How Do We Know How Far The Stars Are?

Photos on this page courtesy NASA. 

My grandparents were into astronomy. Back in the 1950's there were part of the Sky & Telescope Society. The members not only talked about the mysteries of the universe, they also made telescopes to study the night sky. These were very sophisticated telescopes and various members contributed in various ways. One man, for example, ground the lenses.

When the Internet came along, a girl from my sixth grade class contacted me to say she still remembers the night the class came to our house to look at the planets through a telescope. My most vivid memories revolve around the counting of moons on Jupiter and looking at the moons and rings of Saturn. The constellations, nebulae and manifold wonders of the night sky make an impact that stays with you.

Many questions arise when you look at the stars. How did such a vast universe come into existence? How far does it go? Is there life on other planets? Will we ever know?

One question I don't recall ever asking was how did scientists figure out far those stars are from earth? I mean, we constantly hear people say things like, "The nearest galaxy, Andromeda, is 2.5 million light years away," as if this is a certainty like the weight of a pound of butter. How did they determine this?

Planet earth is part of a solar system that resides in the Milky Way. It's like being a cell in our body. We're a small part of something really big. Andromeda, which scientists call M31, is the Milky Way's nearest neighbor. Andromeda has only a few hundred billion stars. Since the distance light travels in one light year is 6 trillion miles, it becomes really difficult to grasp how far 2.5 million light years is. And that's just the distance to our nearest neighbor-galaxy.

What's amazing to me again is how scientists make these statements with such certitude.When we see the stars of the Big Dipper or Orion or Cassiopeia, how do we know which ones are ten light years away and which ones are five, or fifty, light years away?

Essentially, it's simple mathematics. Scientists used the basic principles of trigonometry that many of us learned in high school. The scientists would view the star from two different places on earth's orbit, say during midwinter and midsummer. This would form a triangle, or what they trigonometric parallax.

It's the same principle used for observing distances and depth with our eyes. Our eyes are set apart so that our brain automatically registers depth without our even thinking about it. Imagine your nose as the sun and each eyeball is on the two sides of the sun during the two parts of the year. Obviously more calculations have gone into determining how far Saturn and Andromeda are than the distance to the tree out front, or the mailbox. But it's pretty interesting when you think about it.

We live in an amazing universe. Take a few minutes to see some of the incredible images captured by the Hubble Telescope, along with interpretations of what we're looking at.

Meantime, life goes on all around you. Celebrate it.

EdNote: The seed for today's blog post was sown by a passage from Mario Livio's The Equation That Couldn't Be Solved.