23rd September 1846 - Discovery of Neptune
172 years-ago today the planet Neptune was discovered. It was found by Johann Galle (09/06/1812 – 10/07/1910) and his then assistant Heinrich Louis d’Arrest (13/08/1832 – 14/06/1875) at the Berlin Observatory after one of the most intriguing and politically charged stories of mathematical analysis.
From the earliest of times, humankind had known of 6 planets…Mercury, Venus, Earth, Mars, Jupiter and Saturn. (Although strictly, the term planet is Latin for wandering star and the Earth was only recognised as a planet during the 16th century). Then in 1781, William Herschel, a German-born musician living in Bath, and a keen amateur astronomer, serendipitously observed the planet Uranus.
This had major ‘cosmology’ implications at the time. Not only did it extend the scale of the solar system, but it also appeared to reinforce a relationship, known as Bode’s law, for the distance of the planets from the Sun. This initiated the search for the asteroids, the first of which (Ceres) was found in 1801. However, the discovery of Uranus also led directly to the discovery of Neptune.
Neptune imaged by the Voyage 2 probe in August 1989
Courtesy of NASA
Il y a 172 ans aujourd’hui Johann Galle (09/06/1812-10/07/1910) a découvert la planète Neptune avec son adjoint Heinrich Louis d’Arrest (13/08/1832 – 14/061875) à l’Observatoire de Berlin à la suite d’une des histoires les plus intriguantes de l’analytique mathématique.
Les six planètes: Mercure, Vénus, Terre, Mars, Jupiter et Saturne sont connues deupuis très longtemps. Puis en 1781 William Herschel, un musicien (et astronome amateur) qui habitait à Bath a observé la planète Uranus.
Cette découverte d’Uranus a élargi les frontières connues du système solaire, a confirmé la loi de Bode pour le distance des planètes du Soleil, a conduit à la recherche des astéroïdes et la découverte de Neptune.
Uranus orbits the sun once every 84 years. By the first decade of the 19th century many positional observations of Uranus had been made and its orbit evaluated. The French astronomer Alexis Bouvard (27/06/1767 – 07/06/1843) working at the Bureau de Longitudes at the Paris Observatory, combined these with earlier, pre-discovery historic observations of the planet. The positional data did not compute to a stable orbit, even after taking into account the gravitational disruptions on the new planet by the known masses within the solar system.
This led Bouvard to the hypothesis in 1821 that the orbit of Uranus was being perturbed (gravitationally disturbed) by an unknown planet beyond the orbit of Uranus. The well-known German mathematician and astronomer, Friedrich Wilhelm Bessel promoted this hypothesis in 1830.
The story of how the English mathematician John Couch Adams and the French astronomer Urbain Le Verrier both independently calculated positional predictions for where this missing planet would be found is well known and documented (see for example further reading ).
The then Director of the Paris Observatory, François Arago, had commissioned Le Verrier to undertake predictive mathematical modelling for where the missing planet was to be found, whilst in England, Adams had secured to support of James Challis at Cambridge to do the same.
Both Le Verrier and Adams had difficulties in securing telescope search-time, with several months elapsing between their independent research conclusions and observational searching starting. The observatory in Cambridge initiated the search on 1st July 1846 but was hampered by a lack of detail in the star maps they had available for the predicted location (the constellation of Aquarius).
After, rightly, becoming impatient with the inertia at Paris, Le Verrier sought and received the support of Galle at Berlin; whom commenced telescopic searches on the 18th Sept. Five days later Galle and d’Arrest identified the planet and made confirmation observations over the following nights.
Whilst national prides led to some controversy as to whom should be credited with the discovery, Adams and Le Verrier acknowledged each other’s works, and became correspondents and friends.
 Le Verrier – Magnificent and detestable Astronomer
James Lequeux, 2013. https://www.springer.com/gb/book/9781461455646
Chapter 2 specifically deals with the discovery of Neptune and is available to view online at:
 The Search for Planet X
Tony Simon. 1962. Scholastic Book Services
This is an introductory level text and was one of the first astronomy books I read! It is out-of-print but copies are available on Abebooks, Ebay and Amazon.
18th September - Birthdate of Octave Callandreau
Octave Callandreau was a French astronomer born in Angoulême the main town in the Charente department of France. He graduated from the prestigious Ecole Polytechnique in 1874 to join the Paris Observatory where he became astronomer adjoint in 1881 (and astronomer titulaire in 1897). By 1880 he presented his doctoral thesis on the Application of the methods of Gylden to the perturbation of the minor planets. Naturally hardworking and modest he managed to impress Urbain le Verrier who was then director at the Paris Observatory and known for being difficult to work with.
Octave Callandreau (18/09/1852-13/02/1904) était un astronome français né à Angoulême le chef-lieu du département de la Charente. Il est diplômé de l’Ecole Polytechnique en 1874 avant de rejoindre l’Observatoire de Paris où il est devenu astronome adjoint en 1881 (et astronome titulaire en 1897). En 1880 il a soutenu sa thèse doctorale avec une étude des perturbations d’une petite planète par les méthodes de M Gylden. Naturellement travailleur et modeste il arrive à impressioner le directeur de l’observatoire Urbain le Verrier, (11/03/1811-23.09/1877) qui était connu pour sa caractère difficile.
His main interests were in celestial mechanics and he contributed to the development of this subject publishing many articles in the Bulletin astronomique of which he was one of the founding members and which he continued to edit until his death in 1904. He left Paris in 1882 to lead one of the ten French expedition (Haiti) to observe the transit of Venue which took place on 6 December 1882. Sadly, his life was cut short when he died suddenly at the age of 51 leaving a wife and seven children
Il a contribué au développement de la mécanique céleste publiant beaucoup d’articles dans le bulletin astronomique, une revue importante dirigeé par sa collègue Félix Tisserand (13/01/1845-20/10/1896) et dont il était membre du comité de rédaction jusqu’à sa mort en 1904.
Il est allé en Haïti en 1882 pour diriger l’une des dix expéditions françaises d’observation du transit de Vénus (6/12/1882). Sa mort inattendue à l’âge de 51 ans a bouleversé la communauté scientifique, sa femme et ses sept enfants