Stop Using GMT!

TL;DR: GMT is the obsolete precursor of UTC. GMT literally does not exist anymore—and hasn't for almost a century. I do not care what your phone, API, or law says—all modern timekeeping is based on UTC!

The science of time is complicated, but basically we can measure it from atomic clocks (the TAI time scale) or from the (notional) position of the sun in the sky (UT1). Marrying up the best of both worlds gives us UTC, offsets of which give us the local time we see on our devices and use in our lives.

Notice that GMT doesn't enter into this anywhere. That is because GMT is obsolete.

Unfortunately, there exist today a huge number of people who persist in the wholly erroneous belief that GMT still exists. These people are generally ignorant. Less forgivable are the British nationalists who evidently care more about the historical role the English played in developing chronometry than the fact of accurate time itself. These people are actively undermining the very nature of consensus time, out of an apparent, misguided sense of patriotism.

What Even Was GMT, Anyway?

At any particular instant, the position of the sun in the sky is determined by your longitude (distance from a particular meridian (imaginary "vertical" / north–south reference line) where longitude is 0°). At the Royal Observatory, Greenwich (ROG), the British defined a series of such meridians (effectively changing the longitude of every point on the whole Earth each time they got a new instrument^[1]). The last of these meridians was officially standardized in 1884 at the International Meridian Conference as the "Prime Meridian".

The Greenwich Meridians

Before 1721, the exact details are not clear. The first official Greenwich Meridian was defined by Edmond Halley in 1721, and was in the northwest corner of the observatory complex. James Bradley set up a new instrument 37 meters to the east, defining a new meridian (now also called the Bradley Meridian) in 1750. Another instrument on the same spot replaced it in 1816. George Airy's instrument set up in 1851, another 6 meters east. The Greenwich Meridian, as defined by the Airy Transit Circle (ATC) (i.e. the 1851 meridian) was standardized on 1884-10-22 as the world's Prime Meridian (France used a meridian through Paris for a few decades more).

Technological improvements have allowed us to make further revisions; since the 1980s, the ITRF Zero Meridian is the worldwide standard that defines longitude, and is 102.478 meters east of the 1884 line. The British maintain what can only be described as deliberate ignorance of this fact.

The position of the sun in the sky at a particular point in the day changes with what day of the year it is. We average this over the course of a year to get something called "mean solar time". With the new formal definition of longitude, the mean solar time at longitude 0° could be reasoned about, and this formalized the notion of Greenwich Mean Time (GMT)^[2].

Chronometry Has Advanced Since the 1880s.

The British nationalists would have us stop there, but that state of affairs is more than a century out-of-date.

Confusion over whether GMT was reckoned from noon or from midnight made GMT unreliable. The IAU evidently^[3] feverishly discussed the issue in 1925, the chaos punctuated with the publication of the 1925 British Nautical Almanac^[4], in which British standards were further changed. The debacle led, in 1928, to the term "Greenwich Mean Time" being retired by the 3rd General Assembly of the IAU Commission 4 (see page 5, source in French^[5]). In its place, one was to use other terms—namely "Universal Time" (UT)—for the purposes of astronomy, and therefore timekeeping. This should have been the end of the term "GMT" forever.

In 1929, UT was redefined as the result at a "mean observatory"—an average of several observatories' measurements worldwide were averaged, boosting accuracy^[6]. Thus, Greenwich stopped being even the sole source for defining time.

Eventually, observatories' individual timescales would be called UT0, averaging them while correcting for polar motion would be (an early version of) UT1, and UT would become an umbrella term—but the Greenwich Observatory was by this time already on the decline. By the 1930s, technological limitations and atmospheric (and magnetic!) pollution were hobbling astronomers at the Greenwich observatory. With WWII, the facility was all but abandoned. In 1948, the Office of the Astronomer Royal was moved to Herstmonceux in East Sussex, which likely meant the cessation of any timekeeping measurements at the facility. In any case, by 1957 the facility was closed completely. It became a museum in 1960.

Throughout the 1960s, 70s, and 80s, systems of measurement and reference standards advanced still further. 1973 brought the correction of UT1 to handle the motion of the Earth's pole. In 1984, optical astronomy was abandoned entirely in favor of VLBI, SLR, and LLR (Very-Long Baseline Interferometry, Satellite Laser Ranging, and Lunar Laser Ranging) systems, which are far more accurate. This made UT0 obsolete, and forever decoupled time from direct measurements of the Sun.

One of the more significant developments of this era was another redefinition of the meridian. Due to local anomalies, gravity at the observatory site does not point exactly toward the center of the Earth; the plane defined by "up" (measured from local gravity) and the ground line through the Airy Transit Arc does not intersect the Earth's axis, but actually goes off to one side. To correct this error, that plane was kept in the same orientation, but shifted so that it passes through the Earth's center^[7]. This resulted in a further shift east of more than 100 meters to essentially where the modern meridian—the ITRF Zero Meridian (but more-commonly "IERS Reference Meridian" or "International Reference Meridian" ("IRM"))—is today^[8]. This explains why, if you stand on the big stainless steel line outside the museum, your GPS will (correctly!) tell you that you are 102.478 meters west of true 0°^[9].

