Analemma Revisited

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January 11th, 2018 at 5:24:15 AM permalink
odiousgambit
Member since: Oct 28, 2012
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We discussed the analemma a bit but dropped it, I'd kind of like to go back to it.

I often mention it to people this time of year, especially if they too have noted that the Sun keeps rising at the same time every day till about now, yet the days have been getting longer since Dec 21. The fact is that this longer day is fully a matter only of the sunset occurring later, according to our clocks. The way I put it comes down to explaining that if you take a time lapse photo over a year of the Sun at noon, that photo will show the Sun making a figure eight. Soon enough, though, usually I have to admit I don't fully understand it.

In science, I think it is also true that thinking that intuition needs to match fact is a trap. For one thing, what was counter-intuitive later becomes intuitive often, the intuition so many times just coming from some silly source. Trying to absorb Einstein's concepts, on the other hand, for example, always runs into a stubborn intuition wall with me at some point. Someone interested in Science really at some point has to drop those defenses. However, in the case of the Analemma, I'd like to make some progress.

From the Wikipedia article,

Quote:
The asymmetry of the lobes of the figure-eight form arises mainly from the fact that the equinoxes do not occur at the perihelion and aphelion.


This writer confused me, I am going to try to change 'equinoxes' to 'solstices' at Wikipedia to get things started. The winter solstice occurs about Dec 21st and a google search says the perihelion occurs Jan 3. Let's see what happens with this attempted edit, it can get shitty.
Mustard:You like Kipling, Miss Scarlet? Sure, I'll eat anything [from movie]
January 11th, 2018 at 6:41:31 AM permalink
odiousgambit
Member since: Oct 28, 2012
Threads: 92
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So far so good on the Wikipedia edit, maybe there won't be a fuss. I used to participate a bit with Wikipedia, I kind of like the idea that a contribution can be something you leave behind .... after your death even, perhaps. But I've never created an article.

Next step will be to enlist intuition for the figure eight phenomenon at that article; I've got some ideas but you have to avoid "original research" and that can spill over into things that aren't research. Stay tuned.
Mustard:You like Kipling, Miss Scarlet? Sure, I'll eat anything [from movie]
January 11th, 2018 at 7:27:53 AM permalink
JimRockford
Member since: Sep 18, 2015
Threads: 2
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In the movie Castaway, Tom Hanks appears to plot the location of a spot of sunlight on his cave wall every day, thereby constructing an analemma. However he has no working timepiece to know when to take his reading, so it is impossible. I puzzled over it for the rest of the movie wondering if I had overlooked something. That kind of thing can be a huge distraction for me. I'm such a nerd sometimes.
Where is Joseph Mifsud?
January 11th, 2018 at 8:09:56 AM permalink
OnceDear
Member since: Nov 21, 2017
Threads: 6
Posts: 785
Quote: JimRockford
In the movie Castaway, Tom Hanks appears to plot the location of a spot of sunlight on his cave wall every day, thereby constructing an analemma. However he has no working timepiece to know when to take his reading, so it is impossible. I puzzled over it for the rest of the movie wondering if I had overlooked something. That kind of thing can be a huge distraction for me. I'm such a nerd sometimes.

I spent the whole movie waiting for the FedEx advert to finish.
January 11th, 2018 at 12:40:25 PM permalink
Wizard
Administrator
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Quote: JimRockford
In the movie Castaway, Tom Hanks appears to plot the location of a spot of sunlight on his cave wall every day, thereby constructing an analemma. However he has no working timepiece to know when to take his reading, so it is impossible. I puzzled over it for the rest of the movie wondering if I had overlooked something. That kind of thing can be a huge distraction for me. I'm such a nerd sometimes.


That bothered me too. I believe it is correctly listed as an error on IMDB.
Knowledge is Good -- Emil Faber
January 11th, 2018 at 3:25:32 PM permalink
Pacomartin
Member since: Oct 24, 2012
Threads: 850
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Quote: Wizard
That bothered me too. I believe it is correctly listed as an error on IMDB.

YOU WOULD BE CORRECT


Plot holes
In the cave, Chuck has made an analemma, which is impossible without a precise watch, which is stated he didn't have. The analemma is a graph of the difference between solar time and civil time, in any case he only had access to a solar time reading.

Chuck plans for his launch by looking at where the Sun is shining on a rock in his cave. The spot of light makes a figure 8 as the year progresses. But the light spot would have to be marked at the same time each day, as it would also move across the rock throughout the day. It is unlikely that it just happened to be that time of day as he looked at his "calendar", and it is unlikely that he knew what time it was since his watch did not work.

January 11th, 2018 at 3:41:55 PM permalink
Evenbob
Member since: Oct 24, 2012
Threads: 121
Posts: 15568
There are people who watched that
ridiculous movie to the end? Did he
get off the island? I stopped caring
an hour into it and turned it off..
If you take a risk, you may lose. If you never take a risk, you will always lose.
January 11th, 2018 at 4:39:27 PM permalink
OnceDear
Member since: Nov 21, 2017
Threads: 6
Posts: 785
Quote: Evenbob
There are people who watched that
ridiculous movie to the end? Did he
get off the island? I stopped caring
an hour into it and turned it off..
He had to deliver that one slightly late, rather water damaged but unopened parcel. . . 'Cos that's the FedEx way.
January 12th, 2018 at 3:18:15 AM permalink
odiousgambit
Member since: Oct 28, 2012
Threads: 92
Posts: 2312
So, I think I have this down.

Making the figure 8 with time lapse photography, why this happens for the up and down motion is easy to understand. It's a matter of understanding that the sun is high in the summer and low in the winter.

The article notes,
Quote:
For an object with a circular orbit but significant axial tilt, the analemma would be a figure of eight with northern and southern lobes equal in size


Now, I believe this is wrong. If the orbit was a perfect circle, the analemma would be a straight line going up and down, no figure eight. Correct me if I am wrong, but see next.

The side to side motion is harder to understand intuitively. But imagine if the eccentricity of the orbit was so severe it resembled a paper clip somewhat. If the sun was in the middle of that paper clip, as the Earth approached on one of the legs, the bearing of the sun could be noted. If then 24,48 etc. hours later, after an exact full rotation, the bearing would change as the Sun is first approached and continue to do so later as the Earth heads away. It's now easy for me at least to get this video in my head and see that this analemma on an Earth without any tilt would be a straight line going East or West or vice versa.

The combination of the up and down with the east to west changes produces the figure eight.

On this latter matter, the article gets it right,
Quote:
For an object with an eccentric orbit but no axial tilt, the analemma would be a straight east–west line along the celestial equator


I can tell you if we make a major change in the article we better be right. So I would like to know if anyone thinks I have it wrong.
Mustard:You like Kipling, Miss Scarlet? Sure, I'll eat anything [from movie]
January 12th, 2018 at 3:37:58 AM permalink
odiousgambit
Member since: Oct 28, 2012
Threads: 92
Posts: 2312
PS, somebody came along and changed back my original change at wikipedia. I dunno about that.

PPS an MIT guy he says. I won't win the argument, not being an astronomer, and he could be; so I added about the solstices, we'll see how he likes that.
Mustard:You like Kipling, Miss Scarlet? Sure, I'll eat anything [from movie]
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