Visit your local video store and you'll see a lot of science-fiction movies out on DVD right now. Put the stress on fiction, though. The science in those films can be pretty suspect.

Although you don't need great science to make a great movie, bad movies generally have a lot of bad science. Sure, they're just movies, but it can still be enlightening to gauge their accuracy.

Planet of the Apes
Tim Burton's remake of the classic "Planet" had a lot of promise, but ultimately was based on silly premises:

• By the year 2029 we'll have huge space stations orbiting other planets.
  
• We'll use genetically engineered chimpanzees to pilot tiny "worker bee" spaceships in situations too dangerous for a man.
  
• Random electromagnetic storms can send unwary space travelers travelling through time.

First, we can hardly get a space station built to orbit the Earth -- let alone some ringed planet far, far away -- in just 30 years. Second, why use chimps when remotely controlled computers are even cheaper? Third, if storms that let you travel in time were common enough to be seen floating around in our neck of the galaxy, the universe would be an even more mixed up place than it is.

The Mummy Returns
Yes, we're talking about a movie where dead Egyptians come to life, and where Brendan Fraser is cast as the action hero. But even so, there's a lot of scientific silliness in "Return of the Mummy."

In a scene near the end of the movie, our hero must outrun the rising sun to reach the base of a pyramid. If the sunlight gets there first, his son will die.

The problem: The light from the sun is going in the wrong direction. The way the scene is set up, the light from the rising sun would hit the top of the pyramid first and then move down. The movie had that exactly backward.

I know movie directors think they're important, but making the Earth spin the wrong way just to make a sequel is going too far.

Space Cowboys
This movie about a group of retired astronauts training to save a Russian satellite was silly and fun. However, it had its share of scientific errors.

For example, an engineer says that to throw a baseball at the moon, all you need to do is "just knock it halfway there, about 100,000 miles, and the moon's gravity will take it from there." That's way off, for two reasons:

• The moon's mass is 1/80th of the Earth's, so you'd need to get it much closer than halfway there.
  
• It takes about as much energy to get into low Earth orbit as it does to get from low Earth orbit to the moon. If you can get the baseball even a few hundred miles off the Earth, it's relatively easy to get it the rest of the way to the moon.

Watch the movie and then find more science faux pas here.

Evolution
Ivan Reitman directs this David Duchovny film about a meteorite that brings alien life to Earth. The life evolves very quickly and soon threatens humanity by attacking a shopping mall.

Right.

One big science error is when Duchovny realizes that the element selenium will poison the aliens, because arsenic kills us. You see, we're carbon based, and on the periodic table of elements if you go one element to the right and down two more elements, you get arsenic.

Still with me? The alien's chemistry is based on nitrogen, so if you go over one and down two -- selenium! This won't work in reality.

Arsenic is poisonous to us not because we're carbon based, but because it attaches itself in various ways to chemicals we need. For all we know, arsenic might kill nitrogen-based aliens too. So might oxygen. I would certainly think a fair-sized thermal nuclear bomb would work just as well. But that's just me.