nearer the stars

Some of the world’s summits are adorned with strange constructions, often rounded in shape: astronomical observatories. These buildings, erected on mountain tops as if to be closer to the sky, allow scientists to try to unlock the mysteries of the universe. Many observatories are also open to the general public wishing to observe the milky way. A unique experience!

What is an astronomical observatory?

An astronomical observatory is a place designed to observe the universe, from the solar system and various galaxies to the outer reaches of the cosmos, to help us better understand the laws of physics. Observatories are equipped with numerous scientific pieces of equipment, most notably, extremely powerful telescopes. The largest current telescope is the Gran Telescope Canarias (10.4m in diameter) but telescopes can also be linked to a network. When it comes to networked telescopes, the VLTi holds the record (diameter equivalent to 16.4 m)

Depending on their equipment, observatories may specialize in different types of observations: imaging, spectroscopy, polarimetry etc.

pic du midi observatoire

Credits photos: Camille Espigat

On mountain tops

Some observatories are constructed on the top of mountains. It’s no coincidence that they’ve been set up at high altitude. Contrary to what one may think, it’s not to be closer to the stars. In truth, we’re dealing here with distances of trillions of kilometres so a mere 2 or 4 kilometres won’t make a great deal of difference In the end, the gain in proximity achieved by working at this altitude is negligible.

So why has man gone and set up these enormous architectural structures on the mountain peaks?

The first reason is to allow them to get beyond one of the atmospheric zones responsible for creating a lot of turbulence. This turbulence is caused by differences in temperature on the ground. However, turbulence is mostly caused by the temperature differences between the various atmospheric layers: the greater the difference, the greater the turbulence etc.

Which means the light can’t diffuse uniformly. Which creates, for example, this impression of sparkling stars.

The second reason is the absence of light pollution. Light pollution is reduced in “deserted,” locations, far from “civilisation,” hence the creation of dark sky reserves such as the Pic du Midi.

pic du midi observatoire

Credits photos: Nicolas Bourgeois

To find more about how an astronomical observatory works, we met Philippe Mathias, teacher and researcher at Le Pic du Midi Observatory (Pyrenees/France). Le Pic du Midi, which was opened to the public in 2000, offers everyone a chance to discover the workings of an astronomical observatory, its science facilities and its research work. It’s a unique opportunity to learn about the daily life of the staff, researchers and technicians who work at the summit as well as a chance to enjoy the spectacular panoramic view from 2,877 metres above sea level. In December last year, the Pic du Midi became Europe’s 1st International Dark Sky Reserve and the 6th such reserve in the world. This label serves to protect the starry skies around the Pic du Midi Observatory from light pollution and more generally the natural environment in the Hautes Pyrenees area. 

Which are the largest observatories in the world?

The largest observatories are located in the least populated areas where the weather promises cloudless skies for longer periods. The large international sites include Mauna Kea in Hawaii with 9 telescopes and 3 radiotelescopes, Chile which has no fewer than 5 sites and more than 10 telescopes, as well as the Canary Islands where there are another dozen telescopes including the world’s most powerful telescope: the Gran Telescopio Canarias which is 10.4 m in diameter. Other sites also house 10 m telescopes which either work alone or as part of a network. Also of note is the world’s largest radiotelescope, ALMA, located on the Chajnantor Plateau in Chile, standing at more than 5,000 metres above sea level.

You work at Le Pic du Midi; what do you study specifically?

The research carried out in Le Pic du Midi focuses on 3 key areas:

  • The physics of the sun: 2 lenses scrutinize the sun’s surface, 2 others, fitted with coronographs monitor the sun’s chromosphere and corona. These 4 instruments, fitted with filters which allow them to monitor the interactions between the sun’s surface and corona, take pictures every minute. In addition to certain fundamental physics processes such as magnetohydrodynamics, the observatories can also predict solar weather to help prevent angry solar flares from damaging satellites, for example.
  • The physics of bodies in the solar system: This involves, for example, monitoring the formation of giant planets formed by clouds of matter, using a 1 m diameter telescope, as well as identifying the orbits of lesser known bodies such as comets and asteroids. In particular, we monitor near-earth-objects, bodies which may cross the earth’s orbit.
  • The physics of stars: this field is explored using the Bernard Lyot telescope, the largest telescope in France with a 2 m diameter mirror. This telescope is equipped with a particularly effective instrument, namely a Narval spectropolarimeter. This significant, complex instrument (there are fewer than 5 in the world) can detect and measure the magnetic field on the surface of stars.

pic du midi observatoire

Credits photos: Paul Compere

What is the beautiful thing you have observed to date?

Because to its proximity, the best known star is undoubtedly the sun. However, in spite of its proximity, we don’t really understand the physics of the sun and in particular its 11 year cycle which determines whether there are more or fewer spots on its surface. These sunspots that are found mostly in low latitude areas, are around 10,000 km in diameter and are caused by the presence of a local magnetic field which blocks the movement of the sun’s upper layers. However, and this is what really fascinates researchers, they have been unable to apply this solar model to those stars observed by the Narval spectropolarimeter which display a wide range of characteristics: spots that are ten times larger than those observed on the sun, sometimes located at its magnetic pole, with an overly complex or simple formation etc. We still don’t know all there is to know about the generation of magnetic fields in stars and the solar model is constantly being revised!

pic du midi observatoire

Credits photos: Paul Compere

What’s the best time of year for the public to come and observe the sky?

In theory, the most beautiful nights, in astronomical terms, occur in winter when the temperature of the different atmospheric layers is sufficiently uniform so as to prevent too much turbulence. But winter is also a time of unpredictable weather so you need to make inquiries in advance.

pic du midi observatoire

Credits photos: Sylvain Marmer 

Find this article on the new magazine Hiking on the Moon #9, “BETWEEN HEAVEN AND EARTH”


For more information about Le Pic du Midi, go to 

Credits photos home, 3, 4: Paul Compere

Credits photos 1: Camille Espigat

Credits photos 2: Nicolas Bourgeois

Credits photos 5: Sylvain Marmer





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