New Kilogram, Volt and Ohm are ready for SI-2018 - Page 1
Forum section: electronics / general

author: Dr. Frank
author_rank: Super Contributor
timestamp: July 12, 2017, 08:17:31 pm
content: All expected publications for the precise determination of the Planck and Avogadro constants are now online.You may find the documents, most of them for free download, inmetrologia:http://iopscience.iop.org/journal/0026-1394/page/CODATA-2017_adjustmentThe necessary criterions are fulfilled to the due date of 30th June 2017, obviously:R1: At least 3 different experiments agree within < 50ppb (7 experiments)R2: At least 1 experiment is uncertain to < 20ppb (3 experiments)http://www.bipm.org/utils/common/pdf/SI-roadmap.pdf#page=3I compiled these results in a graph, which is copied from Derek Mullers latest VERITASIUM video, about the redefinition of the kilogram and the NIST-4 Kibble balance.The different numeric results are summarized in this table:A little bit disappointing is the result of the latest PTB measurements, done on new, improved Si - spheres.The numerical value is a bit far outside, and not overlapping with the other most precise WB experiments, from NIST and NRC.Anyhow, the elder results from 2015 agree better.This means, that in a few months time, CODATA 2017 will be published, with adapted values for h, e and N(A) respectively, which will then used for the final fixing of the new kilogram, the Volt and the Ohm, by end of 2018, when the according bipm congress, CGPM, will decide to implement SI-2018. I now believe for certain, that this will happen without any doubts.These three units, plus the Kelvin, will then have quantum based definitions, with zero uncertainty in the SI.Currently, Volt and Ohm both have about 0.2 ppm uncertainty, and the prototype kilogram is drifting 0.05 ppm in hundred years, or so.I'm very excited but also pleased about this outcome, looking forward to the new units.Frank

author: lukier
author_rank: Unknown
timestamp: July 12, 2017, 09:00:00 pm
content: Good news!So by fixing the constants, we'll get 0 ppm error V = nhf/2e - by definition.But the practical realization, that is the JJ array, still has uncertainty, noise floor (due to phase noise etc.). Am I correct?

author: Dr. Frank
author_rank: Super Contributor
timestamp: July 12, 2017, 09:30:55 pm
content: Quote from: lukier on July 12, 2017, 09:00:00 pmThat depends on the environment, where you look on it.Comparing two JJAs, in cryogenic environment, at 4.2K and using superconducting devices, they agree within 1E-19.That is on the same order as other quantum standards, like Cs or Sr clocks.If you compare a JJVA voltage at room temperature, thermal voltages and noise limit the uncertainty to about 1nV, that is about 1E-10 relative to 10V.That will also be the practical uncertainty limit. But currently, we do not know any 'analog' voltage reference (zener) uncertain to better than about 1E-7.In comparison, time can easily be maintained to 1E-13 or better at everybody's home, out of thin air.Frank

author: alm
author_rank: Super Contributor
timestamp: July 12, 2017, 09:52:47 pm
content: Obviously any realization will have some uncertainty. But the advantage of defining in terms of fundamental physical constants is that the definition is stable over time (assuming the laws of physics do not change) and can be reproduced from first principles.

author: quarks
author_rank: Frequent Contributor
timestamp: July 13, 2017, 09:12:31 am
content: thanks for your hint and summing up

author: brucehoult
author_rank: Super Contributor
timestamp: July 13, 2017, 02:38:48 pm
content: Quote from: alm on July 12, 2017, 09:52:47 pmIt's a bit concerning that it depends on the local value of g, which changes both over time and in different parts of the room!What about oscillating the kg at right angles to local gravity, using the precisely known V, I and calibrated (as in the video) coil?

author: HighVoltage
author_rank: Super Contributor
timestamp: July 13, 2017, 03:33:35 pm
content: That is an impressive amount of work that is put in to the new kg definition.Thanks for sharing this.

author: Dr. Frank
author_rank: Super Contributor
timestamp: July 13, 2017, 06:20:21 pm
content: Quote from: brucehoult on July 13, 2017, 02:38:48 pmAs far as I understood, you have no other chance than to precisely measure localGg.The whole principle relies on measurements parallel to the gravity force.If you would try the measurement perpendicular, you would have no effect to measure.Frank

author: zhtoor
author_rank: Frequent Contributor
timestamp: July 13, 2017, 06:38:36 pm
content: Quote from: Dr. Frank on July 13, 2017, 06:20:21 pmhello,an analogy for mass and "volt vs JJA" could be:-establishment of a unit of mass based on measurement of energy in a mass-energy transformation reaction, (E=mc2)so that the unit of mass is determined in terms of c, a known constant (or is it?).ie; m = integ(vi)dt/c2regards.

