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A science-in-everyday-life problem ...

RoyLatham
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11/11/2014 11:13:39 PM
Posted: 2 years ago
Last week I was driving across the Mohave Desert at sunset. As the sun was setting, a full moon was rising in the east. With nice desert-type sunset colors around the horizon, it was a notable event. More so in a desert that has nothing else to look at.

The question is: How often is there a full moon rise at sunset?

The lunar month is about 29.5 days. Barring apocalypse there is one sunset per day, lasting about 15 minutes. We'll say the moon is "full" for a day.
Envisage
Posts: 3,646
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11/12/2014 4:04:44 AM
Posted: 2 years ago
At 11/11/2014 11:13:39 PM, RoyLatham wrote:
Last week I was driving across the Mohave Desert at sunset. As the sun was setting, a full moon was rising in the east. With nice desert-type sunset colors around the horizon, it was a notable event. More so in a desert that has nothing else to look at.

The question is: How often is there a full moon rise at sunset?

The lunar month is about 29.5 days. Barring apocalypse there is one sunset per day, lasting about 15 minutes. We'll say the moon is "full" for a day.

First you want the chances of 'a' Moon rising at sunset. Modeling the moon like the Sun and assuming it rises and sets once every 24 hour period in each location on Earth (it's a bit less than that because the Moon moves forward a bit in it's orbit every day, but nevermind), and assuming the Moon rises at random times each time, then there is a 15 in 1440 chance (15 minutes ever 1440 minutes) that the moon will rise then the Sun rises, as it had to rise within that window of time, which is roughly 1%. So once every ~100 days the Moon rises with the Sun.

Now the chances of the Moon being full at each Sunset is 1 in 29.5 (Since there is only 1 sunset per day, and the moon is full for 1 day every 29.5). So you get an answer of roughly once every ~3000 days you will see a full moon rise at sunset.

.... Hmmm, when is the next one.
v3nesl
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11/12/2014 9:43:06 AM
Posted: 2 years ago
At 11/12/2014 4:04:44 AM, Envisage wrote:
...

.... Hmmm, when is the next one.

I think that comes out to just under 8 years. Hope it's not cloudy.

It's very dependent on defining the sunset window (15 minutes) and what constitutes 'moon rise'. Open those parameters up a bit to "when can I see a setting sun and a rising full moon at the same time" and there's more hope, maybe a couple times a year.
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RoyLatham
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11/12/2014 3:28:23 PM
Posted: 2 years ago
I put in irrelevant info as a distraction. It took me a while to figure out what was relevant.

When the moon is full, where is it with respect to the sun? The moon has to be on exactly the opposite side of the earth from the sun. If it were not on exactly the opposite side, the whole face of the moon would not be illuminated by the sun. At sunset the observer is on the line between day and night, so the sun is on the west horizon. The full moon has to be on the opposite side of the earth from the sun, so that means that it must be on the east horizon. If the earth were large, the full moon would always be in eclipse since the sun and moon are on opposite sides of the earth. That doesn't happen, but only a full moon can be eclipsed.

So the answer is that every 29.5 days, the full moon will rise at sunset. It happens every time there is a full moon. ... Unless, of course, I have overlooked something in my analysis.
mortsdor
Posts: 1,181
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11/12/2014 3:48:48 PM
Posted: 2 years ago
At 11/12/2014 3:28:23 PM, RoyLatham wrote:
When the moon is full, where is it with respect to the sun? The moon has to be on exactly the opposite side of the earth from the sun.
It happens every time there is a full moon.

Ha!

Makes sense.
Mhykiel
Posts: 5,987
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11/12/2014 11:26:57 PM
Posted: 2 years ago
At 11/12/2014 3:28:23 PM, RoyLatham wrote:
I put in irrelevant info as a distraction. It took me a while to figure out what was relevant.

When the moon is full, where is it with respect to the sun? The moon has to be on exactly the opposite side of the earth from the sun. If it were not on exactly the opposite side, the whole face of the moon would not be illuminated by the sun. At sunset the observer is on the line between day and night, so the sun is on the west horizon. The full moon has to be on the opposite side of the earth from the sun, so that means that it must be on the east horizon. If the earth were large, the full moon would always be in eclipse since the sun and moon are on opposite sides of the earth. That doesn't happen, but only a full moon can be eclipsed.

