The La Palma sky, and why it is protected by law

The three factors, and the order matters

1. Altitude. Roque de los Muchachos is over 2,400 metres. Above that there is less atmosphere to look through, and less atmosphere means less turbulence and less absorption.

2. The sea of clouds. This is the factor people do not expect. The trade wind cloud layer sits below the summit and works as a lid: it seals the humidity and the coastal light pollution underneath it. From above you are looking at the sky with the island covered by its own cloud.

3. The law. There is sky protection legislation regulating the island’s outdoor lighting — direction, intensity, hours — precisely so the observatories can work. It is not a municipal recommendation: it is the reason an inhabited place has a sky like this.

What that means for a night of observing

• ·Calima ruins it. It is the one factor that undoes everything: air loaded with Saharan dust and the sky stops being clear. Before driving up, check whether calima is forecast.
• ·A full moon does too. No altitude compensates for a full moon if you are after deep sky.
• ·It is genuinely cold. At 2,400 metres, at night, in any season. Winter clothing, not a jumper.
• ·The road up is long and mountainous. It takes longer than it looks, and longer at night.

The honest part

Access around the observatories is regulated and not everything can be visited freely or at any hour. Check the current conditions before planning the night, because they change.

And a note that saves frustration: the sky is good across the whole island, not only at the top. If you would rather not drive a mountain road at night, any spot away from the towns already gives you a better sky than most of Europe.