author: JoeO
author_rank: Frequent Contributor
timestamp: July 13, 2017, 08:18:54 pm
content: I always wondered what physicists do.

author: brucehoult
author_rank: Super Contributor
timestamp: July 13, 2017, 08:43:51 pm
content: Quote from: Dr. Frank on July 13, 2017, 06:20:21 pmI whipped up a quick spreadsheet to estimate the rate of change of local g due to the apparent movement of the Sun and Moon.https://docs.google.com/spreadsheets/d/1KM6urh6FdOvXzF-nFcXSEUT2DH5a1Cda5evWhQFkZooThe Sun adds an acceleration of +/- 6 mm/s^2 with a period of one day. A little less at Paris, but I'll ignore that. The Moon's effect is around 180 times less.Adding the vectors together, the magnitude of the total changes the fastest when the body is at 90 degrees to the local gravity field i.e. rising or setting.For the Sun, the relative change in the total local gravity is 4.4e-08 per second. For the Moon, 2.4e-10 per second.If you're trying to determine local G -- and then 1 kg -- to 1e-8 then you're going to have to measure g and then use g within a quarter of a second. Or at the very least, include compensation in your calculations. Or work only at noon or midnight, at new moon or full moon, when things will change more slowly.

author: alm
author_rank: Super Contributor
timestamp: July 13, 2017, 08:49:00 pm
content: Local G sounds quite contradictory to me, since by the common definition g is the local gravity constant, and G the global gravity constant (i.e. approximately \$6.67 \times 10^{-11} \cdot { m^{3} \over { s^{2} \cdot kg } }\$).Local g can be measured, so it is 'just' another calibration step that could be measured continuously and would only add a small uncertainty (measurement uncertainty and short term fluctuations).

author: texaspyro
author_rank: Super Contributor
timestamp: July 14, 2017, 03:34:52 am
content: [quote author=brucehoult link=topic=91703.msg1255439#msg1255439It's a bit concerning that it depends on the local value of g, which changes both over time and in different parts of the room!Here is a plot of the effects of solid earth tides and the vertical gravity offset (due to the sun and moon) over 30 hours. The day this run covered was not a particularly wobbly day.If you have a REALLY good (think >$250,000) pendulum clock you can see the effects of gravity changes.

author: Dr. Frank
author_rank: Super Contributor
timestamp: July 14, 2017, 06:24:36 am
content: Sorry, I of course meant local g, not the Gravity Constant G.It's explicitly explained in the Veritasium video, that g has to continuously be measured, and that even the heavy permanent magnet inside the balance has its influence.The Kibble balance equation explicitly contains g, at the bottom, to the right :Frank

author: alm
author_rank: Super Contributor
timestamp: July 14, 2017, 06:35:42 am
content: But again, I do not see what the big deal is about measuring g. Sure, it makes the experimental setup more complicated, but it is just another quantity to measure. Apart from a (probably small) contribution to the experimental uncertainty, it does not make the experiment any less valid.

author: Henrik_V
author_rank: Regular Contributor
timestamp: July 14, 2017, 10:39:25 am
content: Quote from: lukier on July 12, 2017, 09:00:00 pmThe uncertainty is 'just' shifted to the uncertainty of how exactly we know the values of these constants. As far as we know these constants are fixed. But our knowledge of the true* value will change, with the consequence that the values even of the most stable artifacts will change too.*) 'true' values only exist in math

author: Dr. Frank
author_rank: Super Contributor
timestamp: July 14, 2017, 11:25:39 am
content: Quote from: Henrik_V on July 14, 2017, 10:39:25 amThat's not correct, in a metrological view.Currently, only the frequency, or time is defined exactly, with zero uncertainty.That's a fixed value for the Cs emission frequency.h and e currently are not fixed, and have to be measured, implying some uncertainties:h= 6.626 070 040 x 10-34 J s; rel. uncertainty = 1.2E-8e= 1.602 176 6208 x 10-19 C, rel. uncertainty = 6.1E-9 and .By the new SI-2018, h and e will be assigned fixed values with zero uncertainty.This will then never change any more, as these constants can not be measured any more by means of other units like formerly by the Prototype Kilogram.Instead h and e themselves will define the other units, like the kilogram.This change of metrological baseline is the biggest intellectual change in the new SI-2018.Such an evolution already happened many years ago, in 1983, when the meter was defined by using fixed values for c (speed of light) and s (second), with zero uncertainty.Frank

