Maximilianus Hell (1720-1792) - Munin

valuable data as well. 82 In fact the expeditions of Niebuhr and Bugge/Aascow are not even
mentioned.
As regards the publicity of the Danish observations, the Astronomer Royal curiously
postponed publication of the exact moments of contact at egress – the only information that
really mattered in the ongoing efforts to calculate the parallax. Only a few weeks after the
transit had taken place, however, Chr. Horrebow published a Latin dissertation „On the Path
Drawn by Venus upon the Disc of the Sun‟, where he tries to define the exact path of Venus
by means of three different methods. 83 The procedure involved three trained observers – each
with their assistant, in addition to a fourth assistant paying attention to the clocks – all of
whom were kept busy over several hours in the early morning of 6 June 1761. In the
introduction, Chr. Horrebow states that: 84
I will say nothing yet on the observations made with larger telescopes for the
determination of the egress of Venus, or with a quadrant and a meridian circle
for the verification of the time-keeping and the meridian both before and after
the conjunction took place; I reserve all this to its proper time and place.
As has already been explained, the adjustments to the time-keeping were not published until
1765, in an article in the vernacular. Besides, only two among the crew of observers had
attempted to observe the moments of contact at egress (see Section II.2.5). None of the other
important aspects of the transit – the “black drop” effect, the possibility of an atmosphere
surrounding Venus, the determination of the visibility of the forthcoming transit of 1769, etc.
– are known to have been discussed in any publication by the brothers Horrebow or their
assistants in the aftermath of June 1761.
However, one aspect of the theory of Venus that did receive attention in Copenhagen was the
question of the “moon of Venus”. During the spring of 1764, one of Christian Horrebow‟s
assistants, Peder Rødkier (or Roedkiær, ?-1767), believed he observed the notorious “moon of
Venus” on two occasions in the month of March. The instrument maker Johannes Ahl (or
Johan Ahl, 1729-1795) also saw the same object. A few days later, the Astronomer Royal
82 P. Horrebow 1761, pp. 24-29.
83 Chr. Horrebow, Dissertatio de semita, qvam in Sole descripsit Venus […] Die 6 Junii Ao. 1761 (in two parts,
presented as a dissertation at the University of Copenhagen on 28 and 29 July 1761), 1761a and 1761b.
84 Chr. Horrebow 1761a, p. 3: “Nihil heic jam loqvor de observationibus per tubos majores pro determinando
Veneris egressu, & per qvadranten & Rotam meridianam pro verificatione temporis & meridiani ante & post
conjunctionem factis, has omnes suo loco & tempori reservo”.
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