am out of practice so all I do use Stellarium. After of course checking the box for "Atmospheric virtualization" and selecting the values for pressure and temperature. Then I get hours of fun checking and unchecking virtualization and watching objects near the horizon move up and down. Hours of fun.bounce.gif  


Edited by barbarosa, 13 July 2020 - 03:42 AM.


#6 Tony Flanders

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Posted 13 July 2020 - 03:02 PM

I am in Dallas, Texas. I went out to the lake today to possibly view the Neowise comet. The tree line on the other side of the lake is quite far and there are no high rises in the direction I was looking. I have seen the sunset from there many times and the time the sun actually disappears behind the tree line is very close to the published sunset time. I was using my phone app to get the landmarks. On the phone the horizon line appeared much below the viewed tree line. The position of the comet at the time was shown in the online planetarium to be about 10 degrees altitude. Which means in my effort to view it, the comet may already be near or below the tree line. Please comment whatever useful info on this issue. Thanks.

If the ephemerides that Sky Safari is using are correct, Comet C/2020 F3 NEOWISE could not possibly have been visible from Dallas last night (July 12). It would have been 10 degrees above the northwest horizon just about 9 p.m. -- is that when you were out?

 

At that time, the Sun was just 5 degrees below the horizon, meaning that it would still be bright enough to read a fine-print book with ease. The sky near the Sun would be far too bright even to see a first-magnitude star, much less a comet, no matter how bright. In other words, you failed to see the comet because it was too dim, not because it was too low.

 

If this doesn't match your experience, it's possible that your app is an hour off, possibly due to ignoring daylight saving time.

 

I agree that the horizon on the far side of the lake is almost certainly much lower than 10 degrees. All the other factors that people are mentioning, such as refraction, are noise at this level of detail; they affect the altitude of the horizon about a half degree except in extraordinary cases.

 

The comet will be getting rapidly higher in the evening sky every night from now on. You might be able to spot it tomorrow or the next evening.



#7 kathyastro

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Posted 13 July 2020 - 03:21 PM

If you are in Texas, the comet will set soon after sunset.  Here in Nova Scotia, it is just barely circumpolar, skimming the weeds as it passes North.  Farther north, it is easily circumpolar.

 

Evening viewing should get better in the next week or two.



#8 starblue

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Posted 15 July 2020 - 10:25 PM

Last night (7/14) I looked for the comet at 10pm MDT, about the boundary between nautical and astronomical twilight--the western sky was clear and getting darker but nearer the horizon it was still aglow with sunset colors. The comet's alt/az was 10* high and 325* around. I started my scanning with 7x35 binoculars from there. However, my body doesn't have digital setting circles built-in, so I was off somewhat. But in scanning I stumbled onto a distinctive pair of stars, and since I know my constellations well, I knew it was the Arabs' "3rd leap of the gazelle" in Ursa Major. Checking their location in SS6, I saw the comet was halfway between them and the horizon and 5* to the right, which is half my binocular's FOV of 10*. Looking there with the binoculars, there it was!

 

The point is, use as many landmarks as possible, on earth and in the sky, to locate an object you're not familiar with. It's also getting higher, which will work to your advantage. And use binoculars--it's not *that* bright.


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#9 unn

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Posted 26 July 2020 - 02:13 PM

Thank you every body.

 

I was finally able to see it last week in the evening. It was easier to find it with binoculars by the landmarks rather than use the auto controller with coordinates. Thanks.


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