MAPLESS IN
CAMBRIDGE?
Soon the ideal excuse appeared, as to why Challis had failed to
find Neptune. He hadn’t got the map. The new Berlin Sternkalendar
(Hour 21, by Bremiker) covering the zone of the heavens in which
it was discovered, had not yet been distributed: only the Berlin
Observatory owned a copy. This explanation was mentioned by Challis
in a Oct. 17th letter to the Athenaeum, and developed in his presentation
to the RAS on November 13th, 1846:
‘If I had had this map [Hour 21], a first sweep would have
been unnecessary: I should have compared my field of view with the
map at once.’
The Dec 5th editorial of The Athenaeum made this issue central:
‘if the Cambridge library had possessed the twenty-first hour
of the Berlin star-maps, Adams and Leverrier would have changed
places.’
(This Editorial was by Augustus de Morgan, as his wife Sophie later
disclosed in her biography). It became a central part of the story,
e.g. Smart wrote in 1946:
‘There can be no doubt that if the star-chart had been distributed
immediately after it had been engraved, Challis’s labours
would have been immeasurably reduced and discovery at Cambridge
would have been almost certain.’ (p.80)
Meanwhile, back in the real world, Challis owned the Hour 22 Berlin
star-map, and Neptune was on that map for the first four weeks of
his sky-search. ‘Hours’ here allude to Greenwich Hour
Angles, of which twenty-four comprise the circle of Right Ascension.
Each map therefore spanned fifteen degrees, with the hours subdivided
into sixty ‘minutes’ and each map having an overlap
zone of four minutes (‘Four minutes’ here equal one
degree). Neptune, moving retrograde, exited from the map ‘Hour
22’ onto the ‘Hour 21’ towards the end of August
1846. Thus Challis had a good four weeks of his sky-search while
Neptune was on the map in his possession. Only after this, in September,
was map ‘Hour 21’ required.
His October 17th letter to the Athenaeum specified the two positions
of what he later recognized as Neptune:
Aug 4th : RA 21h 58min, Aug 12th : RA 21h 57 m
These parameters were on the ‘Hour 22’ map which he
presumably had in front of him, because the ‘Hour 22’
map extended out to 21h 56m (For comparison, when Challis started
on July 29th Neptune was at 21h 59m GHA, then by Sept 23, still
moving retrograde, it was at 21h 53m GHA).
For Galle and D’Arrest In Berlin the new star-map was indeed
crucial. As Encke, the Berlin Observatory’s Director, wrote
to M.Arago in Paris:
‘Sans cette circonstance infiniment favourable
... sans une carte où l’on pût être sûr
de trouver les étoiles fixes jusqu’à la dixième
grandeur, je ne crois pas qu’on eût trouvé la
planète.’ (Comptes Rendues 12th October, p.662)
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