calling all astronomers ...

[ChrisGreaves]

I am reading ASTRONOMY The Science of the Heavenly Bodies, and have made the following notes:-
(1) “Bradley and Aberration”, Chapter 17 sixth paragraph
“The apparent places of all stars ... very nearly 20".5.
(2) “Bradley and Aberration”, Chapter 17 seventh paragraph
“In fact, this is one ... 8".80, a value ...”
(3) “Bradley and Aberration”, Chapter 17 eighth paragraph
“In whatever part ... very small circles, 41" in diameter. “

I read these values as "twenty point five seconds" [of an arc], not "twenty point five minutes [of an arc]"

But at the point (2) my mind shudders: In fact, this is one of the more accurate modern methods of ascertaining the distance of the sun. As early as 1880 it enabled the writer to calculate the sun's parallax equal to 8".80, a value absolutely identical with that adopted by the Paris Conference of 1896, and now universally accepted as the standard.

My mind tells me that we see the sun where it was eight minutes ago, that is about one seventh of an hour, and I am now confused.
As a child I was told that as the sun dipped below the horizon, it "had really set eight minutes ago; it is long gone below the horizon".

Please and thank you: what is going on here?
Am I confusing parallax with aberration?
Is the use of 8".80 a typographical error? Should it be printed as 8'.80?

Thanks, Chris

[ChrisGreaves]

And while you have that book bookmarked, Chapter 22 "Mountain Observatories":-
Mountains in the beautiful climate of California ... especially when fog banks rolling in from the Pacific, cover the valleys below like a blanket, preventing harmful radiation from the soil below.

I can understand that fog/clouds would inhibit visible-light observations, but what is the radiation in " harmful radiation from the soil below"?
Is this what we today would call "light pollution" from the cities?
Or is it a reference to some other radiation outside the frequency of visible light?

Thanks
Chris

[HansV]

I think the fog prevents heat to radiate from the soil below, which would cause turbulence in the air.

[ChrisGreaves]

I think the fog prevents heat to radiate from the soil below, which would cause turbulence in the air.

Ah, yes, he said, hesitantly.
I continue to think: some artificial light (from, say, Los Angeles) might seep upwards through the clouds, and some heat in the infra-red must, too, seep upwards. That said, in both cases they would be both subdued and diffused compared to the ground-level sources of that electro-magnetic radiation.
I think.
Cheers
Chris

[BobH]

What does your book say about the Mount Davis Observatory in Southwest Texas?

I was last there about 20 years ago. Light pollution from Mexico has increased causing problems for the observatory and limiting its effectiveness. The light is coming from the many maquiladores built near the US border as a result of NAFTA and US companies moving operations there.

[Graeme]

That's an interesting read Chris. I'm a big fan of the history of astronomy. Bradley must have been a tenacious observer to have measured Gamma Draconis all year, especially in the winter when it would have passed through the Zenith at Kew during the day!

I think there are two things going on here. One is the Parallax of the Sun which is 8.8" of arc and which is the radial difference between the position of the centre of the Sun observed from Earth and its position when seen from the surface of the Earth (1 Earth radius difference). The other thing is the time it takes for light to reach us from the Sun which is 8 minutes 20 seconds (93,000,000(miles)/186,000(miles per second))

I've not heard of the aberration caused by the speed of light causing a change in velocity of Earth in it's orbit, so I'll have to read your link a few more times for it to make sense! But you need shudder no more, the Sun setting thing is 8 minutes!

Regards

Graeme

[ChrisGreaves]

What does your book ...

Flattery will get you every where, Bob.
By a truly amazing coincidence (really!) my next task was to read Chapter 22 - Mountain Observatories.
I couldn't recall seeing Mount Davis in there, but a lot has happened since then.
I did at quick Edit, Find and it didnt turn up.

It is a good read, this chapter, because it starts with Sir Isaac Newton and ends with the "100-inch reflector on Mount Wilson", although I remember reading in my Wonder Book Of Things To Do in 1955, about a 200-inch being ground for Mt Palomar, and in 1990 got to drive there from Borrego Springs, but arrived 15 minutes after closing time.
Still, it was nice to have closure on another niggling idea.

P.S. Texas gets one mention that I can find in the book: "... the total eclipse of July 29, 1878, which passed over the United States from Pike's Peak to Texas". Pikes Peak is pretty high, what with it being a peak in the first place, so I imagine that Texas must have been a bit of a let-down.
Cheers
Chris

[ChrisGreaves]

Bradley must have been a tenacious observer to have measured Gamma Draconis all year, especially in the winter when it would have passed through the Zenith at Kew during the day!

