From our viewpoint on Earth, a planet will gradually shift from rising before the Sun to setting after the Sun. From observations, in some instances, the Sun may appear to catch up and pass an object, and in other instances an object may appear to move from one side of the Sun to the other. These are all a result of the relative orbital speeds of an object and the Earth - both play a part in how the object's motion is perceived.
   You can get a sense of this motion by observing the planet Mercury for the first few weeks of this month when Mercury moves into the evening sky. As it moves along its orbit from superior conjunction it is gradually moving further east out away from the Sun, following the setting Sun toward the western horizon. During the first week of the month Mercury wil pass by the open star cluster the Pleiades and then past the open star cluster the Hyades. This animated image shows Mercury moving past the Pleiades during the early evening hours between May 1st and 7th.
   The separation between Mercury and the Sun comes to an end when Mercury, from our perspective, stops moving eastward on June 9th and starts moving west back toward the Sun during June.
   This 'turning point' in Mercury's orbit is referred to as an elongation, and in particular this is the eastern elongation, as Mercury is to the left, east side of the Sun. At elongation the inner planets (Mercury or Venus) are as far out to the side from the Sun as they can be from our perspective on Earth. Following elongation Mercury will quickly catch up with the Sun and reappear on the right or west side of the Sun as a morning planet during July.