So the answer is that every 29.5 days, the full moon will rise at sunset. It happens every time there is a full moon. ... Unless, of course, I have overlooked something in my analysis.

If the moon were on the exact opposite of the earth you would have a lunar eclipse. The moon is also closer to the earth so it would have to be at a different angle from the dividing line.

I didn't check the math of Envisage, but using the odds can not tell you how often such an event happens. It is a general odds of such an event happening in a single location on any random day.

Taking the numbers and adding up 3000 minutes won't tell you about the events occurrence.

If I were to make an educated guess i would look at what times are recorded for sunsets. Days are usually 24 hours long so the sun sets fairly consistently in a 3 hour window. 1630-1930. The question is when does the moon rise within those 3 hours.

Given that the moons cycle is 29.5 days and we average a month to be around 30 days. Then the moon rises and advances in it's rising times by ~50 minutes each day. the 3 hour window segmented by those times is 3.6. So for on average 3.6 days a month the moon is rising when the sun is setting.

Now the question is out of those 3.6 days how often is the moon full? Take the cycle of the moon 29.5 and multiply by the number of days a month the sun and moon rise at the same time. this way we know from a event of them matching how often they will match again.

106.2 days/ 30days for months = 3.5

My estimation would be for the same spot on earth every 3.5 months the sun sets when a full moon is rising. the estimate could be more accurate and the spot is assumed to be close to the equator.
Envisage
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11/13/2014 5:35:06 AM
Posted: 2 years ago
At 11/12/2014 3:28:23 PM, RoyLatham wrote:
I put in irrelevant info as a distraction. It took me a while to figure out what was relevant.

When the moon is full, where is it with respect to the sun? The moon has to be on exactly the opposite side of the earth from the sun. If it were not on exactly the opposite side, the whole face of the moon would not be illuminated by the sun. At sunset the observer is on the line between day and night, so the sun is on the west horizon. The full moon has to be on the opposite side of the earth from the sun, so that means that it must be on the east horizon. If the earth were large, the full moon would always be in eclipse since the sun and moon are on opposite sides of the earth. That doesn't happen, but only a full moon can be eclipsed.

So the answer is that every 29.5 days, the full moon will rise at sunset. It happens every time there is a full moon. ... Unless, of course, I have overlooked something in my analysis.

*facepalm*

Didn't think of that.... It seems reasonable enough.
RoyLatham
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11/13/2014 9:18:05 AM
Posted: 2 years ago
I never really thought about it, but the reason we don't have an eclipse at every full moon is that the moon is usually either slightly above or slightly below the shadow of the earth:

"The moon takes about a month to orbit around the Earth. If the moon orbited in the same plane as the ecliptic " Earth"s orbital plane " we would have two eclipses every month. There"d be an eclipse of the moon at every full moon. And, two weeks later, there"d be an eclipse of the sun at new moon for a total of at least 24 eclipses every year.

But the moon"s orbit is inclined to Earth"s orbit by about 5 degrees. Twice a month the moon intersects the ecliptic " Earth"s orbital plane " at points called nodes. If the moon is going from south to north in its orbit, it"s called an ascending node. If the moon is going form north to south, it"s a descending node. If the full moon or new moon is appreciably close to one of these nodes, then an eclipse is not only possible " but inevitable."
http://earthsky.org...
v3nesl
Posts: 4,500
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11/13/2014 9:27:51 AM
Posted: 2 years ago
At 11/12/2014 3:28:23 PM, RoyLatham wrote:
I put in irrelevant info as a distraction. It took me a while to figure out what was relevant.

When the moon is full, where is it with respect to the sun? The moon has to be on exactly the opposite side of the earth from the sun. If it were not on exactly the opposite side, the whole face of the moon would not be illuminated by the sun. At sunset the observer is on the line between day and night, so the sun is on the west horizon. The full moon has to be on the opposite side of the earth from the sun, so that means that it must be on the east horizon. If the earth were large, the full moon would always be in eclipse since the sun and moon are on opposite sides of the earth. That doesn't happen, but only a full moon can be eclipsed.

