In response to a column I wrote awhile back regarding the question of whether space travel is really feasible, a reader e-mailed me asking just how do we measure stellar distances.

The answer to that question can invariably create a great deal of confusion since it is very difficult for us to comprehend the distances in space.

One method used by astronomers to measure the distance of nearby stars is to measure what’s known as parallax. By definition, parallax is the apparent shift in position of an object when viewed from two difference locations.

Perhaps the simplest demonstration of parallax requires nothing more than a clear night and the fingers on your hands.

Once outside, hold up one of your fingers in front of your face while looking at the stars. Close your left eye and position the tip of the finger you are looking at on a star.

Without moving your finger, close the right eye and open the left. In doing this, you will note the position of the finger against the background of the stars will appear to have changed. This positional change is known as parallactic displacement.

If you would like, this experiment can be further refined if you hold the finger close to your face and blink your eyes (one at a time), and then holding the finger at arm’s length and repeating the blinking. The farther the finger is from you face, the less it appears to jump against the background of stars.

As the Earth orbits the Sun, astronomers often use the diameter of this orbit, 185 million miles, as a line of position.

At one time of the year, Jan. 1, for example we are on one side of the Sun, and six months later, we’ll be on the other side.

If we observe a star from these two positions, we have the ingredients necessary to determine the measure of a triangle whose baseline is the diameter of the Earth’s orbit and the angles measured at the Earth’s position for the two dates.

Although the mathematics are easy, the angles which are measured are incredibly minute and thus must be measured with the utmost precision.

The angles are determined by the shift of the target star against the very faint background stars which, on the average, will be much more distant that the target star.

It’s a simple as that.

On another subject, the brilliant light in the southwestern sky shortly after sunset is marked by Venus, which I mentioned in last week’s column.

As planets move around the Sun, they occasionally will approach one another in what astronomers refer to as a planetary conjunction. Although it’s still a couple of week’s away, observers will see a very close conjunction when Venus and Neptune appear to almost touch each other on Jan. 27. Neptune now has the recognition as the last planet in the line of our solar system.

As far as the former last in line Pluto is concerned, it has been demoted to the realm of “dwarf” planets when the Minor Planet Center (MPC) has added “insult” to injury by taking away its name and giving it a number.

Pluto, as it has been known since its discovery over 60 years ago, has been given the number 134340 by the MPC that is the official organization responsible for collecting data about asteroids and comets in our solar system.

More about the Venus/Neptune conjunction in a later column.

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