Distance
by Doug Tanaka
While reading Chet Raymo's book, "An Intimate Look At the Night Sky", the thought occurred to me that almost all amateur astronomers are drawn to the hobby from a very basic need to know about the Universe - and more specifically, what our place is within it. Fundamental to that knowledge is knowing how far away things are.
The basic unit of astronomical measurement is the light year. We're so familiar with the term when we say something like, "The Andromeda Galaxy is 2 million light years away," or, "The Hercules Cluster is about 160 light years across," we have a feeling for relative distance or size, but can we truly understand what this distance or size means? For example, we know that a light year is the distance that light travels in one year, and that light travels 186,282 miles per second in a vacuum. But to understand the light year, we first need to understand how fast 186,282 miles per second really is. What do we compare that kind of speed to? What if we compare it to someone who we know is really fast, say, Ichiro Suzuki? Well, the comparison doesn't work at all, because, compared to the speed of light, Ichiro Suzuki isn't any faster than Lou Piniella (although, for that statement to be true, we begin to realize that the speed of light really MUST be fast!). So, while the light year is a very convenient term – and the only real measure that works for "Universal" distances - I think a better feel for distances comes from scale models.
The planet walk around our observatory at Battle Point Park gives a good example and a feel for the distances of the planets. In his book,
Chet Raymo also has a good example of distance and scale. Using common objects and a football field, he starts by having us imagine a small grapefruit, representing the Sun, on the goal line. On the four yard line would be a fleck of sugar representing Mercury, on the seven yard line would be a grain of salt representing Venus, and on the ten yard line, another grain of salt, representing the Earth. On the fifteen yard line would be another fleck of sugar for Mars, on the fifty yard line a pea for Jupiter, and at the end of the field, a smaller pea for Saturn. Uranus would be a peppercorn at the end of a second football field and Neptune, another peppercorn, would be at the end of a third football field. Pluto would be a mote at the end of a fourth football field. The binary stars, Alpha Centauri, the closest stars to the Sun, would be a pair of grapefruits one thousand miles away.
Just how vast the distances are at this scale took on a new meaning when I stopped to think that, as fast as light is, it still takes over eight minutes to go from the Sun, at the goal line, to the Earth, at the ten yard line. Ten yards in over eight minutes! I've seen banana slugs move that fast. It would take about 5.5 hours to reach Pluto, four football fields away, and 4.3 years to travel the one thousand miles to Alpha Centauri.
However, while this little twist in thinking about scale made me think about the light year in a different way, I can't really say I have any better feel for how big "160 light years across" really is. The only thing I'm sure of is that Lou would take twice as long as Ichiro to get to Alpha Centauri.