Southern vs Northern constellations, any difference in visibility? [No]

[Abishua]

I heard a statement and not sure how to verify it, so asking here. So the statement is that from lets say 40 degrees north lat you can see all the northern constellations from any longitude, but the same is not true if you are at -40 degrees lat.. or southern hemisphere. So at south at that latitude you cannot see all the southern constellations like you can see all the northern ones from the north lat.

Is this true or bunk?

 

[Henk001]

I heard a statement and not sure how to verify it, so asking here. So the statement is that from lets say 40 degrees north lat you can see all the northern constellations from any longitude, but the same is not true if you are at -40 degrees lat.. or southern hemisphere. So at south at that latitude you cannot see all the southern constellations like you can see all the northern ones from the north lat.

Is this true or bunk?

Its bunk.
The northern and southern hemisphere are completely symmetrical. As soon as you cross the equator you will be able to see the whole southern hemisphere (with the south celestial pole initially on the horizon and all the constellations circling around it).

 

[Henk001]

(Amateur) astronomers all over the world use Stellarium. Try it. Choose any location you like and watch what is visable.
Don't worry about the fact that it is a computer program. Those amateur astronomers would immediately notice if things were wrong

 

[Abishua]

Yes.. I was just looking at stelarium for the past hour or so.. to put it more clearly, the claim is that from all meridians at the equator simultaneously you can see ursa minor, major and polaris when looking north, but when looking south you cannot see simultaneously from all of them the south pole star and southern cross constellation.

 

[Trailspotter]

Yes.. I was just looking at stelarium for the past hour or so.. to put it more clearly, the claim is that from all meridians at the equator simultaneously you can see ursa minor, major and polaris when looking north, but when looking south you cannot see simultaneously from all of them the south pole star and southern cross constellation.

No, it is not possible to see Ursa Major from all meridians at the Equator simultaneously. However, it is possible to see both Ursa Major and Crux (Southern Cross) from the Equator simultaneously, just not all time. I actually have seen them both simultaneously from Amboseli NP, Kenya, 2.5° south of the Equator.

 

[Mick West]

This type of claim dates back to Samuel Rowbotham, and is repeated by modern Flat Earth promoters. It was very hard to check back in the 1800s, but is very easy to check now.

Here's an example from Dubay's book, and his supporting quotes:

Polaris, situated almost straight over the North Pole, should not be visible anywhere in the Southern Hemisphere. For Polaris to be seen from the Southern Hemisphere of a globular Earth, the observer would have to be somehow looking "through the globe," and miles of land and sea would have to be transparent. Polaris can be seen, however, up to approximately 23.5 degrees South latitude.

"If the Earth is a sphere and the pole star hangs over the northern axis, it would be impossible to see it for a single degree beyond the equator, or 90 degrees from the pole. The line-of-sight would become a tangent to the sphere, and consequently several thousand miles out of and divergent from the direction of the pole star. Many cases, however, are on record of the north polar star being visible far beyond the equator, as far even as the tropic of Capricorn." -Dr. Samuel Rowbotham, "Earth Not a Globe, 2nd Edition" (41)

"The astronomers' theory of a globular Earth necessitates the conclusion that, if we travel south of the equator, to see the North Star is an impossibility. Yet it is well known this star has been seen by navigators when they have been more than 20 degrees south of the equator. This fact, like hundreds of other facts, puts the theory to shame, and gives us a proof that the Earth is not a globe." -William Carpenter, "100 Proofs the Earth is Not a Globe" (71)

Dubay, Eric (2014-11-09). The Flat Earth Conspiracy (Kindle Locations 1346-1358). Lulu.com. Kindle Edition.

Totally wrong. the latitudes a star is visible from depends only on its declination, equivalent to latitude in the celestial sphere (and maybe a couple of degrees due to refraction at the horizon). So Polaris, at 89° can only be viewed down to 2 or 3 degrees below the equator (under ideal viewing conditions).

Dubay seems to be repeating a misconception that the tilt of the Earth allows the stars to be seen. The tilt of the earth is only relevant to the Solar System, the axis of the the Earth points in a relatively fixed position year to year, and the pole stars are just the stars closest to the point where the axis happens to point.

