Stellar Parallax Definition: Lesson for Kids

Instructor: Shoshana Yarin

Shoshana has taught all grades with an emphasis in science and has a master's degree in science.

How do astronomers know the distances to stars? One method called stellar parallax involves your thumb, angles, triangles, and Earth's orbit around the Sun. Read this lesson to learn more!

How Do We Know Distances in Space?

Have you ever stared up at the stars at night and wondered how far away they were? The distances in space are a little hard to understand, but astronomers have interesting ways of figuring it out.

One way is using what's called stellar parallax. A very simple explanation is a shift in position of a nearby object against a far-away object. Because 'stellar' means 'star,' the nearby object in this case is a close star. Can you guess what the far-away object would be? Stars that are even further away!

Stars at Night

Hold your thumb out in front of you. Close one eye and notice where your thumb is in relation to something far away. Maybe compare it to a tree, a chair, or anything in the distance. Don't move your thumb, but switch open the other eye and close the first eye.

Is your thumb in the same place? It should look like it's moved compared to the far-away object. Switch back and forth between closing each eye and it looks like your thumb moves. That's a parallax and it has to do with seeing something far away from two different points. In this case, the two different points are your two eyes!

Stellar Parallax in Space

Let's look at stellar parallax and how it's used in space. Earth does not have two eyes, but it can be in two different places in its orbit around the Sun. We know the distance of Earth to the Sun is 93,000,000 miles or one AU (astronomical unit). If we observed a star from two different places of orbit, it would be just like your two eyes seeing an object. What actually happens is that from the two different places on Earth's orbit to the object, it creates a triangle. Triangles mean geometry and that means calculations.

Stellar Parallax from Earth
Stellar Parallax

Astronomers can only use this method for the closest stars. The angle measurements of the triangles formed are very tiny. The farther away something is, the longer the triangle is and the smaller the angle. If you try drawing two triangles; one long and skinny and the other short and fat, you'll see that the angle at the tip of the long triangle is much smaller.

Angle of Stellar Parallax
Parallax Angle

Astronomy and Math

Because the angle measures are so small, astronomers use a unit called an arcsecond to measure them. An arcsecond is a measurement of a tiny angle used in calculating parallax. You can remember that 'arcsecond' and 'angle' both start with the letter 'A.'

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