It’s that time of year when many of us will be putting up Christmas lights and decorations: on the Christmas tree, around the house and, in many cases, outside it as well.
It’s those outside lights that can sometimes be the cause of inter-neighbourly conflict, because while some folks might put a simple ‘Happy Xmas’ sign in their window, others really go to town!
We’ve reached an accord with the people over the road now – they put up as many lights as they like, but turn the flashing ones and the really bright ones at bedtime (and for our late-night observing sessions in the back garden).
But we’re certainly not the first people to grumble that "those Christmas lights must be visible from space."
And, surprisingly, it turns out that we were right all along!
Christmas lights really are visible from space. And we can say that for certain, because proof has been supplied by NASA and the US National Oceanic and Atmospheric Administration (NOAA).
Christmas lights make cities brighter from space
The two bodies are jointly responsible for the operation of the Suomi National Polar-orbiting Partnership (Suomi NPP) – a climate-tracking satellite that launched in 2011.
Suomi NPP’s Visible Infrared Imaging Radiometer Suite (VIIRS) instrument measures emissions of visible and infrared light from Earth, and while analysing its first few years’ worth of data, researchers noticed something.
Around the Christmas/holiday period, cities in the US and Canada, Europe and other majority-Christian regions shone out around 20-30% brighter, at night, than they did for the rest of the year.
In suburban and semi-rural areas that figure increased to as much as 50%.
Applying all sorts of clever filters, algorithms and analytical tools to the data, the researchers were able to discount various other sources of light – moonlight reflecting off cloud tops, that kind of thing – and determined that, yes, this increased brightness as seen from space could indeed be attributed to people’s Christmas lights.
The video below shows what the Suomi NPP data revealed, with the observed percentage lighting changes mapped out and the locations of major cities and their environs clearly identifiable.
Interestingly, the same effect was seen in pretty much all US cities, regardless of their demographic make-up.
As NASA researcher Miguel Roman – who is interviewed in the video below – put it: "Despite being ethnically and religiously diverse, we found that the U.S. experiences a holiday increase that is present across most urban communities."
As if to prove there’s more that unites us and divides us, the same effect is also seen over cities in majority-Muslim countries during the month of Ramadan.
A bigger difference was observed between cities and the suburbs.
Yes, cities have more people – but many of them live in apartment blocks (where outside lighting isn’t usually feasible) and are likely will return to suburban-dwelling families for Christmas.
Conversely, in suburban areas, there are far more single-family homes with enough room for lots of outside lights, and the population temporarily increases during the festive period as people return home to visit family.
But can MY Christmas lights be seen from space?
"Okay," we hear you cry, "Christmas lights are visible from space.
"But you’re talking about the cumulative effects of all the Christmas lights in an entire town… what about the lights on that house at No 43?"
Well, actually, those might be, too!
The idea that any one house’s decorations could be seen from space might seem ridiculous – it’s even the premise of the 2006 Christmas movie ‘Deck The Halls’, starring Danny De Vito and Matthew Broderick.
But in 2017, a group of physics students at the University of Leicester decided to find out how ridiculous it really was.
They defined 'from space' as meaning 'from the International Space Station' and calculated that to be visible from the ISS’s altitude, a house would need an apparent magnitude of at least +6.5.
That equates to light emissions of around 10,600 lumens.
Given that the light emitted by a single LED bulb is around 4 lumens, that means you’d need 2,638 LEDs to make your house visible from space.
Or as Unilad calculated it, about £500’s worth – though the researches also assumed zero light pollution, so you might need another few hundred bulbs to be on the safe side!
The students’ findings were published in the University of Leicester’s own in-house, student-run Journal of Physics Special Topics… giving them experience of the peer-review process, which was really the point of the whole exercise.
NASA’s research likewise has a more serious side: as NASA’s Miguel Roman put it: "By looking at the lights we can see changes in human behaviour throughout the seasons … more than 70% of emissions [come from] cities.
"So NASA is putting a lot of emphasis on understanding cities and [how they’re] interacting with the climate and our system as a whole."