To account for this glaring problem in their model, desperate heliocentrists since the late 19th century have claimed the ball-Earth actually tilts a convenient 23.5 degrees back on its vertical axis. Even this brilliant revision to their theory cannot account for the visibility of many other constellations though. For instance, Ursa Major, very close to Polaris, can be seen from 90 degrees North latitude (the North Pole) all the way down to 30 degrees South latitude.

Dubay, Eric (2014-11-09). The Flat Earth Conspiracy (Kindle Locations 1358-1362). Lulu.com. Kindle Edition.

Again, the visibility of any star is directly related to the its declination. The star in Ursa Major closest to the celestial North Pole is the amusingly named Dubhe, which has a declination of about 61.5° So from 30° South, at it's highest, Dubhe still fails to rise above the horizon (unless refraction bumps it up a degree or so). Here's the view showing the horizon. This intermittent visibility of (most of ) Ursa Major is entirely expected and very straightforward.

 

[Henk001]

bit off topic, but I couldn't resist this picture of Ursa Major from Chile
(taken from Phil Plait's Bad Astronomy (http://www.blastr.com/2017-5-9/astrophoto-big-dipper-over-chilebarely)
The lake shown is Laguna Lejia in Northern Chili, latitude 23°30' S)

 

[AmberRobot]

"The astronomers' theory of a globular Earth necessitates the conclusion that, if we travel south of the equator, to see the North Star is an impossibility. Yet it is well known this star has been seen by navigators when they have been more than 20 degrees south of the equator. This fact, like hundreds of other facts, puts the theory to shame, and gives us a proof that the Earth is not a globe." -William Carpenter, "100 Proofs the Earth is Not a Globe" (71)

Dubay, Eric (2014-11-09). The Flat Earth Conspiracy (Kindle Locations 1346-1358). Lulu.com. Kindle Edition.

Totally wrong. the latitudes a star is visible from depends only on its declination, equivalent to latitude in the celestial sphere (and maybe a couple of degrees due to refraction at the horizon). So Polaris, at 89° can only be viewed down to 2 or 3 degrees below the equator (under ideal viewing conditions).

Further, it is very easy to show with simple trigonometry that in Dubay's model the elevation angle of Polaris as a function of latitude doesn't agree with reality, disproving his flat earth model. He may mention observability, but he doesn't mention the actual angles they're observed at.

 

[Mick West]

This all related to the "Ground Truth" idea I wrote about here:
https://www.metabunk.org/ground-truth-verifying-stellariums-model-of-the-solar-system.t8678/

Also, because we know that the Stellarium predictions match the view from any arbitrary time and place, we can use Stellarium to observe the sky. We don't need to travel, we don't need to go outside. We have extensively verified that Stellarium is correct using ground truth, so we can use it to make observations about how things look from various positions. For example we can use to to demonstrate that the pole star, polaris, has an angle in the sky equal to latitude, and that it's not visible from the southern hemisphere, and that the Earth does not appear to be flat.

Stellarium matches observed reality. Stellarium shows a round Earth. Case closed.

 

[Abishua]

Thanks for all your responses.. lovely picture Henk! if some people from equator do not provide photographic evidence of this I agree.. case closed.

 

[Rory]

The latitudes a star is visible from depends only on its declination, equivalent to latitude in the celestial sphere (and maybe a couple of degrees due to refraction at the horizon). So Polaris, at 89° can only be viewed down to 2 or 3 degrees below the equator (under ideal viewing conditions).

There is another factor in this, which I discovered when challenged by a flat earther that he had evidence that Polaris could be seen from below 3°S - that of elevation.

It turns out he was right: but the image he was citing was taken from Kilimanjaro, at over 19,000 feet above sea level - an elevation which would give a view, by rough calculations, something similar to what would be seen from about 182 miles, or 2.7 degrees, further north.

Not sure if there are any mountains further south than that from which Polaris could be seen. All the big ones in Ecuador are a little closer to the equator. And, of course, that still leaves another 86.9 degrees worth of globe from which the north star can't be seen.

 

[Enricks]

This all related to the "Ground Truth" idea I wrote about here:
https://www.metabunk.org/ground-truth-verifying-stellariums-model-of-the-solar-system.t8678/


Stellarium matches observed reality. Stellarium shows a round Earth. Case closed.

