01
A baseline gives two viewpoints
Astronomers observe from opposite sides of Earth's orbit, six months apart.
Earth & space · 24 minutes
Measure a star by watching it move
Hold up a finger and switch eyes. A nearby finger jumps against the distant wall. Earth repeats the same geometry with nearby stars.
Keep this question in mindHow do we measure things that are far beyond Earth?
01
Notice the problem
Earth's orbit becomes a ruler.
A nearby star shifts against the distant sky. The smaller the angle, the farther the star.
Six months apartEarth's orbit becomes a ruler.
A nearby star shifts against the distant sky. The smaller the angle, the farther the star.
02
Three things to notice
Look at these three ideas.
Each picture shows one part of the question. You will test the ideas next.
02
Nearby objects shift more
The same baseline makes a larger viewing angle for a closer object.
03
Distant background stars make a grid
A nearby star's tiny apparent motion is measured against stars much farther away.
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Your prediction
How should parallax angle change as star distance increases?
Choose what you think will happen. It is fine to be wrong. The experiment will help you find out.
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