Hi all.
This is my first post; I have a Q I am hoping someone can answer for me please:
If you were to stand, say, in the Gobi desert, with a plumb line, the mass of the Himalayas and their root would deflect the line slightly - and this would cause errors in triangulation, identifying exact positions etc.
So:
Would you be able to witness this with the naked eye?
(Without using other methods such as triangulation, fixing on stars, counting swings on a pendulum etc?)
I gather the deflection is only of the order of arc-seconds - I know this is small, but is this a visible deflection? How long would the plumb-line have to be to make it visible? And even so, what would you measure the deflection against, given that ‘down’ itself is altered by the presence of the nearby mass?
The problem is that the gravitational shift affects everything. So it’s not as if you’d see the bob hanging slightly away from strauight down – all the things you use to orient your sense of directly up and down will be affected, too. If you read the first link, you’ll see that the reason Maskelyne knew about the effect was the change in apparent separation between points.
It’s not just heavy masses that do this – the rotation of the earth causes the earth to bulge at the equator. at all [points on the earth you get the fictitious cebtrifugal force being added at an angle to the downward force of gravity. But you don’t see the angular error because the ground and everything else adjusts itself to the local vector sum of the two.
so, no, you won’t see it, but you shouldn’t expect to. You can deduce it, though.
I hate this characterization of the centrifugal force. I’ll agree with you that in the strict Newtonian sense, that is, in the inertial frame of reference, there is no such force. However, in the accelerational or rotating frame it is perfectly valid to call it a force and make calculations based on this. I’d prefer a less dismissive term, such as “apparent force”.
Would you be able to witness this with the naked eye? Well, you really have no choice but to observe the gravitational distortion. It’s there - it’s happening. Is the effect discernable to the naked eye, compared to what the view would look like without the gravitational shift? It stands to reason. One hundred and fifty years ago, during the British survey of the India subcontinent, field operators with manual theodolites and office computers (the people, not the machines) had to account for minute visual shifts caused by the gravitational force of the mountains, as well as the prismatic effect of relative humidity in the air. Such corrections were crucial for accuracy, as each successive measurement was based on the presumed accuracy of all the measurements that had been made previously. If these atmospheric illusions were not observable, there would have been no need to account for them mathematically in their measurements. Reasonable?
They are observable in that sense. After all, they have been measured somehow but I don’t think that that is the sense of the OP.
The OP says: “Would you be able to witness this with the naked eye? (Without using other methods such as triangulation, fixing on stars, counting swings on a pendulum etc?)” I get the impression that MisterL is asking whether these deflections would be noticeable to the casual observer, like at Mystery Hill or Tumble Down Town. The answer, in that sense, is “no” since there is no local reference to contrast the obsevation.
Mentock is pretty much saying the same as I did above, but his mention of “Mystery Hill” brings up an interesting point.
The “Mystery Spots” where gravity seems to point in the wrong direction are the result of miscues in the vicinity throwing off your idea of what constitutes “up” and “down”.
At Clarke’s Trading Post in New Hampshire, there’s the **Topsy Tury House
http://ourworld.compuserve.com/homepages/robfriedman/OLDSOUTH.HTM
I’ve been to both. The house itself is set unevenly, and you know it. But the things you use to decide exactly which way is up have been subtly miscued, so that you don’t really judge correctly. You then get freaked out when things apparently roll “uphill”.
The same thing happens with thec many “mystery hills” around the country, where if you put your car in neutral, it apparently drifts “uphill”. It’s really rolling downhill, but your idea of uphill has been screwed up by the surroundings. (These hills have been covered in the pages of the Skeptical Enquirer. CSICOP or somebody probably has something about them on their website.)
So you won’t see the real effect of gravitational anomalies because it affects everything in the area, and plumb bobs seem t point radially into the earth when they actually don’t, but they do hang “down” seen against the local force direction. “Mystery sites” seem to show you things not hanging or rolling the correct way because the local cues have been “tilted”, giving you an incorrect idea about which way “down” is.
{semijack]If you compute the the gravitational acceleration without rotation and the same acceleration at the equator with rotation and take the ratio, that ratio multiplied by the polar diameter results in a number that is 26 miles greater than the polar diameter.
Is that a coincidence or does the earth act sort of like a collection of particles held together mainly by mutual gravitation?[/semijack]
Perhaps the question that David Simmons is asking, is whether or not the Earth bulge is a result of its intrinsic strength. If so, it’s a good question, there is actually a few dozen meters of extra bulge (beyond the 26 miles) that is unaccounted for by that, and up until the sixties that was thought to be remnant bulge from when the earth was rotating faster than it is now–which would have indicated a much higher viscosity for the earth’s mantle. At the time, that was a major argument against continental drift, but it was disproved.