Lies People Continue to Tell

The history above is rather condensed, and it tracks the science, not contemporary usage. Apparently since the advent of GMT, England has taken great pride in its role, to the extent that it continues, even in the modern scientific age, to promulgate blatantly false propaganda, to the effect of confusing everyone.

Overhead view of the Royal Observatory, Greenwich. — Figure 1
: Map of the Royal Observatory, Greenwich. The Prime Meridian is at left, the tourist attraction with the incorrect labeling and the gaudy line on the ground. The real ITRF Zero Meridian is the yellow line (also labeled "Prime Meridian"). (Image via Google Earth.)

A sign incorrectly announcing its location to be 0° longitude. — Figure 2
: Absolutely horrible sign formerly^[10] labeling the 1884 Greenwich Meridian. It incorrectly suggests that longitude and west/east are centered around the Greenwich Meridian. This sign's actual location was 0°0'5.3"W. (Image source gratefully acknowledged.)

The museum at what was once the Greenwich Observatory? The building has an all-capitals sign advertising the "PRIME MERIDIAN OF THE WORLD", with one side labeled "WEST" and one side labeled "EAST" (Figure 2). The line itself is indicated by a steel line embedded in the pavement, with golden letters showing (no doubt incorrect) longitude displacements to other great cities. The real meridian is completely unmarked.

The deception is deliberate, of course. Any idiot with a cell phone and a map app can see that the alleged meridian is in the wrong place; in fact it confuses "thousands" (of the millions) of visitors to the site each year. It's not like no one notices.

The museum has erected a sign (Figure 3) to address this point. The sign suggestively asks whether the viewer's GPS is broken, before dismissively acknowledging a "satellite meridian". The fact that this "satellite meridian" is the real, actual IERS Reference Meridian is not mentioned, and the sign heavily implies that the 1884 meridian ("the line") is the basis of longitude—a blatantly false conceit which, at the URL on the sign, is repeated explicitly^[11].

A sign gaslighting you about your GPS. — Figure 3
: Deeply misleading sign just to the east of the 1884 meridian, which strongly (and incorrectly) implies that the viewer's GPS is broken and that the 1884 meridian is the basis of longitude today. (Image by me from Google Earth.)

The misinformation extends directly and analogously into bogus ideas about GMT.

The following code, which prints the current date, is in JavaScript, a language invented in 1995:

console.log( new Date().toString() );

As I write this, what do you suppose it outputs?

> "Sat Dec 18 2021 19:30:52 GMT-0800 (Pacific Standard Time)"

Hisssssss how did this happen??? GMT hasn't existed since 1928! And using it with UTC offsets??? That has never been correct, even historically!

Okay, so maybe it's just for backward compatibility. What would happen if we told it explicitly to report as UTC instead?

console.log( new Date().toUTCString() );

> "Sun, 19 Dec 2021 03:35:22 GMT"

There is no excuse for this. It is literally just wrong. Not only is it outputting values in a time standard that hasn't existed for a century (and predates the language itself, to say nothing of the computer it's running on), it is deliberately, explicitly conflating the two time scales!!!

As per the Interpretation Act of 1978, §9, the UK is legally bound to have civil time be (a function of) GMT—which of course is preposterous, since GMT, once again, doesn't exist anymore, and isn't measured by the UK. The so-called "Greenwich Time Signal" (the official, though not practical, method by which civil time is disseminated), is actually broadcast from BBC radio stations, whose clocks are, of course, synchronized from UTC^[12]—as all clocks ultimately are. The problem is that the British people are lied to, and told that this is GMT, when in fact it is UTC.

Many countries, particularly in Europe and in the current/former British Commonwealth, legislate much the same—and all of them are just as wrong.

Not a Synonym

Faced with the incontrovertible fact that GMT doesn't exist, the UK will generally claim, if pressed, that GMT today is a synonym for the UTC+00:00 timezone. Surely, this is even more madness!

The "mean" in "Greenwich Mean Time" refers to solar mean time—that is, the (average) position of the actual Sun. Yet UTC is not measured off of the Sun. It ticks with atomic clocks and is only close to solar time (UT1, which is also not GMT) for convenience. UT1 and UTC are most definitely not the same—they differ by up to 0.9 seconds! Since clocks these days are accurate to a quintillionth that, that kind of error is really unacceptable. Furthermore, the Greenwich observatory is not at longitude 0°, and so mean solar time there is useless for defining time anyway. Solar time isn't even an integer number of seconds, so it doesn't even get the number of seconds in a day right.

Greenwich Mean Time is not measured at Greenwich (or anywhere), civil time is not a measurement of solar mean, and such seconds wouldn't be valid to keep time. It is literally wrong in each of its three terms.

We could define words to mean whatever we want, but re-defining GMT as UTC is bogus on literally every level, and the only reason people are trying to do that is to lie to you about history, because their prioritize their national glory over historical truth.

