In Would alignment of the planets cause catastrophe?, Cecil suggests that planetary alignments can be discounted because, even when planets appear to be aligned from Earth, they are typically spread across wide solar-centered arcs.
Now, just to play devil’s advocate here, if I did believe that the planets exerted a significant gravitational (or other more mysterious) effect on us poor hapless Earthlings, why would the solar-centered arc be at all relevant? Surely any summation of forces resulting from the alignment would be more sensitive to the Earth-centered arc. And isn’t a small Earth-centered arc exactly what we mean by “alignment” of planets?
In fact, if I really feared the gravitational effects of an alignment, I think I would be most concerned when a number of planets lined up with the Sun itself (solar-centered arc near 180 degrees if both inner and outer planets are involved) or perhaps when a number of (outer) planets were aligned at solar midnight.
I realize that planetary alignment fans might not want to hear about inverse square laws for gravitation (yecch - math!) and, if gravity isn’t the force they have in mind, it’s frustrating to try and debunk the completely unspecified. But I think the solar-centered arc argument is a strawman.
In the book “The Jupiter Effect”, the planets were supposed to affect the Sun, not the Earth, so a Sun-centered arc is what’s relevant. If you want to talk about effects on the Earth, then you use an Earth-centered arc.
In either case, the effects are essentially zero. The fact that the Moon is on an elliptical orbit means that every month its gravity on the Earth varies by far more than the planets’ gravity combined. Since we aren’t torn apart every two weeks, it’s safe to assume the planets have no deleterious effect on the Earth.
If a dilletante may be permitted to throw in one rule-of-thumb:
Two objects of the same density have the same gravitational pull as their apparent size.
That is, gravity decreases as the square of the distance, and so does apparent size.
So in order for something of moon-like density to have the same gravitational effect on the Earth as the moon does, it would have to be close enough so that it looked as big as the moon.
I don’t know how dense Jupiter is, but I know Saturn would float. I WAG that Mars is no less than 50% nor more than 200% as dense as the moon. Given how small they look in the sky, even if they were all lined up, you can imagine how small the gravitational effect is, compared with the moon or sun.
Yes, but the volume increases with the cube of the radius, or the apparent size raised to the (3/2) power. So, even though the sun and the moon occupy about the same amount of sky, the sun has a much larger gravitational pull.
However, although the Sun has roughly half the density of the Moon, its gravitational effect on the Earth is still far greater. On the other hand, the tidal effect of the Moon is much greater, because the diameter of the Earth is much bigger compared to the distance from the Moon to the Earth, than it is compared to the distance from the Sun to the Earth.
What bup said actually applies to tidal forces, not the direct graavitational force. Two objects of the same apparant size will exert tidal forces proportional to their density. For instance, the Moon exerts about twice the tidal effect of the Sun, and they have the same apparant size, so the Moon is about twice as dense.
The proportionality between angular size and tidal force is not direct, though, there being a power of 3/2 in there.
No, I got that, and in fact it’s implicit in my post, because I refer to volume, not mass. I assumed bup was talking about mass-volume density, not mass column density. Which I think is reasonable, no?
Unfortunately, running with rules of thumb is as dangerous as scissors. You can put your eye out with those thumbs.
The Sun and moon both have the same apparent size, but the gravity of the Sun is 200 times that of the moon, as Achernar alludes to.
OK, I guess that’s another way of saying that the moon is closer? The tide is proportional to the inverse cube of distance, whereas gravity is inverse square.
Power of 3, isn’t it?
The tidal effect is proportional to the body’s volume times density, and so is proportional to the cube of the body’s radius, and inversely to the cube of the distance to the body. The radius divided by the distance is angular size. The tidal effect is proportional to the cube of the angular size times density.
hello everyone, here goes my first post! I had to see what people have been saying about this subject as a few years back I read several books that indicated a “doom date” of the end of the world due to astrological events. In “Fingerprints of the Gods,” by Graham Hancock (http://www.grahamhancock.com/library/fotg/default.htm)
he talks of a day that passed I believe last year on My 5th. He stated that due to all the planets being lined up on the opposite side of the sun would have triggered a gravitational shift on Earth that would basically shift the planets outer core 1/3 around itself. He backed up his story with some extravegence, the book was almost 1000 pages. He believes that ancient civilzations were as advanced as we were and were devastated the last time this lining up of the planets took place. I have to say some of the evidence was quite convincing and mysterious stuff. I thought the book was a fun read and when that date arrived we took note and laughed about it the next day. I don’t subscribe myself to Mr. Hancocks theories, but some of the material really intrigued me.
Thats really all I had to say, if you think this subject is a good on ethen I would definately recommend that book.
The date in question was 5 May 2000. No, I don’t remember anything earth-shattering happening on that day either.
As so often with Hancock, the really depressing thing about what he writes is not so much that it’s nonsense as that it’s old and already discredited nonsense. You’d have thought that the fact that nothing had happened in 1982 would have made him think twice before uncritically accepting the similar predictions of others about 2000.
<< If Hancock said that anything would happen on 5/5/2000 >>
Although it didn’t get much publicity at the time, in fact there was a cataclysmic event on 5/5/2000: time was completely distorted and reversed, so that 11:04 PM EDT came before 11:03 PM, rather than after. By 11:05 PM, things were back to normal. Most clocks were unable to reflect this subtle cataclysm, but I was in a bar watching a digital clock closely between drinks, and I saw it, clear as a bell.
Graham Hancock wrote of fascinating things in the book. He indicated that very few survivors came out of the cataclysm triggered by the same event (27,000 years ago). He said mass graves existed with all kinds of animals and humans mixed together. He spoke of whale carcasses in the mountains and Antartica possibly being Atlantis. One of the scariest things wa about the pyramids and how in the great pyramid a tunnel that pointed at Sirius. Egyptians knew Sirius was two stars, but we didn’t find this out until the last 100 years or so (I think). It was also aligned to be a warning about the date in question. He believed that ancient civilizations were advanced as we are and the survivors left warnings about the globe. Edward Cayce has said many similar things. Really crazy stuff, but interesting nonetheless.
Not quite. Saying the closer an object is, the more it affects tides doesn’t explain why being closer affects the tides. Besides, of course, that the effect weakens with distance because the pull weakens with distance, being closer to the object means the differential pull on the near vs. far sides of Earth is greater.
Think of it this way - put an asteroid (say 100 km diameter) orbiting the Sun at the same distance as Earth. Even though the distance between the Sun and Earth is roughly the same, the tidal effect on the asteroid will be smaller because there is less distance between the near and far sides of the asteroid.
Yahbut, in the two instances under discussion, we are talking about the same radius.
I understand why and how the radius affects the tidal force–it’s just that it doesn’t make any difference since the radius is the same in both cases. It’s the distance that’s different–and of course, that is reflected in the different ratios.