Does refraction affect the visibility? On the wikipedia article about circumpolar stars it reads:

Vega (+38° 47') is technically circumpolar north of latitude +51° 13' (just south of London); taking atmospheric refraction into account it will probably only be seen to set at sea level from Cornwall and the Scilly Isles

 

[Mick West]

Does refraction affect the visibility? On the wikipedia article about circumpolar stars it reads:

Vega (+38° 47') is technically circumpolar north of latitude +51° 13' (just south of London); taking atmospheric refraction into account it will probably only be seen to set at sea level from Cornwall and the Scilly Isles

It does, but not a lot. The easiest way to remember and visualize the effect of refraction near the horizon is to note that when the sun first touches the horizon, it has (geometrically) just gone below the horizon. i.e. the refraction at the horizon is (on average) about the angular size of the sun, or half a degree.

Even that's an oversimplification though, stars are a lot smaller, so when a star stars sets over the ocean (theoretically instantly) it's at the absolute maximum position for refraction, and so it's theoretically possible for it to be visible for a bit more. In practice though if refraction boosts it another degree then it's likely to be lost in the haze anyway.

Here's a video showing stars seeming to slow down a bit as they get closer to the horizon.

Source: https://vimeo.com/188149183

And an analysis:

Source: https://www.youtube.com/watch?v=m-xXhrTG3Sk

 

[Enricks]

Thank you. So, because of refraction near the horizon, it is possible to see stars which have a lower declination and you cannot theoretically see from your latitude, like stated in the article I quoted?

 

[Mick West]

And a more quantative answer can be found here:
http://iopscience.iop.org/article/10.1086/132705/pdf

We have studied the variation of astronomical refraction near the horizon. We have collected 144 measurements of refraction from seven sites by three techniques and have found that the variation of refraction on the horizon is substantially larger than has previously been realized. The rms deviation of our observations is 0°.16, while the individual measurements range from 0.°234 to 1.°678. At the 95% confidence level the total refraction should vary over a range of 0°.64.

A range of extreme observations though history has been collected by Andrew Young
http://iopscience.iop.org/article/10.1086/420806/pdf

Indeed, much larger variations than these have been observed occasionally, particularly at high latitudes, beginning with the famous observations of the Dutch explorers led by Willem Barents, in 1597. They observed the first sunrise in spring 2 weeks earlier than expected, corresponding to a horizontal refraction of over 4 . The typical horizontal refraction near Hudson's Bay was found to be ''more than a degree'' by Captain Middleton (see Coats 1852, p. 132) in the winter of 1741–1742, a result confirmed by James Isham (see Rich & Johnson 1949, p. 73) in the same area the next year. In modern times, Nansen (1897) observed the Sun when it was 2°11' below the horizon, and Shackleton (1962) reported a refraction of 2°37' . Recently, Sampson et al. (2003) reported refraction exceeding 2° at Edmonton, Alberta. But large refractions are not confined to high latitudes: Bouris (1859) reported ''whole series of stars regularly observed with the meridian circle that culminate at Athens up to 4° below the horizon, such as Lupi, 1 Arae, 2 Arae, Arae, ... and Canopus,'' and I myself have observed sunsets delayed by more than 5 minutes in San Diego, California, corresponding to refraction more than a degree greater than normal.

So 1 to 2 degrees happens occasionally, over 2° is very unlikely and there are no really reliable measurements of this. Over 4° can probably be consider to be a hard limit.

 

[Mick West]

Thank you. So it is possible to see stars which have a lower declination and you cannot theoretically see from your latitude, like stated in the article I quoted?

Yes it is, but as noted above, only by a few degrees.

 

[Enricks]

So, applying this to a circumpolar star. You would see Vega as circumpolar between 51° 13' and Cornwall (roughly 50°) even if you shouldn't be able to, because of refraction (like stated in the article).
As it starts to go up in the sky, its position relatively to the other stars should change, as refraction is no longer there far from the horizon. Conversely, as it starts to go down, its position should change again, appearing on the horizon instead of going down. (to put is simply, its circular path in the sky is not regular).

Am I right?

 

[Mick West]

So, applying this to a circumpolar star. You would see Vega as circumpolar between 51° 13' and Cornwall (roughly 50°) even if you shouldn't be able to, because of refraction (like stated in the article).
As it starts to go up in the sky, its position relatively to the other stars should change, as refraction is no longer there far from the horizon. Conversely, as it starts to go down, its position should change again, appearing on the horizon instead of going down. (to put is simply, its circular path in the sky is not regular).

