Stars appear to twinkle because their light passes through moving air in Earth’s atmosphere. That air bends the light in changing ways before it reaches you, making the star seem to flicker.
The flicker is along the route
NASA’s StarChild explanation places the cause in atmospheric turbulence. Some light is deflected slightly toward you and some away as conditions along the path change. The effect does not require the star itself to switch on and off.
NASA also notes that stars near the horizon can appear to twinkle more because their light crosses more atmosphere than light from a star higher overhead. A clear-looking sky can still contain moving air.
Why that matters to telescopes
The same atmosphere affects astronomical images. NASA’s Hubble overview explains that shifting pockets of air distort the view from the ground. Hubble’s position above the atmosphere avoids that source of distortion.
That connects a familiar backyard observation to an engineering choice. The pretty flicker you notice with your eyes can make fine detail harder to capture through a telescope.
A small observing exercise
On a clear evening, find a safe place to stand and compare a star relatively high in the sky with one closer to the horizon. Note whether either appears to flicker more. Do not turn one observation into a universal rule: record what happened on that evening.
If you cannot confidently identify what you are looking at, label it “unidentified light” in your notes. A blinking light by itself is not enough to identify a star. A sky map or a knowledgeable local astronomy group can help with identification.
For a useful record, include the time, direction and approximate height above the horizon. “Low in the west at 9 p.m.” gives a future comparison more context than “a bright thing.” This is an observation suggestion, not a report of a test we performed.
The night sky contains several different light stories. Explore the Moon’s changing phases or the glow of an aurora.