author: alm
author_rank: Super Contributor
timestamp: July 14, 2017, 11:55:53 am
content: And to get rid of the constants altogether, we could redefine the SI base units as fractions of fundamental constants, just like how it is common to use c (speed of light in vacuum) as unit of speed (so c = 1) in some parts of physics. Plus it would let us use those obscure metric prefixes like exa and zetta.

author: Vtile
author_rank: Super Contributor
timestamp: July 15, 2017, 08:48:24 am
content: Isn't the defining uncertainty in all these measurements the uncertainty of definition of Ampere, isn't it. Everything else today is derived from it (everything is measured with precise ampere derived measuring instruments aka electronics). As a mere mortal it is something I wonder at times, though I'm sure there is some relatively simple solution (which I possibly could see, if I truely thought the subject over 5 minutes).

author: saturnin
author_rank: Regular Contributor
timestamp: July 15, 2017, 02:42:20 pm
content: Dr. Frank, thank you for very interesting and informative post! It is amazing how accurate measurements they are able to achieve...Quote from: Dr. Frank on July 14, 2017, 11:25:39 amI would not say that today's definitions are not exact. Let's take kilogram as an example:"The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram (IPK)".The definition itself is exact. 1 kg is (exactly, with zero uncertainty) the mass of the IPK. (Uncertainty of unit realization is of course something different and always non-zero.) I know that most evidence suggest its mass varies in reality, but as follows from the definition it is exactly 1.000000... kg whatever happens (cleaning, washing - it is a real science how to maintain the IPK):http://www.bipm.org/metrology/mass/units.html#si-brochure. Still, its drift ~0.05 ppm/century is just remarkable! What other artefact standard does have such stability?The funny consequence of the kilogram definition is: When the IPK is cleaned (and its mass is changed little bit), mass of all objects in the universe expressed in kg units is changed too (there is no real change of course) It is evident the kilogram definition is not suitable for modern applications, so the solution is to get rid of drifting IPK, measure elementary physical constants as accurately as we can now and assign them fixed values...

author: branadic
author_rank: Super Contributor
timestamp: November 06, 2017, 10:23:56 pm
content: Since the volt will be newly defined in 2018 is it worth to calibrate multimeters at the end of this year / at the beginning of next year or just wait till the new definition is implemented by all calibration labs during 2018? How long will that take?-branadic-

author: glarsson
author_rank: Frequent Contributor
timestamp: November 06, 2017, 10:43:09 pm
content: Quote from: branadic on November 06, 2017, 10:23:56 pmHow many digits of precision does your multimeters have?

author: Andreas
author_rank: Super Contributor
timestamp: November 07, 2017, 05:08:03 am
content: Quote from: Dr. Frank on July 12, 2017, 08:17:31 pmHello branadic,I fear that your question is (at least) one year too early for the annual calibrations.And the question is how much will the change be compared to the (>=4 ppm?) uncertainity of your instruments.With best regardsAndreas

author: Dr. Frank
author_rank: Super Contributor
timestamp: November 07, 2017, 11:38:27 am
content: The SI-2018 Volt will change by 0.00844 ppm, and the Ohm will change by 0.00015ppm, compared to the current definition of the Josephson and the Klitzing constants.The new fixed values of constants of nature will be:h = 6,62607015E-34e = 1,602176634E-19k = 1,380649E-23N(A) = 6,02214076E+23These will have no error any more.This CODATA-2017 Special Adjustment has been published 20th October 2017 in Metrologia:http://iopscience.iop.org/article/10.1088/1681-7575/aa950aThere's also the final recommendation to change the SI to these new constants, and to draw a new baseline, eliminating the IPK. This will then be officially decided end of 2018.Hurray!Frank

author: EEVblog
author_rank: Unknown
timestamp: November 07, 2017, 12:54:30 pm
content: Quote from: Dr. Frank on November 07, 2017, 11:38:27 amCalibrated 3458A's will be going cheap on ebay soon

author: Cerebus
author_rank: Super Contributor
timestamp: November 07, 2017, 01:43:10 pm
content: Quote from: EEVblog on November 07, 2017, 12:54:30 pmIn your dreams buddy!cf

author: dr.diesel
author_rank: Super Contributor
timestamp: November 07, 2017, 03:24:54 pm
content: I frequently dream of cheap 3458As, 732s and Data Proof scanners.

