It was Johannes Kepler who documented his three laws of planetary motion back in the first few decades of the 17th Century. The laws are all pretty self explanatory:
1. Planets move in elliptical orbits with the sun at one of the foci.
2. A line joining the sun to a planet -- the radius vector -- sweeps out equal areas of space at equal intervals of time (basically means planets travel faster nearer the sun and slower further away)
3. The square of the orbital period of a planet is proportional to the cube of its mean distance from the sun.
ANALYSIS: Don't Freeze Your Telescope Off!
Using these laws, which fundamentally come from the laws of gravitation, allows us to predict with incredible accuracy how the planets move so we can tell that on May 28th, 2013, Venus and Jupiter will pass within one degree of one other (about twice the apparent size of the full moon). To add a little bit of sparkle, a little further above the horizon Mercury will put in an appearance.
Although they may look close from our point of view, in reality Jupiter will be 660 million kilometers further away than Venus, and 744 million kilometers further away than Mercury, which is the closest of all of them come to us. Mercury is always an elusive planet but the 28th will be a great evening to try and spot it low in the west after sunset.