Am I right?

Yes, but if you look at star trail timelapse you'd see this is a pretty small effect over the ocean horizon. You really won't see much with the naked eye - other than stars occasionally being visible a bit past where they should not be.

In the image below there's a slight flattening of the trails behind the lighthouse as they are raised up very slightly.

Image source: http://www.photosbykev.com/wordpress/2014/01/18/star-trail-photography/

 

[Enricks]

The slight flattening on the horizon also happens for non-circumpolar star-trails? If I'm not mistaken, the video you showed me a few post before (the one with the green lines) showed a slight changing in the star trails' path near the horizon. It could be that.

 

[Mick West]

The slight flattening on the horizon also happens for non-circumpolar star-trails?

It's a compression of the entire (unrefracted) image near the horizon. It's non-linear, so the closer the horizon you get, the more distorted it is.

So take this image, imagine these are the perfect geometrical paths of the stars, like you'se see if there was no atmosphere, and the earth was invisible.

add a horizon

And approximate the refraction - exaggerated a bit so you can see what is happening.

The entire portion of the image near the horizon is raised up and compressed a bit. The circumpolar stars (the stars that describe a full circle around the pole without being hidden) have a flattened bottom to their paths, but little to no distortion higher up. The other stars (that set) has a slight deviation to their path - but the steeper the setting angle the less noticeable this is

Slider comparison:
[compare]

[/compare]

 

[JFDee]

The entire portion of the image near the horizon is raised up and compressed a bit

Isn't the horizon itself 'elevated' by refraction? I think it should be higher compared to the one in the unrefracted version.

 

[Mick West]

Isn't the horizon itself 'elevated' by refraction? I think it should be higher compared to the one in the unrefracted version.

It is, yes, but less than the stars as it's a lot closer (i.e goes through less atmosphere)

 

[Henk001]

Isn't the horizon itself 'elevated' by refraction? I think it should be higher compared to the one in the unrefracted version.

I think you could be right considering this schematic drawing:

The grey line corresponds with the direction of the "non-refracted"horizon; the purple with with the refracted horizon; there is an angle between the two: the refracted horizon appears to be higher. I don't know in what amount though

 

[Enricks]

The other stars (that set) has a slight deviation to their path - but the steeper the setting angle the less noticeable this is

Does the sun have this deviation too? (ie. the rise/set azimuths are different from the expected ones)

 

[Henk001]

Yes. And so does the moon. This is the general idea:

 

[Enricks]

Yes. And so does the moon. This is the general idea:
View attachment 27653

Thanks. So, are the azimuths reported in rise/set charts already adjusted for this?

 

[Mick West]

Thanks. So, are the azimuths reported in rise/set charts already adjusted for this?

Generally yes. Remember these charts originally came from ground-based observations, rather than some mathematical model of the Earth and sun viewed from space.

http://www.sunrisesunset.com/faqsAstronomy.asp

Q: What is the definition of sunrise and sunset?

A: Generally speaking, when referring to the sun, the definition is when the leading or trailing edge of the sun passes the horizon. This time is somewhat different from the more specific definition which is called the geometric rise or set. The geometric rise or set of a celestial body is when the center of the object, such as a star, passes the horizon and there is no atmospheric refraction.

Please see the next few questions.

Q: What is atmospheric refraction and how does it affect times calculated on this site?

Here on Earth we have an atmosphere which actually bends light down toward the surface when the object is near the horizon, allowing us to see the sun (and the moon, planets and stars) before and after they would normally be visible if there were no atmosphere. This is called atmospheric refraction.

The amount of 'bending', called the refractive index, varies due to many factors and is not constant from day to day or even by the minute. The two main factors that affect the refractive index are the amount of water vapor in the atmosphere and the temperature of the atmosphere. A third, smaller factor is the atmospheric pressure and then there are many much smaller factors, such as the presence and amount dust in the air. All these factors together will affect the refractive index and when taking into account that we are interested in sunrise and sunset, when the light goes through much more atmosphere than at noon, we realize that the refractive index will have quite an effect on the times, making them off by many seconds. Because these factors are always changing it makes no sense to claim that the times are accurate to a second. It is also now apparent that we cannot say that any specific day has exactly 12 hours (or some other number) of sunlight.