The initiative came from the Astronomical Institute of the University of Wrocław, and it grew into an unusually crowded partnership: the Czech Academy of Sciences, the Czech nature protection agency, the administration of the Jizera Mountains protected area, forest districts on both sides of the frontier, and the regional authority in Liberec. What they were protecting is a stretch of the Western Sudetes with almost nobody in it. On the Czech side the park runs from the hamlet of Jizerka up to Smrk, at 1,124 metres the highest summit of the Bohemian Jizera Mountains. On the Polish side it follows the High Jizera ridge and wraps around the Jizera Meadow and the settlement of Orle. The international border runs straight through the middle of it, which is why the two countries had to agree - a light switch on one side of a line does not respect the line.

Astronomers grade night skies on the Bortle scale, running from a genuinely primeval sky at one end to the washed-out orange of a city centre at the other. The Jizera park sits at level four, and reaches level three when conditions are exceptional. That means the Milky Way is unmistakably there, structure and all, while the horizon still glows in several directions. Stand on the ridge on a clear night and you can name the sources: Liberec and Jablonec nad Nisou to the south, Tanvald beyond them, Jelenia Góra to the east. The influence of a city's lighting reaches tens of kilometres into country that has none of its own. The park exists partly so that people can come and see that for themselves, which is a harder argument to dismiss than a chart.

The Jizera Meadow lies at 840 to 880 metres in the valley of the Jizera River, and it is one of the coldest places in Poland - the country's cold pole, by most reckonings. On 3 February 2012 the meadow recorded minus 36 degrees Celsius. Sub-zero temperatures have been logged in every month of the year, and snow can lie into May. The cause is not altitude but shape: at night, cold air drains off the surrounding slopes and pools in the basin, which has nowhere to send it. Climatically the meadow behaves like ground 600 metres higher in the Karkonoše. For an observer this is a mixed blessing. Cold, dry, settled air is exactly the air you want between your eye and a galaxy. It is also the air that ends the evening early.

The darkness here was not designed. It was left behind. People first came to Jizerka in the 15th century hunting gemstones, calling the place Klein Iser; glassworks opened in 1828, and at its peak the settlement held around 450 people with a school, a pub, a sawmill, a bakery and a blacksmith. The wood-fired furnaces closed in 1911, undercut by coal-fired factories down in the valleys, and tourism moved into the empty houses. On the Polish meadow the village of Gross-Iser was demolished in the 1950s; only the mountain hostel now known as Chata Górzystów still stands. Along this whole frontier, communities emptied when their German-speaking inhabitants were expelled after 1945, and much of the high ground was never resettled. Jizerka today has no public street lighting at all. What looks like conservation is, in large part, the shape of an absence.

On the Polish side the preserve anchors an astro-tourism project called Astro Izery; on the Czech side, Jizerka's Panský dům and the Chata Pyramida sit in a settlement that simply goes dark after sunset. Paved paths reach the meadow from Świeradów-Zdrój, and hiking and ski trails cross the ridge in every direction - these are the mountains the Jizerská 50 cross-country race is run in every winter. The lookout tower on Smrk has been rebuilt, and from the top the view carries east to Sněžka in the Giant Mountains and west across the Lusatian Highlands as far as the cooling towers at Boxberg in Germany. Which is the park's argument in one glance: from the darkest ground in the region, you can still see what is making the rest of it bright.