One night I noticed a star changing colors and moving perceptibly. But through binoculars, a few dimmer stars in the same field appeared steady. The behavior continued throughout the night, but not a couple of nights later. Why?

You’re absolutely right that stars twinkle — and sometimes appear to move around — due to our atmosphere “scrambling” their light as it travels from the top of Earth’s atmosphere to the ground. This phenomenon, also called scintillation, tends to occur more obviously in bright stars. Your eye isn’t sensitive enough to pick up the changes as easily in a dim star, even through binoculars or your telescope. 

A star’s brightness aside, scintillation can actually be a localized effect in a few cases. First, stars close to the horizon twinkle more because their light must travel through more air to reach your eye. As they rise and the light travels through progressively less air, the effect lessens, and stars at the zenith (the overhead point) are affected least.

Second, localized atmospheric disturbances can be created as heat dissipates from warm areas of the ground; looking at the sky in the direction of a concrete building or asphalt parking lot may cause stars in that direction to twinkle more, while stars in other parts of the sky might appear more stable. 

Finally, because the amount of scintillation depends so much on what the atmosphere is doing at any particular moment, differences in wind speed, humidity, temperature, and other factors from day to day can affect how much stars twinkle on different nights.

Alison Klesman 
Associate Editor