author: Echo88
author_rank: Frequent Contributor
timestamp: November 07, 2017, 03:52:14 pm
content: Dont you already have two 3458A?But a 732B and a Scanner would be nice, yes. Hint: On ebay.de theres at the moment a pricey Keithley 7168.

author: dr.diesel
author_rank: Super Contributor
timestamp: November 07, 2017, 04:07:31 pm
content: Quote from: Echo88 on November 07, 2017, 03:52:14 pmGood things come in pairs!

author: texaspyro
author_rank: Super Contributor
timestamp: November 07, 2017, 06:13:56 pm
content: Quote from: dr.diesel on November 07, 2017, 03:24:54 pmI intercepted your dreams before you had them...I have bought (all working properly):4 x 3458A's at $2000 - $2400 ea. Two with the high-spec Vref.1 x 732B at $6002 x 732A at $300 ea (one missing battery assembly)1 x SR104 for $200!Good things come to those who wait. It helps when sellers mis-label their auctions.

author: Andreas
author_rank: Super Contributor
timestamp: November 07, 2017, 07:45:09 pm
content: Quote from: Dr. Frank on November 07, 2017, 11:38:27 amMhm,so I will have to correct all my LTZ reference readings and my ADC reference voltage calibration values: but in which direction?Assuming the LTZ now reads 7.200000000V will it be 61nV more or less after the change?With best regardsAndreas

author: HighVoltage
author_rank: Super Contributor
timestamp: November 08, 2017, 08:06:50 am
content: Quote from: texaspyro on November 07, 2017, 06:13:56 pmWOW!I am also looking for miss - labeled items but you seem to have a skill 10 fold better!Congratulations.Are you willing to sell the 732B at a reasonable profit?

author: saturnin
author_rank: Regular Contributor
timestamp: November 08, 2017, 10:36:29 am
content: Quote from: Andreas on November 07, 2017, 07:45:09 pmJVS generates a voltage that is given by following formula: UJ(n) = nf/KJ, wheren, an integer, is the step number of the voltage and KJ= 2e/h is the Josephson constant (e- elementary charge andh- Planck's constant). If I am not mistaken, the current volt is defined by using Josephson constant called KJ-90which is agreed to be 483 597.9 x 109Hz*V-1(https://physics.nist.gov/cgi-bin/cuu/Value?kj90).Newly defined SI units will be based on exactly defined fundamental constants (values ofeandhwill shift slightly). Therefore, there will be also new value of exactly defined Josephson constant (you can calculate it based on the proposed values ofeandh). Comparison of new KJand actual KJ-90will give you magnitude and direction of change of the Volt unit (and your LTZ reference too).Please, correct me if I made mistake anywhere...

author: Dr. Frank
author_rank: Super Contributor
timestamp: November 08, 2017, 03:31:08 pm
content: Quote from: Andreas on November 07, 2017, 07:45:09 pmHey, that's a really good question, which brings another very important feature to my mind.The change of -8.4E-9 is from the latest CODATA-2014 definition of Kj to the upcoming one.As saturnin argues, 'conventional units' for Volt and Ohm have been used, that is an 'adopted value' for each of Kjand Rk, called Kj90and Rk90!Therefore, these conventional Volt90and Ohm90deviate from the SI-Volt and the SI-OhmDefinitely this definition of conventional units had been used in the calibration of instruments from 1.1.1990 onwards, also creating a jump of 7..8ppm with that date. (I have reported my experience with my then 2 months old hp3458A @ university).Reference here:https://www.bipm.org/en/publications/mises-en-pratique/electrical-units.htmlThe deviation between the Volt90and the new SI-2018 definition is as big as -0.11ppm, and for the Ohm it's +0.018ppm.Both deviations are very well noticeable on the calibration data of high-grade Volt and Ohms standards, and measurable (in difference mode) with 8.5 digits DMMs.. provided, if this old 1990 definition of conventional units is still in use as of today.At the moment, I can't find any hint at NIST, to which definition Volt calibrations are currently performed, I'll have to ask them.For the future, from 2019 onwards, there already exists a draft for a new mise-en-pratique of the electrical units, which definitely foresees usage of Kjand Rk. That's logical, because from then on, the kilogram will be mise-en-pratique by the Kibble balance, and therefore the correct SI-definition must be used.The final definition of e, h is not yet incorporated, see paragraphs 5. and 6.:https://www.bipm.org/cc/CCEM/Allowed/26/CCEM-09-05.pdfTherefore, I have to cancel my statement, maybe in 2019 there will be a considerable step in calibration of Volt and Ohm.FrankPS: on the PTB site, there's a new hint: The new SI will formally go effective on the World Metrology Day, that will be 20th May 2019.

