We have now also included on our web page a gallery section showing geological samples and structures associated with the Limousin asteroid impact 201 million years ago.
In this month’s blog we take a closer look at the Meudon observatory. One of the world’s leading Solar observatories, and steeped in history, it is also one of the easiest for UK and French visitors to access. The town, chateau and park make a delightful visit for astronomers and non-astronomers!
Nous avons maintenant sur notre page une galerie de photos vous présente des images d’échantillons géologiques associés à l'impact de l’astéroïde qui est tombé sur le Limousin il y a 201 millions d’années.
Ce mois nous allons regarder de plus près l’Observatoire de Meudon, un des observatoires solaires les plus importants dans le monde. Meudon est chargé d’histoire et très accessible pour les visiteurs français et anglais.
A French observatory devoted to solar physics
It is best to ask the way (or get your smart phone to guide you) and the uphill walk through a pleasant residential area and it takes 15 to 20 minutes, the slope being fairly steep. However if you prefer you can take the bus number 289 in the direction of Clamart and alight at the stop Eglise de Meudon; cross the road and walk 5 minutes uphill. The address of the observatoire is 5 Place Jules Janssen, 92190, Meudon, France. As you walk just beyond place Jules Janssen a landscape of greenery meets you and the observatory building sits like a country manor house at the edge of a manicured lawn.
At the moment work is being carried out to upgrade the security of the observatory building and until this is complete the interior is closed to the public. However you can go upon the terrace which overlooks the city giving magnificent view of Paris. It is one of the main places in Meudon for locals to take a stroll. It also offers a wonderful view of the 14th July fireworks taking place at the Eiffel Tower. Afterwards you can explore the neighbouring forest which is renowned for its biodiversity.
Ou en arrivant à Meudon-Val Fleury il est possible de prendre le bus 289 direction Clamart et descendre à l’arrêt Église de Meudon; traversez la rue puis montez la colline (environ 5 min). L’addresse de L’Observatoire est 5 Place Jules Janssen 92190 Meudon France
Malheueusement en ce moment il y a des travaux sur le site de Meudon pour que les bâtiments repondent aux règles de sécurité appliquées à l’accueil du public et donc l’observatoire est fermé aux visites. Mais vous pouver monter sur la terrasse de Meudon (61 565m2) qui surplombe la ville et qui permet une magnifique vue sur Paris.
C’est un des lieux principaux de la ville de Meudon et ou les Meudonnais se promenent ou observent souvent les feux d’artifices de Paris le 14 juillet. Vous povez également vous promener dans la forêt voisine qui est renommée pour sa biodiversité.
Pour une explication des instruments qui se trouvent à l’observatoire de Meudon voir les paragraphes en anglais ci-dessous ou les liens au pied de l’article.
Some amazing instruments were and are housed at Meudon, many of which are still there today. These include:
La Grande lunette.
A lunette is the preferred French word for a refracting telescope, one which uses a lens; the word télescope usually refers to a reflector; i.e. one which use mirrors. At the turn of the 19th century, many of the world’s larger observatories were investing in ‘Grande lunettes’ – large refracting telescopes. The instrument commissioned and deployed at Meudon was built by the Henry brothers and is housed in the large 18 metre dome erected above the château building after its restoration. This instrument is an 83cm diameter lens (the ‘aperture’) and a focal length of about 16 metres long. The instrument is in fact two refractors, co-aligned and with a common mounting and external tubing. The smaller refractor was added in 1891 and has a 62cm aperture. The purpose of the twin instrument was to allow simultaneous observation for both visual (via the 83cm) and photography via the 62cm. At the time of its commissioning, the Meudon facilities were the largest refractors in Europe and only the Lick Observatory, California, had a larger aperture.
Janssen and Delandres’s interests were in astro-cartography, photography and spectrography and it’s no surprise that micrometers (to measure very small angular separation of e.g. double stars), polarimeters (to measure the degree of polarisation of light from celestial objects) and spectrographs (to observe stellar spectra) were also commissioned at Meudon. Indeed the spectroheliograph was invented and first commissioned at Meudon, and Deslandres was the first to produce spectral images and observations of the solar chromosphere. The instruments were used not only in many scientific advances and discoveries, but also as a teaching facility.
Developments in mirror technology and the inherent difficulty in manufacturing, and mounting, of large lenses, meant that in the early part of the twentieth century the focus for large aperture telescopes was on reflectors. The Meudon Grande Lunette remains to this day one the largest refracting telescopes in Europe (only exceeded in aperture by the 1m SST (Swedish Solar Telescope) solar instrument built in 2001 on La Palma).
In 1999 the Meudon dome was damaged by a storm, and although it had ceased to be used in 1990 by 2005 a decision was made to begin its restoration. When this work is complete the refractor will again be used for teaching and showing to the public. For now the dome is closed to the public.
Le Spectroheliographe, Grand commun et cœlostat
In an outbuilding of the Meudon château a spectral analysis laboratory (a ‘grand commun’) was built in order to be able to identify the elements and compounds in stellar spectra. Here in 1892, a spectroheliograph was built commissioned by Deslandres, and used a siderostat (or in French, ‘un cœlostat’). The cœlostat was first designed by Léon Foucault, and is basically a flat mirror on a mechanical mounting so that the moving mirror sends the light of a moving celestial object towards a fixed instrument, such as a spectrograph. The mirror tracks the path of the celestial object (usually the sun) and is aligned so that continuous observation of the object can be made without moving the telescope/instrument. This separation of tracking from instrument allows larger instruments to be used. The Observatory’s first cœlostat was installed in 1910 and the same basic technology is used in many of the world larger solar telescopes (including the SST). Meudon continues to be today one of the foremost of the world’s solar observatories, with spectral observations and images dating back to 1909. Many of the technological advances in spectrographs (for example the Lyot coronagraph and the more recent 4-quadrant phase mask coronagraph) have been made at Meudon.
Jules Janssen 1m Reflector
Keeping apace with telescope technology, in 1891 the 1 meter objective sized reflecting telescope, named the Jules Janssen telescope was commissioned at Meudon. The extra light gathering power of this larger instrument was used to observe and moreover photograph fainter objects, in particular the nebulae, star clusters and the diffuse comets.
The Solar Tower
The site has continued to be a centre for solar astronomical since its inception, and in 1969 the 35m high Solar tower, a ground-breaking development in solar observatory design, was commissioned in the chateau grounds. This houses a 60cm telescope (‘supplied’ by a roof-top cœlostat) and a large spectroheliograph. This is used not only to image the solar spectra but also to measure small scale velocity changes in the solar atmosphere and photosphere (by means of Doppler shifting of spectral lines). By measuring small velocity changes, not only solar atmospheric dynamics but also the very recent science of helioseismology has been advanced.
Meudon observatory remains a leading solar observatory today. Daily solar observations have been carried out there for more than 100 years and the centre has available a unique record of over 100,000 images taken over 10 solar cycles.
The photograph shows the present building viewed from the lawn. But you really don't need to be interested in astrophysics to appreciate this place. Meudon is a breath of fresh air and only 10 minutes outside the city centre. For the artistic members of the family, Meudon also host the ‘musée Rodin’ but perhaps that is for next time.
How to get to Meudon observatory
The Meudon Spectrroheliograph
The Jules Janssen 1M Telescope
The Solar observatory at Meudon today
A brief history of the Meudon observatory
The Rodin museum
Next month we will return to our series looking specifically at the physics of the Sun and we will focus on how hot the sun is…and how we know!