Science history is full of amateur scientists who spent fifty years of their lives making daily measurements on the diameter of the largest buttercup blossom in a square foot of earth selected by a consistently random toss of a one-foot-square frame into a cow pasture.
Darwin spent 40(?) years watching rocks descend into the earth by virtue of being undermined by earthworms. Watching grass grow would have been exciting for him.

I think there are two things going on here. One is the Parallax of the Sun which is 8.8" of arc and which is the radial difference between the position of the centre of the Sun observed from Earth and its position when seen from the surface of the Earth (1 Earth radius difference).

So, roughly, if you stood at the North Pole, and I was sipping a Pina Colada in Ecuador, and we set Hans down on the surface of the sun closest to Earth (so on a direct line towards the centre of the sun) then Hans would see you and I as the base of a triangle whose angle was almost 9 seconds of an arc, or

Code: Select all

8.8	seconds
0.146666667	minutes
0.002444444	degrees
0.0000067901	of a circle

Also, I believe, the inverse need not be true, that if You and Hans set up positions on the sun, and I ordered a second Pina Colada in Ecuador, the angle would not necessarily be 8.8 degrees. I think

The other thing is the time it takes for light to reach us from the Sun which is 8 minutes 20 seconds (93,000,000(miles)/186,000(miles per second))

AhHah! So these values of "8" are just an evil coincidence, unrelated to each other, like the apparent sizes of our Moon and Sun.

I've not heard of the aberration caused by the speed of light causing a change in velocity of Earth in it's orbit, so I'll have to read your link a few more times for it to make sense! But you need shudder no more, the Sun setting thing is 8 minutes!

I was plucking at straws with "aberration", mainly because that word was used a lot in chapter 17, so I reasoned that it might be relevant.
Thanks
Chris

[HansV]

Hans would be incinerated within milliseconds...

[ChrisGreaves]

Hans would be incinerated within milliseconds...

That's OK. Graeme and I could still imagine that you were there; or imagine that you were still there.
I( mean, can you imagine me sipping on a Pina Colada? I'd much rather a ginger beer.
Evily Yours
Chris

[Graeme]

I was plucking at straws with "aberration", mainly because that word was used a lot in chapter 17, so I reasoned that it might be relevant.

No straws needed plucking! astronomical aberration is a thing, who knew! I'm familiar with aberration with respect to chromatic aberration and different types of that with achromatic doublet and apochromatic triplex refractor telescopes. This though is from Isaac Newton's (et al) work measuring buttercups. The aberration being discussed by Bradley is about the difference of where the sun is and where it appears to be due to the distance the Earth has moved in its orbit and the time taken for light to reach us at a finite speed. Best described in Special Relativity. This stuff is hugely interesting and must be at least Masters degree level, which I don't have! So thanks for the ebook. I owe you a ginger beer.

https://en.wikipedia.org/wiki/Aberration_(astronomy)

Thanks

Graeme

[ChrisGreaves]

... the difference of where the sun is and where it appears to be due to the distance the Earth has moved in its orbit and the time taken for light to reach us at a finite speed. ...

So I was right, again more by accident.
Although - maybe I understood what Bradley was saying.
That eight seconds of what he calls aberration corresponds to the eight minutes delay I learned as a child.
Again, the respective values of "eight" and "eight" are an evil co-incidence which managed to make my brain hiccough.

If the eight-minutes-of-time delay is due to eight minutes of the earth's orbit, then as a fraction of a year it is

Code: Select all

8	minutes of time
0.133333333	of an hour
0.005555556	of a day
0.0000154321	of a year

which is roughly 2.25 times my earlier figure (Book1 updated and attached)

I am still uneasy. That "eight" minutes of time-delay is describing in a roundabout way, the base of an extremely thin triangle whose apex is our Sun.
But that is what we were describing until Hans vaporized a few hours ago.
Isn't that the same triangle?
And the argument that "Well, yes, but minutes, hours, and days are human ideas, not solar concepts" is weak because there is nothing human about the period of "one day" or of "one year". Those units are part of the solar system.

(Later) Hang about. I have TWO extremely thin triangles, but the base of one is the radius of the earth (North Pole to Ecuador), whereas the base of this new triangle is "orbital arc in eight minutes". My factor of 2.5 then ought to mean that eight minutes of Earth's orbital time is equivalent to 2.7 earth radii.
That doesn't seem right.


