[Aztlan] repeat transits of Venus

[Lynda Manning-Schwartz]

A transit occurs when a planet reaches inferior conjunction (that is, when
it lies directly between the Earth and the Sun) within a day or so of the
planet crossing the ecliptic. (Note that only planets between the Earth and
the Sun--Mercury and Venus--can transit.) Venus crosses the ecliptic at 77
degrees longitude (currently around June 8) or at 247 degrees longitude
(currently around December 9). Therefore, all transits of Venus currently
fall within several days of either June 8 or December 9. 

Note that precession changes these dates slightly over time. Precession
moves any point along the ecliptic backwards with regards to the fixed star
field, making the date of the event later in the year, at a rate of one day
(one degree) per 71.67 years. See The Astronomical Companion by Guy Ottewell
at http://www.universalworkshop.com/pages/astrolist.html and Wikipedia's
entry at http://en.wikipedia.org/wiki/Precession . (Isn't it handy to have a
72-year Maya calendric count for precession [72 x 365 days = 73 x 360 days]?
)

Transits of Venus have a recurring pattern of two pairs of transits 8 years
apart, followed by more than a century. Thus, transits occur at 8 years,
105.5 years later, 8 years later, and 121.5 years later in sequence. Then
the sequence starts over again. Therefore, the Venus transit cycle is about
243 years. During this time, precession has moved the date of the event to
3.4 days later. For example, in 2004, the date is June 6 at 5:13 UT; 972
years earlier, in 1032, the date is thirteen days earlier on May 24 at 15:19
UT [2004 - 1032 = 972 / 72 = 13.5 days].

The last transit of Venus was on June 8, 2004. The next three transits will
be June 6, 2012, December 11, 2117, and December 8, 2125. A transit in 910
AD would be most similar to a transit in 2125 (5 cycles x 243 years = 1215
years). 

Transits of Venus last several hours and are therefore visible, at least in
part, throughout much of the day, and throughout much of the world. Whether
or not the transit is visible at a certain location on Earth depends on the
time of day and other factors, such as the season (June or December) and the
viewing latitude and longitude. See http://www.nao.rl.ac.uk/nao/transit/ for
Venus transits from 1000 AD to 2700 AD with visibility maps.

For more information, see www.transitofvenus.org and Astronomy Magazine's
June 2004 issue (Vol 32, Issue 6) at www.astronomy.com, particularly
articles by Michael Bakich, "Viewing Venus in transit," and Ray
Jayawardhana, "Chasing the shadow of Venus."

Lynda Manning-Schwartz

-----Original Message-----
From: aztlan-bounces at lists.famsi.org [mailto:aztlan-bounces at lists.famsi.org]
On Behalf Of martha noyes
Sent: Tuesday, January 30, 2007 8:48 PM
To: aztlan at lists.famsi.org
Subject: [Aztlan] repeat transits of Venus


Aloha - anyone know at what intervals transits of Venus can/could have been
seen from a single location?  For instance, if a transit was visible in
Hawaii in 910 AD, when would it be visible again or when would it have been
visible years earlier?  

Mahalo for any help on this.

Oh, I lost an email from someone on the list who asked for references to
materials available in both Hawaiian and English.  If you'll email me again,
I can send you some references.
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