The concept of distance is something I've struggled with since I was a young boy. I am reminded of this limitation when I take long car trips with my 7-year-old son ("Are we there yet?") or go for a hike and misjudge the time it will take to complete my journey because the points on my topographical map seem so close together.

On a clear, dark night, the dense carpet of stars sprayed across the sky may give an impression that they are close enough to pluck right out of the firmament. A casual glance may suggest the stars are on a two-dimensional plane even though we know better. Modern-day astronomers use a variety of techniques to provide insight to the immense scale of the universe.

Here on Earth, measuring things is fairly simple and direct. But in space, we can't just zip out to a nearby star and use a tape measure to find its distance from us. We are dependent upon observational methods. One technique is trigonometric parallax.

Surveyors use parallax to measure distance to a target object. First, they create a baseline by sighting a target from two points. Then, using the angle subtended by the apparent shift of a distant object against its background and the known length of the baseline, they calculate the distance to their target. Adapted to the science of astronomy, trigonometric parallax has been used to determine the scale of our solar system and the distance to "nearby" stars.

To get a better sense of how parallax works, try holding a fingertip out at arm's length. Look at it and blink your view back and forth between one eye and the other. Next, try the same thing with the fingertip closer to your face. Do you notice that there is a bigger shift against the visible background when your fingertip is close to your face compared to when you hold it at arm's length?

The standard unit for distance in the solar system is the Astronomical Unit, or AU. The AU is based upon the average distance between the Earth and Sun. One AU equals 93 million miles. Early attempts to measure the AU occurred during Transits of Venus. By timing the ingress and egress of Venus' shadow across the disc of the sun from several locations on Earth, astronomers could find the angle subtended by Earth from the limb of the sun. The distances between observing locations provided the baseline.

For the original measures of stellar parallax, astronomers used opposite sides of Earth's orbit as their baseline. If the target star was relatively nearby, observations made six months apart would show a larger shift against the background star field when compared to more distant stars.

All of this changed in 1989, when the European Space Agency launched the Hipparcos satellite. Free from the distorting effects of Earth's atmosphere, this space-bound surveyor measured distances to over 118,000 stars with unprecedented accuracy out to about 500 light years.

Next week, part two about distance measurements in the universe.

Peter Lipscomb is the director of the Night Sky Program for the New Mexico Heritage Preservation Alliance. Contact him at plipscomb@nmheritage.org.