The inferior conjunction of Venus occurs every 583.93 days as Venus passes between the Earth and the Sun. Most recent was 18 August 2007. The period for every fifth event is eight years minus 2.4 days, so it will happen on 16 August 2015. This regular period, due to the 13/8 relation between the orbits of Venus and Earth, also applies to the superior conjunction (Venus behind Sun), the dates of change to forward and retrograde motion, and the dates when Venus is at maximum apparent angle of 45-47 degrees each side of the sun. Venus was recently at maximum angle on 9 June 2007 and went retrograde on 30 July. In eight years, it will reach these positions on 7 June 2015 and 27 July 2015 respectively, 2.4 days before the corresponding days this year.
For the ancients, the date when Venus as evening or morning star changed direction could have been a main temporal marker, observed 2.4 days earlier against the zodiac stars every eight years. I would be interested in any information on this - did the ancients monitor the apparent 2.4 day precession of the position of Venus each eight years? This movement is certainly much easier to observe than the precession of the zodiac recorded by Hipparchus in the second century BCE.
The ten points of the apparent change of direction of Venus (five each for the evening and morning stars) form two pentagons, each evenly spaced around the zodiac. These pentagons precess with an orbital period of about 1217.5 years, as do the inferior and superior conjunctions. The Pythagorean School of ancient Greece used the pentagon as its symbol, (
http://en.wikipedia.org/wiki/ Pythagoreans. They had a strong interest in the mathematics of celestial observation, and had cultural affinity to the themes of beauty and love symbolized by Venus/Aphrodite. For these reasons, I imagine it is likely the Pythagoreans monitored this nearly exact eight year cycle. Would their secrecy mean we will never know?
Fifty years of Venus Dates
Inferior Conjunction; 10/11/1970; 18/6/1972; 24/1/1974; 28/8/1975; 6/4/1977; 8/11/1978; 15/6/1980; 21/1/1982; 25/8/1983; 4/4/1985;
5/11/1986; 13/6/1988; 19/1/1990; 23/8/1991; 2/4/1993; 3/11/1994;
11/6/1996; 17/1/1998; 20/8/1999; 30/3/2001; 31/10/2002; 8/6/2004;
14/1/2006; 18/8/2007; 28/3/2009; 29/10/2010; 6/6/2012; 12/1/2014;
16/8/2015; 25/3/2017; 27/10/2018;
Eight Year Cycle of Inferior Conjunction illustrating precession: 10/11/1970; 8/11/1978; 5/11/1986; 3/11/1994; 31/10/2002; 29/10/2010; 27/10/2018
Superior Conjunction: 30/8/1971; 6/4/1973; 10/11/1974; 16/6/1976;
21/1/1978; 27/8/1979; 3/4/1981; 7/11/1982; 14/6/1984; 19/1/1986;
25/8/1987; 1/4/1989; 5/11/1990; 12/6/1992; 17/1/1994; 23/8/1995;
30/3/1997; 3/11/1998; 9/6/2000; 14/1/2002; 20/8/2003; 27/3/2005;
31/10/2006; 7/6/2008; 12/1/2010; 18/8/2011; 25/3/2013; 29/10/2014;
4/6/2016; 9/1/2018; 13/8/2019;
Evening star maximum angle: 7/9/1970; 9/4/1972; 15/11/1973;
19/6/1975; 26/1/1977; 30/8/1978; 6/4/1980; 12/11/1981; 16/6/1983;
24/1/1985; 27/8/1986; 4/4/1988; 10/11/1989; 14/6/1991; 22/1/1993;
25/8/1994; 2/4/1996; 8/11/1997; 11/6/1999; 19/1/2001; 22/8/2002;
30/3/2004; 5/11/2005; 9/6/2007; 17/1/2009; 20/8/2010; 28/3/2012;
3/11/2013; 7/6/2015; 14/1/2017; 18/8/2018;
Morning star maximum angle: 20/1/1971; 28/8/1972; 5/4/1974;
7/11/1975; 16/6/1977; 18/1/1979; 25/8/1980; 2/4/1982; 4/11/1983;
14/6/1985; 15/1/1987; 23/8/1988; 31/3/1990; 2/11/1991; 12/6/1993;
13/1/1995; 21/8/1996; 29/3/1998; 30/10/1999; 9/6/2001; 10/1/2003;
18/8/2004; 26/3/2006; 28/10/2007; 7/6/2009; 8/1/2011; 16/8/2012;
24/3/2014; 26/10/2015; 4/6/2017; 6/1/2019
Average Days between aspects: 583.93
Year x 8/5: 584.4 days
Days lost per eight year cycle: 2.4
Years lost per 8 yr cycle: 0.0065708
Cycles per precessional rotation: 152.19
Years per Precessional rotation: 1,217.5
Years per pentagon point precession: 243.5