To hear Sam Davidson tell it, the Romans had an interesting test for drafting military men. "In the Big Dipper, there's a star that is really a double star," the retired engineer and astronomy aficionado explains. "If you could see the double star, you became an archer. If not, you were an infantryman." You can test your own visual acuity at a star party (think Carl Sagan, not Joan Rivers) Saturday night at Joshua Tree National Park, where the Andromeda Society sponsors a look at the "natural planetarium" in the sky. If it's a clear night, there should be just enough light, Davidson says, to view craters on the moon without washing out views of the stars. He's also hoping for clear views of the Andromeda Galaxy, a mere 3 million light-years away, as well as the Milky Way in our galaxy. Those who attend the gaze-in are advised to dress warmly and bring binoculars; telescopes will be set up for viewing. Families are welcome to the monthly event, but the society's Web site warns that "unruly, unmanaged children are captured and sold to the first passing starship from distant Andromeda where we believe they are cooked and eaten!" Meet at the Hidden Valley Nature Trail parking lot. Viewing begins shortly after dark and may last as long as three hours. No charge for the gathering, but participants must pay the $10 vehicle entry fee to the park. For information, go to www.andromedasociety.com.