Look at ephemeris for Sun, Mercury and Venus:



You can see that trajectories of Mercury and Venus  are close to Sun's trajectory.

This astronomical phenomena takes place for interior (Mercury and Venus) planets. Explanation for elongation is here (from website http://en.wikipedia.org/wiki/Elongation_(astronomy) )


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(For other uses of elongation, see Elongation)

This diagram shows the elongations (or angle) of the Earth's position from the Sun.

Elongation is an astronomical term that refers to the angle between the Sun and a planet, as viewed from Earth.

When an inferior planet is visible after sunset, it is near its greatest eastern elongation. When an inferior planet is visible before sunrise, it is near its greatest western elongation. The value of the greatest elongation (west or east), for Mercury, is between 18 and 28; and for Venus between 45 and 47. This value varies because the orbits of the planets are elliptical, rather than perfect circles. Another minor contributor to this inconsistency is orbital inclination: each planet's orbit is in a slightly different plane.

Refer to astronomical tables and websites such as heavens-above to see when the planets reach their next maximum elongations.

In 2008, Venus does not have a greatest elongation - either eastern or western. The planet instead moves from a greatest western elongation on October 26, 2007 to a greatest eastern elongation on January 14, 2009.

In 2008, Mercury has greatest eastern elongations on January 21, May 14, and September 11 (and after that on January 4, 2009). Western ones happen on March 3, July 1, and October 22.

For practical usage you may be interested in the moments when Mercury and Venus reach maximum Eastern or Western elongation.

In order to calculate these moments we recommend to use the standard events library:


and in "Planetary Orbit" section choose one of the items (Eastern and Western elongation):


This is the price chart for Dow Jones Industrial Index together with moments when Mercury reaches maximum Eastern elongation:


To calculate the maximum elongation for Venus you need modify this event in ULE Editor. Just choose Venus there: