Monday, April 12, 2010

How Scientists Measure the Distance Between Stars

(This is just a bulletin-board post for links I don't want to lose.)

The unit of measurement is light years rather than miles.

A light year is a measure of time rather than distance, and equals 5,880,000,000,000 miles (or five trillion eight-hundred eighty billion miles).

How would someone measure the distance that appears to be between the stars from the perspective of a person standing on the Earth? Can we do this?

Parallax is a way of using perspective to measure the distance of things far away, such as stars.

A simple everyday example of parallax can be seen in the dashboard of motor vehicles that use a "needle" type speedometer gauge (when the needle is mounted in front of its dial scale in a way that leaves a noticeable spacing between them). When viewed from directly in front, the speed may show 60 (i.e. the needle appears against the '60' mark on the dial behind); but when viewed from the passenger seat (i.e. from an oblique angle) the needle can appear against a slightly lower or higher mark (depending on whether it is viewed from the left or from the right), because of the combined effect of the spacing and the angle of view.

(This doesn't exactly answer my question, but it is interesting.)

Scientists use parallax to measure the distance between a star and the Earth.

In order to calculate how far away a star is, astronomers use a method called parallax. Because of the Earth's revolution about the sun, near stars seem to shift their position against the farther stars. This is called parallax shift. By observing the distance of the shift and knowing the diameter of the Earth's orbit, astronomers are able to calculate the parallax angle across the sky.
The smaller the parallax shift, the farther away from earth the star is. This method is only accurate for stars within a few hundred light-years of Earth. When the stars are very far away, the parallax shift is too small to measure.

For stars over 100 light-years away, they use Cepheid variable stars, although there are difficulties and complications in using this method. The most famous Cepheid, and also the one nearest to Earth, is Polaris, the North Star.

All this is interesting, but how do you measure the distance *between* stars? How do you measure the distance between two distant objects from a third distant location?


A parsec is a unit of measurement used for very, very great distances, and "relates to the geometric method astronomers commonly use to establish distance. Parsec stands for parallax of one arcsecond." A parsec is equal to 1 astronomicalunit divided by the tangent of an arcsecond.

An astronomical unit is the distance from the Earth to the Sun, approximately 93 million miles.

An arcsecond is a subdivision of a degree in geometry/trigonometry. It is equivalent to the width of a dime as seen from 1 1/4 mile away.

Apparently, a person has to understand trigonometry in order to measure the distance between two distant objects from a third distant location. I am not that person. :D But isn't it fascinating to know that there are logical, rational ways of figuring things out?

This also shows that a person who understands the terms can join in the conversation. I do not understand the terms at this time, and so cannot really participate, but I can study the terms if I want to be involved later. (I tell my kids this all the time-- how important it is to know definitions of terms if they want to participate effectively in a topic of conversation.)


distance measurement said...

Measuring distances to celestial objects is one of the hardest problems in astronomy. Even scientists sometimes have trouble figuring out how far are the objects they study!

Katie said...

I can understand why!