TWICE IN A LIFETIME
FREQUENCY OF VENUS TRANSIT
Abhimanyu Chawla
Department of Physics, Panjab University, Chandigarh
Contents
Frequency of Venus Transits
Synodic Period
Demonstration Activity
Frequency of Venus Transits
Come in pairs that are eight years apart
Followed alternately by spans of 121 ½ years and 105 ½
years.
Hence twice in a lifetime!
But why is it so rare?
Synodic Period
Time required for a planet to move from a certain
position, with respect to the Sun,
back to that same position as seen from Earth.
Synodic Period
For Earth:
365 days  360 degrees
Hence, in one day  360/365 = 0.986 degrees
For Venus:
225 days  360 degrees
Hence in one day  360/225 = 1.6 degrees
Synodic Period
Each day Venus gains (1.6-0.986) degrees on Earth
(1.6-0.99) degrees in  1 day,
So 360 degrees in  360/(1.6-0.986) = 586 days
= 1.6 years
This is the Synodic Period of Venus:
After every 1.6 years, Venus comes in the same
position w.r.t the Sun, as seen from the Earth.
Activity
Two factors
Inclined Orbit
The orbit of Venus is tilted about 3.394° with respect to
the orbit of Earth
Orbital Speed
Time taken by Venus to complete a lap is
not exactly 8/13th the time by Earth, rather
7.997/13
 Inferior Conjunctions shift clockwise.