How Do We Know How Far Away Stars Are?
“The nearest star is 40 trillion kilometres away. No spacecraft has ever come close. No signal we've sent has arrived. We've never touched it, visited it, or sampled it. So how do we know exactly how far it is?”
Close one eye. Hold a finger in front of your face. Line it up with something on the far wall. Now switch eyes — open the closed one, close the open one. Your finger jumps.
You didn't move. Your finger didn't move. But the view from your two eyes is slightly different, because your eyes are a few centimetres apart. The closer your finger is, the more it jumps. Hold it further away and the jump gets smaller. This is parallax, and it's the foundation of how we know where the stars are.
Every six months, Earth moves from one side of the Sun to the other — a shift of about 300 million kilometres. From these two positions, astronomers photograph the sky and look for stars that have shifted their position very slightly against the ultra-distant background of far galaxies.
The nearest star, Proxima Centauri, shifts by 0.77 arcseconds — a tiny angle, 1/4700th of a degree. But it's measurable. And from that angle, the calculation is elegantly simple: distance in parsecs = 1 ÷ parallax in arcseconds.
For Proxima Centauri: 1 ÷ 0.77 = 1.3 parsecs = 4.24 light-years.
That number comes from a geometry problem, not from visiting it.
This technique works for stars within a few thousand light-years. Beyond that, parallax shifts become too small to detect even with the best telescopes. For those stars, astronomers use other methods: comparing a star's apparent brightness to how bright it should be based on its colour and type, or analysing how light shifts as stars move toward or away from us.
Each method has a range. Stacked together, they form a cosmic distance ladder — parallax for nearby stars, brightness comparisons for the galaxy, other signals for the universe beyond. No single rung reaches all the way, but each rung is carefully calibrated against the last.
When you look at the night sky, you're not seeing a flat canvas of stars. You're seeing dots at radically different distances — some hundreds of light-years away, some thousands. And all of them in the past. The light reaching your eyes tonight left those stars before you were born, before your parents were born, in some cases before human civilisation existed.
You are looking back in time, using parallax as a ruler.
Ready to explore?
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If a star 100 light-years away exploded tonight, when would we see it? And if we somehow knew it had exploded, could we warn anyone near it? Why or why not?
Reflect
We cannot visit stars. We cannot touch them or send probes to them in any reasonable timeframe. Yet we know their distance, temperature, mass, and chemical composition. What does this tell you about what you can learn from light alone?