So the answer is that every 29.5 days, the full moon will rise at sunset. It happens every time there is a full moon. ... Unless, of course, I have overlooked something in my analysis.

Well, does it? Does a full moon rise at sunset every month [roughly] - ? I don't think so.

There's some arc within which you perceive the moon as 'full', and the moon traverses this arc over some period of time. We're saying roughly one day, so the arc is 360/29.5 degrees (I think I did that right, you get the point, I trust). So the moon can be anywhere within that arc (statistically speaking) when the sun sets. Sometimes the full moon will already be up when the sun sets, probably more often it comes up after the sun has set (since it's close to 180 degrees from the sun at full full-moon). So what fraction of full-moon-arc will be in that rising-at-sunset-arc? I don't know, but it's not going to be every full moon. The fraction may be pretty close to sunset-time / full-moon-time.

As in all our evo threads, the thing to do is test it. It's going to take a few months, but we should try to pay attention and observe the relative positions of full moons.

This is almost like fun!
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v3nesl
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11/13/2014 9:31:14 AM
Posted: 2 years ago
At 11/12/2014 11:26:57 PM, Mhykiel wrote:
...

106.2 days/ 30days for months = 3.5

My estimation would be for the same spot on earth every 3.5 months the sun sets when a full moon is rising. the estimate could be more accurate and the spot is assumed to be close to the equator.

That sounds pretty good to me, "feels" about right. I'll report back this time next year :-)
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v3nesl
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11/13/2014 10:05:30 AM
Posted: 2 years ago
At 11/12/2014 11:26:57 PM, Mhykiel wrote:
At 11/12/2014 3:28:23 PM, RoyLatham wrote:
I put in irrelevant info as a distraction. It took me a while to figure out what was relevant.

When the moon is full, where is it with respect to the sun? The moon has to be on exactly the opposite side of the earth from the sun. If it were not on exactly the opposite side, the whole face of the moon would not be illuminated by the sun. At sunset the observer is on the line between day and night, so the sun is on the west horizon. The full moon has to be on the opposite side of the earth from the sun, so that means that it must be on the east horizon. If the earth were large, the full moon would always be in eclipse since the sun and moon are on opposite sides of the earth. That doesn't happen, but only a full moon can be eclipsed.

So the answer is that every 29.5 days, the full moon will rise at sunset. It happens every time there is a full moon. ... Unless, of course, I have overlooked something in my analysis.

If the moon were on the exact opposite of the earth you would have a lunar eclipse. The moon is also closer to the earth so it would have to be at a different angle from the dividing line.

I didn't check the math of Envisage, but using the odds can not tell you how often such an event happens. It is a general odds of such an event happening in a single location on any random day.


I agree, btw, that 'odds' are technically not the right tool for the job, but I think you'll find that the math is exactly the same - they both come down to a fraction. It all boils down to finding what fraction of lunar orbit fits our 'full moon rising at sunset' criterion.
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Mhykiel
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11/13/2014 5:36:14 PM
Posted: 2 years ago
At 11/13/2014 9:27:51 AM, v3nesl wrote:
At 11/12/2014 3:28:23 PM, RoyLatham wrote:
I put in irrelevant info as a distraction. It took me a while to figure out what was relevant.

When the moon is full, where is it with respect to the sun? The moon has to be on exactly the opposite side of the earth from the sun. If it were not on exactly the opposite side, the whole face of the moon would not be illuminated by the sun. At sunset the observer is on the line between day and night, so the sun is on the west horizon. The full moon has to be on the opposite side of the earth from the sun, so that means that it must be on the east horizon. If the earth were large, the full moon would always be in eclipse since the sun and moon are on opposite sides of the earth. That doesn't happen, but only a full moon can be eclipsed.

So the answer is that every 29.5 days, the full moon will rise at sunset. It happens every time there is a full moon. ... Unless, of course, I have overlooked something in my analysis.

Well, does it? Does a full moon rise at sunset every month [roughly] - ? I don't think so.

