Look closerSky
The twinkle test: how to tell a planet from a star
When: All year
Stars twinkle because each one is a single point of light that moving air keeps nudging. A bright point that stays steady is often a planet, because a planet is a tiny disk and the nudges average out.
Why this time of year
Try it on any clear night: compare a few bright points and see which ones hold steady.
How to notice it
- Go out on a clear night and pick one bright point of light that is well up in the sky, not hugging the treetops.
- Watch it for ten slow breaths. Note whether it flickers or glows steady.
- Compare it with a few other bright points nearby. Steady next to flickery is the clue that it may be a planet.
- Look at something low near the horizon and then something high overhead. Low objects, even planets, can twinkle more.
Try this with kids: Can you find one light that twinkles and one that holds still? Which one might be a planet?
Good to know: Twinkling (scintillation) is the flicker you see when moving air bends light from a pinpoint source.
Sources
- earthsky.orgStars are pinpoints so atmospheric disturbance makes them twinkle; zigs and zags across a planet's disk cancel so planets shine steadily; planets low in the sky may twinkle; more atmosphere means more twinkling.
- imagine.gsfc.nasa.govNASA Ask an Astrophysicist: stars are point-like and their image moves as turbulent air pockets bend light; planets are much bigger than typical seeing so variations average out.
- starchild.gsfc.nasa.govNASA StarChild: air movement (turbulence) bends starlight; stars near the horizon twinkle more because there is more atmosphere in that direction.
- news.wisc.eduUW-Madison Space Place: starlight bends as it passes warmer or cooler air; planets generally do not twinkle nearly as much as stars.
Checked against these sources on October 13, 2026.