author: texaspyro
author_rank: Super Contributor
timestamp: November 08, 2017, 04:28:44 pm
content: Quote from: HighVoltage on November 08, 2017, 08:06:50 amNot even for an unreasonable profit! Unless the price is very close to a new one. That one came from France. Reminds me, time to change the battery.Oh, and I forgot to mention the $200 Fluke 720 KVD. Listed properly. For who knows what reason, I was the only bidder. Seller was not too pleased with the closing price but accepted the deal.

author: tggzzz
author_rank: Super Contributor
timestamp: November 08, 2017, 05:35:48 pm
content: Quote from: texaspyro on November 08, 2017, 04:28:44 pmI got my 7 decade KVD for £180. Listed as "Vintage Julie Research Laboratories New York VDR 106/7 Voltage Divider Mod A" for collection only, which is correct but doesn't mention it is a KVD.I got it from an antiques dealer that had got it from an acquaintance suffering from dementia. When I entered the house it reeked of fags, and I mentally went "oh no". Fortunately the KVD had been briefly stored in an outdoor garage.I'm still playing around with it using my Solartron 7081 in ration mode. So far thequickexperiments show errors of the order of 0.5ppm, which is very acceptable.No, it won't be for sale for a while, and I'm not entirely sure how to ship it since it weighs ~43lbs/20kg.I could have picked thishttps://www.ebay.co.uk/itm/Vintage-Julie-Research-Laboratories-New-York-Null-Detector-ND106/282626942722up at the same time, but I don't have the space.

author: saturnin
author_rank: Regular Contributor
timestamp: November 08, 2017, 06:27:18 pm
content: Quote from: Dr. Frank on November 08, 2017, 03:31:08 pmThis document (ftp://ftp.nist.gov/pub/physics/neilz/papers/15_8_Measure_on_impacts.pdf) suggests the "conventional" volt, V90, is used for calibrations (since 1990):"...Thus, given the likely relative step change (see attached table) in voltage of 1×10-7, it is likely that there will be a moderate step change for a number of calibrations..."I didn't know there is such huge discrepancy between the volt in use (V90) and SI volt. I thought it would be only a cosmetic (formal) update, but obviously not...

author: Echo88
author_rank: Frequent Contributor
timestamp: November 08, 2017, 09:05:37 pm
content: My 720A came from ebay and was described as working, but the function switch was broken. So i bargained with the seller and paid only about 150€ in the end for it. I was able to repair the function switch with epoxy and love it. Extremely accurate, only the 6. and 7. decade have some slight dial contact corrosion, which produce errors at some settings. Fingers crossed that the function switch doesnt break again. In the future i gotta document the factory-changes it has built in, some reed-relays and connectors on the backside, for calibration purposes i assume.

author: VintageNut
author_rank: Frequent Contributor
timestamp: November 09, 2017, 02:50:16 pm
content: I have two KVDs per my signature. The oldest one was $50 and looks horrible with scratches and missing binding post plastics. The other is a newer D model and looks very clean. I paid $300 for the nice newer one.Both agree with one another very close. I can advance every digit of my calibrated KE 2000 DMM. Its really sweet to see every digit of the 10V and 1V range marching up and down.Both KVDs are aged for decades and they appear to be very stable.At this point I am waiting for a few calibration cycles of my 731Bs before I start to really use the KVDs. I need a good stable well-known voltage source to start adjusting some of the un-calibrated equipment.

author: nixxon
author_rank: Frequent Contributor
timestamp: November 16, 2018, 06:35:55 pm
content: Follow-up video of current event from Veritasium:https://youtu.be/c_e1wITe_ig

author: metrologist
author_rank: Super Contributor
timestamp: November 16, 2018, 09:10:27 pm
content: Did they vote?