Conclusion

The Royal Observatory Greenwich has not participated in any timekeeping activities since 1957 at the latest, and GMT itself hasn't properly existed since 1928. The meridian through the Airy Transit Circle is easy enough to draw on the ground, and one can in principle think about (an average) local solar noon there, but this is not the meridian for longitude zero, and the local solar noon has not, in fact, been measured scientifically there for three generations.

In any case, UTC, the modern time standard, has no relation to optical telescope measurements—and hasn't since 1984. 600+ atomic clocks around the world are combined to form a timescale called TAI, which is offset by an integer number of SI seconds to be close to UT1 (a sidereal measure of the Earth's rotation that doesn't measure the Sun at all), forming UTC.

GMT seconds weren't even one second long. They varied, with the length of the day. Imagine not knowing how long a second is!

GMT has been formally obsolete for almost a century—and utterly incorrect for at least half of that. The people who continue to use it are British nationalists with more patriotism than common sense, or the many many people around the world that they spitefully delude. GMT is not mean solar time at 0° longitude. GMT is not a synonym for UT, UT1, or UTC. GMT is not a timezone. GMT does not even exist anymore—and anyone who tells you otherwise is dead wrong, full stop.

GMT was historically important, and the fact that it is nonexistent today should do nothing to detract from that great legacy, or the genius and dedication of the great British scientists that developed much of the requisite technology. But for goodness sake—it is a century later, and the Britons persist in lying to themselves and to the world. It is an embarrassment.

Stop this madness! Put GMT back into the historical context where it belongs!

Additional Notes

[1]	Depending upon who you ask, the difference was either small enough compared to survey errors that it was deliberately neglected, or it wasn't recognized as a source of error until 1956.
[2]	GMT itself predates this to July 1676: John Flamsteed defined, in 1672 (using tables created in 1667), a notion of local (solar) mean time. When he moved to the observatory in July 1676, this could recognizably be considered GMT.
[3]	The proceedings for 1925 are not publicly available due to copyright issues, but they are quoted many times to this effect.
[4]	See "The Nautical Almanac And Astronomical Ephemeris For The Year 1925, For The Meridian Of The Royal Observatory At Greenwich (With Two Inset Eclipse Maps)" (Google Book and PDF Download).
[5]	"Les termes Greenwich Civil Time (G. C. T.), Weltzeit (W. Z.) et Universal Time (U. T.) indiquent le temps compté à partir de minuit moyen de Greenwich sans ambiguité. La désignation Greenwich Mean Time (G. M. T.) pour les dates antérieures au I janvier 1925 se rapporte au temps compté à partir de midi, mais après cette date elle signifie, dans les publications britanniques, le temps compté à partir de minuit. Il est conseillé aux astronomes de ne plus employer dans aucun sens les lettres G. M. T." Translating idiomatically: "The terms Greenwich Civil Time (GCT), Weltzeit (WZ), and Universal Time (UT) unambiguously indicate time counted from Greenwich mean midnight. Greenwich Mean Time (GMT), for dates before 1925, refers to time counted from noon, but after that date, in British publications, it is time counted from midnight. Astronomers are [therefore ]advised not to use GMT in any way.
[6]	A concept apparently called "l'heure définitive" ("definitive time"). See [Guinot 2000] "History of the Bureau International de l'Heure" pg. 3. See also [Kershaw 2019] "Twentieth-Century Longitude: When Greenwich Moved" and references.
[7]	By shifting the plane into position instead of rotating it, the angular positions of all celestial objects were maintained. The consequence, of course, was that the longitudes did not.
[8]	There were other factors and corrections, but this shift accounts for most of the 100-odd meter distance.
[9]	For more on the exact history of the meridian and the standards, see "Why the Greenwich Meridian Moved" [Malys et al. 2015].
[10]	The current sign is similar, though admittedly it removes the word "longitude", the most blatantly false lie here.
[11]	E.g. it starts with: "The Royal Observatory in Greenwich is where east meets west at Longitude 0°.", which is objectively false. To its credit, there is a brief but clear acknowledgement that the IERS Reference Meridian exists, but it is 90% of the way down the page, and the rest of the text is deliberately crafted to deceive. For example, "The line in Greenwich represents the historic Prime Meridian of the World - Longitude 0º. [...] If you stand with one foot on one side and the other on the left, you are perfectly in the middle of east and west, according to the prime meridian line." is technically true, provided you pick up that the word "historic" actually means "obsolete" instead of "milestone importance", and that "according to" means "erroneously using obsolete references".
[12]	In this particular case, the signal is synchronized from an atomic clock at Broadcasting House (the BBC headquarters), which is itself synchronized to the National Physical Laboratory's "Time from NPL" broadcast standard. Regardless of the BBC's apparent belief to the contrary, the technical report from NPL itself (correctly) notes that the broadcast is UTC. Moreover, BIPM, the (French) standards organization that administers UTC as a whole, currently lists NPL as contributing five (active) atomic clocks to the implementation of UTC (clock codes: 1353167, 1353514, 1412011, 1412050, 1891701).

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