A magnetic observatory 30 km away knows when Bucharest goes to sleep

#One day, minute by minute

The Earth’s magnetic field at Surlari is about 49,000 nanotesla. The observatory measures it non-stop, and its data go into INTERMAGNET, the worldwide network behind magnetic maps, navigation and the tracking of solar storms.

The chart shows how much the field changed from one minute to the next on a calm winter day. Between midnight and 5 am the changes are a few hundredths of a nanotesla. At 05:05 the changes grow and stay large until late in the evening. Over the whole period, on weekdays, the day trembles a median 2.4 times as much as the night, about one part in 200,000 of the whole field.

Below it is the same day at Panagyurishte in Bulgaria, 290 km away and far from any direct-current railway. On the day shown, there is no similar rise there at 5 am. Storms from the Sun show up in both places at once.

#Not the Sun: the clock

It could be the Sun: in the morning the upper air becomes electrically charged and the field moves more. But in Bucharest the Sun rises, in universal time, between 02:30 in summer and 05:51 in winter, 3 h 22 apart. The tremor at Surlari starts at 03:05 universal time in winter and 02:05 in summer. The difference is one hour, to the five-minute precision of the calculation: daylight saving time.

On a Bucharest clock the tremor starts at 05:05 in winter and in summer, on weekdays, Saturdays and Sundays alike. It stops at about 23:25.

In the figure below, the jump of March 2023 shows from one night to the next. Across every clock change since 2009, comparing the median of the seven nights before with the seven after, the start of the tremor moved by 40 to 80 minutes in the direction of the clocks at 24 of the 34. The median: -60 minutes in spring, +61 in autumn.

#The metro or the trams?

Bucharest’s metro and trams run on 750-volt direct current. The current goes out through the third rail or the overhead wire and comes back through the running rails. The rails are not perfectly insulated, so part of the current returns through the ground along wide paths and makes a magnetic field that reaches tens of kilometres. Romania’s mainline trains (CFR) run on 25,000-volt alternating current, which reverses 100 times a second; its periodic part almost entirely disappears from one-minute means.

The first metro trains leave the end stations at 05:00, according to Metrorex’s station sheets. The first trams leave between 04:15 and 05:40, depending on the line. The last metro trains leave their end stations between 22:50 and 23:50; the last trams leave between 22:15 and 23:30. The average weekday of 2010–2026 shows a small rise between 04:25 and 05:00, a jump at 05:00 and a fall until 23:25.

The 5 am jump fits the first metro trains best, and the rise before it fits the first trams. The nights when the metro ran late, further down, show that the metro is heard. How much the trams add, the data cannot say exactly.

#The nights the city stayed up

On Orthodox Easter night, when Romanians go to church at midnight, the field between midnight and 3 am trembled a median 1.4 times as much as on the Saturday nights of the surrounding weeks, over the 20 years in the record. In no year was it quieter than all the comparison Saturdays. For Easter night Metrorex, the metro operator, has announced all-night service every year since 2007 except 2020, in recent years until 5 am; STB keeps tram 41 on the line. In every hour with data between midnight and 5 am the tremor is a median 1.4 to 1.9 times the usual; from 5 am, when the normal timetable starts, the difference almost disappears.

The exception is 2020. Churches were closed to worshippers, going out was restricted, and Metrorex announced that no trains would run on Easter night. The observatory recorded 0.4 times the level of an ordinary Saturday, the quietest Easter night in the whole series. A year later, with the metro open all night again, it was back to 1.6.

New Year’s Eve is louder still: a median 1.7 times the tremor of the nights in neighbouring weeks, and above all the comparison nights in 10 of 18 years. On New Year’s Eve, too, the metro has run all night every year, in recent years until 5 am, alongside a few tram lines. The one documented exception was New Year 2020–21, under a curfew, when Metrorex announced that no trains would run: that night fell to 0.8.

Another test is the ordinary nights when Metrorex announced longer hours, after football matches at the National Arena or during the protests of 4 November 2015. On all 11 such nights found in press archives, the field between midnight and 01:30 trembled more than usual, 1.9 times as much in the median. The events could have changed other things in the city too, but the timing matches the metro’s. After two big concerts where the metro closed at the usual time, Robbie Williams in 2015 and Metallica in 2026, the level was below or close to an ordinary night (0.64 and 1.05).

Holidays are quieter. Between 07:00 and 18:00 on Christmas Day the field trembles a median of about 87% as much as on an ordinary day of the same kind, 92% on Easter Sunday and 89% on New Year’s Day.

Before any search for events, a program picked out unusually busy nights on its own, leaving out magnetic storms. It found 57. They include New Year 2019–20 and 2023–24 and Easter night 2019. The rest have not been matched to an event.

#Which other observatories hear the trains

The same question, put to 37 European observatories: does their tremor move by an hour when the clocks change? For each, the week before is compared with the week after, in spring and autumn 2019, and the same comparison is repeated on three pairs of weeks when the clocks did not change.

Seven observatories pass the test: their tremor moves by 45 to 75 minutes in the direction of the clocks at both changes, and in the comparison weeks at most twice out of six. They are Belsk (Poland), Hel (Poland), Manhay (Belgium), Duronia (Italy), San Fernando (Spain), Chambon-la-Forêt (France), Lviv (Ukraine). At all seven, according to OpenStreetMap, a direct-current railway runs 8 to 28 km away: 3,000 volts in Poland, Belgium, Italy, Spain and western Ukraine, 1,500 south of Paris. Of the 10 observatories with such a line within 30 km, 7 pass; of the other 26, none. Nor does the one in Turkey, where the clocks have not changed since 2016.

Counted week by week, at the observatories near direct-current railways a clock change brought a jump of about an hour in 19 of 20 cases, and the comparison weeks in 4 of 60. Surlari itself, in a single autumn week of 2019, moved only 35 minutes; over 26 years, the median is an hour.

The trouble is old. Electric trams in Tokyo pushed Japan’s observatory out to Kakioka, and the railways around it were later electrified with alternating current so as not to disturb it. Trains near Berlin ended the observatories at Potsdam and Seddin; their successor, Niemegk, was sited well away. At Surlari, the city has caught up with the instruments.