[HN Gopher] The mountain that weighed the Earth
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The mountain that weighed the Earth
Author : surprisetalk
Score : 95 points
Date : 2026-01-26 16:43 UTC (17 hours ago)
(HTM) web link (signoregalilei.com)
(TXT) w3m dump (signoregalilei.com)
| divbzero wrote:
| > _Primary sources:_
|
| > _Maskelyne's notes:https://doi.org/10.1098/rstl.1775.0050_
|
| > _Hutton's notes:https://doi.org/10.1098/rstl.1778.0034_
|
| > _Cavendish's notes on his own
| experiment:https://doi.org/10.1098/rstl.1798.0022_
|
| I got to reproduce Cavendish's experiment when I was a student.
| Love that we can easily read the primary source today, archived
| and indexed by DOI.
| neitsa wrote:
| > Using the stars as a reference, Maskelyne's team found that
| the plumb lines on either side of the mountain pointed just
| 0.0152 degrees apart.
|
| I'm really interested in knowing how they could get such a
| precise measurement (even accounting for errors), especially in
| the field (outdoor). There's no figure depicting the apparatus
| they used, I wonder how it looked like.
|
| Sometimes, I just ponder at how ignorant I am. If I was tasked
| with the same assignment, I'd definitely fail and this was
| performed 250 ago!
| throwway120385 wrote:
| Maybe something similar to a vernier caliper.
|
| From Wikipedia:
|
| > The first caliper with a secondary scale, which contributed
| extra precision, was invented in 1631 by the French
| mathematician Pierre Vernier (1580-1637).[1] Its use was
| described in detail in English in Navigatio Britannica (1750)
| by mathematician and historian John Barrow.[2] While calipers
| are the most typical use of vernier scales today, they were
| originally developed for angle-measuring instruments such as
| astronomical quadrants.
|
| https://en.wikipedia.org/wiki/Vernier_scale
|
| So it would have been a contemporaneous technique with that
| initial angle measurement, and the use of a Vernier scale for
| angular measurements would have itself been common.
| ahazred8ta wrote:
| They had a vertical 'Zenith Telescope' that looked at the
| same star from two locations. They measured how far from
| vertical it shifted in the magnified field of view. https://e
| n.wikipedia.org/wiki/Ramsden_surveying_instruments#...
| Similar instrumends measured the wobble of the Earth's axis.
| https://en.wikipedia.org/wiki/International_Latitude_Service
| CobrastanJorji wrote:
| I'd love to know what a sufficiently high precision plumb bob
| is like. Is it very tall? How on Earth does one calibrate it?
| helterskelter wrote:
| I remember reading about this in _Mason & Dixon_. Mason, who
| worked at the Royal Observatory, was the one who identified this
| mountain as the best place for the experiment (and was asked to
| help with it but declined).
|
| IIRC, it was partly the Mason Dixon line that inspired this
| experiment. They noticed syatematic errors in the line because
| their plumb bobs were deflected by gravitational pull from local
| terrain. At the time they speculated it was because of the
| Alleghenies, though it was probably more localized variations in
| gravity.
| cossatot wrote:
| Interesting...
|
| A few years later, the gravitational deflection of the Himalayas
| on a plumb line by Airy proved less than expected, which
| suggested that mountains have 'roots' that extend below them,
| displacing more dense rock--like icebergs more or less.
|
| I used the gravitational force of the Longmenshan range to
| calculate the perturbations in the elastic stress field of the
| Earth's crust in Sichuan province, China, to estimate the
| tectonic forces in the region, which caused the 2008 Wenchuan
| earthquake:
| https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/201...
| cwmoore wrote:
| How far does it deflect the Sun?
| ck2 wrote:
| can GPS sats figure out the mass of the earth by being able to
| detect its gravitational distortion on their orbit?
|
| or maybe that upcoming space laser interferometer (LISA) since it
| has to figure precisely how all mass is affecting its position?
|
| I love the history of figuring the circumference of the earth,
| imagine getting it right within 2% in 240 BC
|
| (then Columbus effing it up by 25%)
|
| https://en.wikipedia.org/wiki/Earth%27s_circumference#Histor...
| ISL wrote:
| Scientists use pairs of satellites to map the small variations
| in Earth's gravitational field. It is possible to see
| groundwater depletion and changes in distribution of glacial
| ice, among many things.
|
| https://en.wikipedia.org/wiki/GRACE_and_GRACE-FO
|
| The primary challenge in determining the mass of Earth is
| actually measuring the gravitational constant, G, itself.
| Everything else involved is known at much higher precision. The
| product of G and Earth's mass is known to two parts in a
| billion, but the uncertainty in G is ~22 parts per million.
|
| https://en.wikipedia.org/wiki/Gravitational_constant
|
| LISA is primarily sensitive to time-varying gravitational
| gradients on timescales of a fraction of a minute to a few
| hours and won't be terribly useful for determining the orbits
| of objects in our solar system. (but it is very, very cool).
| cjs_ac wrote:
| > The Schiehallion experiment wasn't the state of the art for
| long. A more precise result was achieved in 1798 by Henry
| Cavendish, who was on the committee for the Schiehallion
| experiment. Cavendish's experiment measured the gravity of large
| lead spheres using an extremely precise torsion pendulum, and cut
| the error from 20% down to 1.2%.
|
| Cavendish was a peculiar fellow.
|
| > At his death, Cavendish was the largest depositor in the Bank
| of England. He was a shy man who was uncomfortable in society and
| avoided it when he could. He could speak to only one person at a
| time, and only if the person were known to him and male. He
| conversed little, always dressed in an old-fashioned suit, and
| developed no known deep personal attachments outside his family.
| Cavendish was taciturn and solitary and regarded by many as
| eccentric. He communicated with his female servants only by
| notes. By one account, Cavendish had a back staircase added to
| his house to avoid encountering his housekeeper, because he was
| especially shy of women.
|
| https://en.wikipedia.org/wiki/Henry_Cavendish
| Eddy_Viscosity2 wrote:
| Who was saying that autism rates are increasing because more
| people have it now and not because we are better are
| recognizing it??
| ggm wrote:
| It's interesting that a device based on specifically constructed
| weights, at a scale to fit in a lab bench experiment (or at least
| a room) were capable of providing this much accuracy compared to
| a field experiment which used significantly larger masses, but
| was probably subject to many many more distorting qualities and
| estimation/rounding errors.
|
| I can imagine that given enough motivation to chase down
| accuracy, they could have re-scaled the lead weight experiment to
| fit larger spaces, larger pendulums, assuming they could control
| for drafts, pigeons living in St Pauls Cathedral...
| augusteo wrote:
| The precision they achieved with 18th century tools is
| remarkable. Measuring 0.0032 degrees of deflection without modern
| instruments, then getting within 20% of the correct answer.
|
| I love stories where the constraint forces creative problem-
| solving. They couldn't measure gravity directly, so they found a
| mountain-sized workaround.
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(page generated 2026-01-27 10:01 UTC)