author: schmitt trigger
author_rank: Super Contributor
timestamp: November 16, 2018, 09:47:35 pm
content: This is one tattoo that I would wear:

author: tomato
author_rank: Regular Contributor
timestamp: November 16, 2018, 10:55:00 pm
content: Quote from: brucehoult on July 13, 2017, 08:43:51 pmYou need to check your calculations. Tidal effects on local gravity (g) are less than 1 mgal, which is about 10e-6 relative. (1 gal = 1 cm/s2, g ~ 980 gal). Tide models are good to about 1 µgal and commercial instruments can measure g with an absolute accuracy of 1 µgal in minutes.

author: texaspyro
author_rank: Super Contributor
timestamp: November 16, 2018, 11:03:22 pm
content: Lady Heather can calculate and plot the vertical gravity offset in micro-gals. Also the solid earth tides in mm.

author: LapTop006
author_rank: Unknown
timestamp: November 17, 2018, 12:48:37 pm
content: Quote from: metrologist on November 16, 2018, 09:10:27 pmIt starts about two hours and 10 minutes into this:

author: brucehoult
author_rank: Super Contributor
timestamp: November 17, 2018, 07:20:23 pm
content: Quote from: tomato on November 16, 2018, 10:55:00 pmWhere is the error? It's a very simple calculation.- mass of Earth, Moon, Sun are well established, as are the distances from the surface of the Earth to the centres of those bodies- force due to gravity = G*m1*m2/r^2, acceleration = F/m1, so reduces to k*m/r^2- the force from a spherically symmetrical body is the same as if the entire mass is concentrated at its centre

author: tomato
author_rank: Regular Contributor
timestamp: November 17, 2018, 08:59:41 pm
content: Quote from: brucehoult on November 17, 2018, 07:20:23 pmYou calculated the forces at the mean earth-sun and earth-moon distances, and then assumed that is how much they vary over the course of a day. The forces actually vary by a small fraction of that as the earth rotates (and revolves around the sun), because the earth-sun and earth-moon distances vary by a small fraction.

author: brucehoult
author_rank: Super Contributor
timestamp: November 18, 2018, 11:03:49 am
content: Quote from: tomato on November 17, 2018, 08:59:41 pmI assumed that the magnitude of the acceleration from, for example, the Sun is near enough to constant over the course of a day, but varies in direction relative to the local vertical for someone standing on the surface of the Earth in the tropics.

author: texaspyro
author_rank: Super Contributor
timestamp: November 18, 2018, 06:15:50 pm
content: Here is a plot showing the vertical gravity offset and solid earth tides over a 24 hour period in Dallas, Texas.

author: tomato
author_rank: Regular Contributor
timestamp: November 18, 2018, 06:41:42 pm
content: Quote from: brucehoult on November 18, 2018, 11:03:49 amLet's look at a simple case. You calculated a change of ± 6mm/s2 due to the sun, which is roughly 0.1% relative change. At noon everything should weigh about 0.1% less (because the sun would pull "up" on you) and at midnight everything should weigh about 0.1% (because the sun would pull "down" on you.) That's would be huge change -- one that would disrupt all kinds of activities.The problem is that your calculation basically assumes that the earth and sun are separated by a distance (r), but they are motionless in space. Of course, they are not motionless, they are orbiting around their center of mass. From our perspective, we are in a constant state of free fall (acceleration) towards the sun, and this free fall is just the amount to cancel the force you have calculated.Tidal forces arise because the gravitational force from the sundoesvary slightly on the surface of the earth, because the distance from the sun varies slightly as the earth spins. This force is much smaller, however, than the 6 mm/s2 you calculated. (Basically, you need to scale your result by (r/R), where r= radius of earth and R = earth-sun distance.)

author: brucehoult
author_rank: Super Contributor
timestamp: November 20, 2018, 08:07:03 am
content: Aha! The Earth itself is in free-fall with respect to the Moon and Sun, so doesn't feel anything. Someone in a place with the Sun overhead or underfoot is not quite in free-fall -- they are constrained to the same path as the Earth (by being pressed on the surface of the Earth), even though they have slightly higher or slightly lower acceleration towards the Sun than the rigid body of the Earth does.Thanks.