|MadSci Network: Astronomy|
Both Venus and Mars circle the Sun. Since Venus is much closer to the Sun than Mars, it travels in its orbit around the Sun much faster than Mars. Venus takes almost 225 days to complete one turn around the Sun while Mars takes almost 687 days. These means that the distance from Venus to Mars changes a lot as they move around the Sun. The distance from the Sun to a planet does not change so much. The distance from the Sun to Venus is 67,205,000 miles, and the distance from the Sun to Mars is 141,578,000 miles. Suppose that at some time Venus and Mars are on opposite points from the Sun. That would be the farthest distance from each other, which is the sum of their distances from the Sun: 208,783,000 miles. As they move in their orbits, they approach each other until Venus is exactly between Mars and the Sun. That would be the moment of shortest distance between each other, and their distance will be the difference between 141,578,000 miles and 67,205,000 miles. You can find a view of the current positions of the planets in http://www.fourmilab.to/solar/ To calculate the time that light travels a distance, we need to know the light speed, which is 670,337,890 miles per hour. That means that the light travels 670,337,890 miles in one hour. To know how many hours does the light takes to travel a distance, we have to divide that distance by 670,337,890. Think about it: we want to know how many times 670,337,890 fits in that distance. That will be the number of hours that the light takes to cover that distance. When Venus and Mars are on opposite sides from the Sun, the light beam takes 208,783,000/670,337,890=0.311 hours. That's less than one hour, or about 18 minutes and 40 seconds. I'll let you do the calculation for the case when Venus and Mars are on the same side of the Sun (it has to be a shorter time, doesn't it?).
Try the links in the MadSci Library for more information on Astronomy.