There's some arc within which you perceive the moon as 'full', and the moon traverses this arc over some period of time. We're saying roughly one day, so the arc is 360/29.5 degrees (I think I did that right, you get the point, I trust). So the moon can be anywhere within that arc (statistically speaking) when the sun sets. Sometimes the full moon will already be up when the sun sets, probably more often it comes up after the sun has set (since it's close to 180 degrees from the sun at full full-moon). So what fraction of full-moon-arc will be in that rising-at-sunset-arc? I don't know, but it's not going to be every full moon. The fraction may be pretty close to sunset-time / full-moon-time.

As in all our evo threads, the thing to do is test it. It's going to take a few months, but we should try to pay attention and observe the relative positions of full moons.

This is almost like fun!

The test is looking at observed data

http://aa.usno.navy.mil...

I did a quick glance at some times and seems 3 months is right.
RoyLatham
Posts: 4,488
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11/13/2014 11:56:19 PM
Posted: 2 years ago
So the answer is that every 29.5 days, the full moon will rise at sunset. It happens every time there is a full moon. ... Unless, of course, I have overlooked something in my analysis.

Well, does it? Does a full moon rise at sunset every month [roughly] - ? I don't think so.

I have seen at least four or five that I can recall, so I'm sure they are not 8000 years apart. But it's testable. The problem is finding a place where both horizons are visible. Deserts and oceans are good. There are also simulation programs that will create the night sky for any date, but I don't have one of those.

As in all our evo threads, the thing to do is test it. It's going to take a few months, but we should try to pay attention and observe the relative positions of full moons.

This is almost like fun!

Yup. I think it's hard to believe too, but I don't see any flaw in the theory.
RoyLatham
Posts: 4,488
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11/14/2014 12:04:04 AM
Posted: 2 years ago
Why didn't I think of doing a web search before? Searching "does the full moon always rise at sunset" produces dozens of hits like this one:

" Many people are surprised to learn that the full moon always rises at sunset and wonder why."
http://www.bubblews.com...
v3nesl
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11/14/2014 7:58:31 AM
Posted: 2 years ago
At 11/13/2014 11:56:19 PM, RoyLatham wrote:
So the answer is that every 29.5 days, the full moon will rise at sunset. It happens every time there is a full moon. ... Unless, of course, I have overlooked something in my analysis.

Well, does it? Does a full moon rise at sunset every month [roughly] - ? I don't think so.

I have seen at least four or five that I can recall, so I'm sure they are not 8000 years apart. But it's testable. The problem is finding a place where both horizons are visible. Deserts and oceans are good. There are also simulation programs that will create the night sky for any date, but I don't have one of those.

I don't think anybody said 8000 years, fwiw.


As in all our evo threads, the thing to do is test it. It's going to take a few months, but we should try to pay attention and observe the relative positions of full moons.

This is almost like fun!

Yup. I think it's hard to believe too, but I don't see any flaw in the theory.

But it's not tested, that's all. Nobody's ever seen anything evolve, the sort of evolve that would serve as a test of the theory. And it's not within a galaxy of being mathematically model-able, and you see how error prone a simple thing like this can be. So no, evolution remains just a sketch of an outline of a theory (don't be dazzled by the quantity of data, the testing just isn't there), and in my opinion, a Ptolemaic style misinterpretation of the data.
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v3nesl
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11/14/2014 8:16:00 AM
Posted: 2 years ago
At 11/13/2014 5:36:14 PM, Mhykiel wrote:
...

The test is looking at observed data

http://aa.usno.navy.mil...

I did a quick glance at some times and seems 3 months is right.

So, since I have too much real work to do :-), I decided to correlate this to a full moon table for my location. So here's the dates that table gives for full moon, and the moonrise time for each of those dates:

1/16___7:33
2/14___7:08
3/16___6:25
4/15___5:38
5/14___5:02
6/13___4:48
7/12___4:59
8/10___5:23
9/9____5:51
10/8___6:18
11/6___6:49
12/6___7:21

So you have a sine wave of moonrises if you plotted this. The overall wavelength there is one year, so the moon will pass through whatever time window equals sunset twice per year. But it may be in that window for successive months. 12 successive months if you define sunset as 5 - 7:30PM, in this example.
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