Transit of Venus documents
Comments concerning my 'A Transit of Venus Primer' Southern Stars
51 (1) 3-18 & covers (2012)
Download (PDF, 8.5 Mb)
To fit within the available space, various footnotes were edited out:
- For the same reason that a transit of Venus does not occur at every
conjunction, a solar eclipse does not occur at every New Moon.
- It is difficult to know what adjective to use for Venus.
Venereal, besides causing sniggers, is incorrect because it derives
most directly from the Latin venereus. meaning sexual love. The
New Zealand Oxford Dictionary accepts Venusian as an
adjective pertaining to the planet, but the term invites confusion with an
inhabitant of Venus. Cytherean is elegant, but little-known.
- Concerning the shift of Venus's path in a transit pair: Conceptualizing the
direction of the shift can be difficult because both Venus and Earth are
moving. What I find useful is to consider Venus at a node and then again
exactly 13 orbits later when the Earth will not quite have returned ti the same
position as 8 years before. In June (see Figure 4) the path of Venus willt
herefore be more northerly than previosuly, and more southerly in December.
- Some authors write 'Horrox', but they have not understoof that this is
the Latinized form of his name.
- Nineteenth-century authors promoted Horrocks to the clergy, but there is no
evidence he was ordained. In addition, he was too young to be an Anglican
deacon, for which the minimum age, then as now, was 23 (Chapman 2005).
- Pre-Gassendi and Horrocks, planetary angular diamaters were calculated
from presumptions about planetary size and an incorrect proximity resulting
from the severely-underestimated distance to the Sun. Values reduced to the
solar distance were typically 2 arcmin or more for Mercury, and 3 arcmin or
more for Venus (e.g. Houzeau 1882). I am mystified that astronomers could have
believed these whopping values, as was Horrocks: 'Still it surprises me that
they should all have been so negligent as not to perceive a deception so
remarkable as to be detected even by the naked eye' (in Whatton 1859).
- Concerning Horrocks' assumption that the diameter of a planet might be
proportional to its distance from the Sun, Kepler had proposed that the
volume of a planet might be proportional to its distance from the
Sun.
- Concerning absolute directional accuracies, the value for 1700
(±10 arcsec) refers to a time ninety years after the invention of
the telescope and six
decades after the invention of the eypiece micrometer.
- Some modern popularizers have claimed that the solar distance was important
because it forms the basis of extra-solar distance scales. This is
anachronistic nonsense. Distances only began to be determined to stars a few
decades before the 19th-century transits, and to galaxies in the 20th century.
Further, no sane person would lavish time and gold to determine the unit of
distance to high accuracy while the distances themselves can only be measured to
low accuracy.
- It is ifficult to know exatly how many observing sites were readied for the
18th- and 19th-century transits because different authors count on different
bases: sites where observations were attempted, sites where observations were
made, sites where usable observations were made...
- A heliometer is a special angle-measuring telescope with
an objective sawn in two. Heliometers were easier to use on alt-azimuth and
undriven or poorly-driven telescopes than eyepiece micrometers.
- On Earth, the greenhouse effect raises temperatures by about 35°C.
- To see Venus during the day, you must also know exactly where to look.
- In Table 4, flattening is defined as (Polar radius−Equatorial
radius)/Equatorial radius.
Note that because of the 243-year transit cycle, the 2012 transit will be very
similar to the one observed by Cook from Tahiti in 1769. That is, Venus's track across
the disc of the Sun will be very similar to that seen by Cook. However the
visibility will be different because the Earth is at a different rotation angle.
In 1769 Cook was able to see the whole of the transit, while in 2012 only the
end of the transit will be visible from Tahiti.
Extracts of Cook's observations appear to have been multiply-copied by a
secretary, with each copy signed by Cook. The extracts include sketches
showing the arc produced by refration in Venus's atmosphere and the black drop.
- Example at the Library of New South Wales
[link]
- Better copy at Te Puna Mātauranga O Aeotearoa
[link]
and high-resolution scan in JP2 format
[link] (This is version reproduced by Salmond (2007))
![[link]](http://www.transitofvenus.nl/images/burnham.jpg){kind=link}