Re Bradley: I have accumulated and read many old books (as in second-hand bookshops and junk sales), basically outdated scientific texts, as old as Bradley. I enjoy them because they are out of date ideas, and as such are easier for me to understand than the latest whiz-bang in Nature. In reading them I know that their findings (in whatever topic) will be superseded by today's knowledge, but still they seem to give me a good foundation of the problems we face.
My studies of the origins of the Third Balkan War still serve me in good stead for understanding human behaviour, diplomacy, fears etc.

Cheers
Chris
P.S. enjoy the signature line for two more days, then it changes. Again.

[ChrisGreaves]

(Later) ...
That doesn't seem right.

(Book1 updated and attached)

Code: Select all

"... eight minutes of Earth's orbital time is equivalent to 2.7 earth radii..."	
 4,000 	miles is Earth's radius
 10,800 	miles ought to be eight minutes of orbital time
 92,000,000 	miles radius of Earth's orbit
 578,285,714 	miles circumference of Earth's orbit
	
0.00001868 	eight minutes orbital distance as fraction of the circumference which is one year
 365 	days in a year
 8,760 	hours in a year
 525,600 	minutes of time in a year
 9.82 	my fraction of a year orbital time expressed as minutes

Hah hah!
Suddenly I feel better, but I don't know what i have proved!

I have managed to calculate 9.82 minutes of time, which sounds pretty close to "eight minutes of time", but Mr. Feld (1962-63) would snort and suggest that the difference is just rounding error by calculating outwards and then calculating backwards.
I think it is time for another
Cheers
Chris

[BobH]

Chris, the Texas observatory might be Davis Mountains, not Mount Davis.

[ChrisGreaves]

Chris, the Texas observatory might be Davis Mountains, not Mount Davis.

Why? What is the Texas observatory trying to hide by this worrying name change?
Did you look in the text of the book?
I thought that Texas was lucky to get a mention, being relatively small compared to some other states with clear skies
Cheers
Chris

[Graeme]

If the eight-minutes-of-time delay is due to eight minutes of the earth's orbit, then as a fraction of a year it is

I'm not sure it is! The 8 minutes delay is due simply to the Sun Earth distance (1AU) and the finite nature of the speed of light. The 8 seconds of arc is the radial measurement of the difference between where the Sun is and where it appears to be as seen from the Earth. (I've read some more since my last post!)

Screenshot 2022-01-30 205931.png

The Special Relativity example of this would be someone standing still in vertical rain fall and someone else moving perpendicular to the rain would experience the rain coming at them at an angle.

Mind you, as Einstein said, "if you can't explain something to a six year old then you don't understand it yourself" I knew about parallax and all it's trigonometry but astronomical aberration is new to me. I must go find out more!

Regards

Graeme

[ChrisGreaves]

I'm not sure it is! The 8 minutes delay is due simply to the Sun Earth distance (1AU) and the finite nature of the speed of light.

OK. This I get. It takes eight minutes for light to do the trip. And even though when the light reaches us, it is lying to us, it is still an eight-minute trip.

The 8 seconds of arc is the radial measurement of the difference between where the Sun is and where it appears to be as seen from the Earth.

But this is not the aberration we first spoke of, is it? This "8 seconds" is, again, the effect of the visual trick brought about by the eight-minute trip from the sun.
I stand my ground, and say that your 8-second-arc and my eight-minute trip are just a nasty coincidence of "eights" BUT they are directly related by an act of Earth's orbit around the Sun. It is just the one triangle, extremely narrow, with an 8-second angle and eight-minute sides.
And I ate to bring this up at this difficult time, but the earth's diameter is about eight thousand miles.

The Special Relativity example of this would be someone standing still in vertical rain fall and someone else moving perpendicular to the rain would experience the rain coming at them at an angle.

Last night in Bonavista, you could be perpendicular to the rain if you stood still. Not one of our favourite topics right now, I am afraid.

I knew about parallax and all it's trigonometry but astronomical aberration is new to me. I must go find out more!

Graeme, you do not have to go find out more. Especially about light-time aberration of relativistic correction.

Cheers
Chris

[ChrisGreaves]

What does your book say about the Mount Davis Observatory in Southwest Texas?

Not my book, but now catalogued and available for download.
My dulcet tones appear only in Section 17.
Cheers, Chris