Ramani yako ingekosea kabla ya chakula cha mchana

Simu yako inajua ulipo kwa sababu inasikiliza saa zilizoko angani. Setilaiti hutangaza wakati, tena na tena, na simu yako huhesabu mahali ilipo kutokana na muda ambao kila ishara ilichukua kufika. Kwa kweli ni tatizo la muda lililovaa vazi la ramani.

Your phone knows where you are because it is listening to clocks in space. Satellites broadcast the time, over and over, and your phone works out its position from how long each signal took to arrive. It is really a timing problem wearing a map.

Sehemu ngumu ni hii. Saa hizo hazipigi kwa kasi sawa na saa iliyoko mkononi mwako. Huko juu, mbali zaidi na uzito wa sayari, muda hupita kwa kasi zaidi kidogo. Kwenda kasi kwenye obiti hufanya kinyume, na kupunguza kasi hiyo kidogo. Athari hizo mbili hazifutani, na setilaiti hizo huishia kupata sekunde thelathini na nane za milioni kwa siku.

Here is the awkward part. Those clocks do not tick at the same rate as the clock on your wrist. Up there, further from the mass of the planet, time runs slightly faster. Moving fast in orbit pulls the other way and slows them slightly. The two effects do not cancel, and the satellites end up gaining about thirty eight millionths of a second every day.

Hilo linasikika kama si kitu. Si kweli. Mwanga husafiri sentimita thelathini hivi katika sehemu ya bilioni ya sekunde, hivyo kosa lisiloweza kusahihishwa la ukubwa huo hukua na kuwa kilomita kumi hivi za kosa la eneo kwa siku. Alama yako ya bluu ingetoka barabarani, kisha ingetoka mjini, kabla ya chakula cha jioni. Saa hizo hutengenezwa kwa makusudi kupiga kwa kasi isiyo sahihi zikiwa ardhini, ili zipige kwa kasi sahihi zikiwa juu huko.

That sounds like nothing. It is not. Light travels about thirty centimetres in a billionth of a second, so an uncorrected error of that size grows into roughly ten kilometres of position error per day. Your blue dot would drift off the street, then off the town, before dinner. The clocks are deliberately built to tick at the wrong rate on the ground, so that they tick at the right one once they are up there.