Science

Aurora Australis over Cradle Mountain

WHY DO CITY LIGHTS AND STARS TWINKLE?

Words: Stephanie Williams Image: Pierre Destribats

Have you ever wondered why city lights sparkle in the distance? Or why the stars pulse in the night sky?

If it’s a clear dark night, you can see thousands of stars that look like they’re pulsating and changing in their brightness. You might be surprised to know that most stars actually shine with a steady light - it’s other factors that cause the sparkle on the way to our eyes.

When you look up, you’re not looking into blank space - you’re looking into the atmosphere. It’s the movement of air (or turbulence) in the atmosphere of Earth that causes the starlight to go a little rogue. Hot and cold air gets in the way and bounces the light around as it travels from the star in the distance through the atmosphere and down to us on the ground. Some of that light reaches us directly and some of it gets bent along the way. Because of that bendiness, our eyes perceive it as twinkling.

Stars that are closer to the horizon appear to twinkle more than other stars higher in the night sky - there’s a lot more atmosphere between you and a star that’s high up. Large astronomical objects closer to Earth, like the moon and other planets, don’t seem to twinkle because there’s not as much atmosphere between us and them. 36 Twinkling, or scintillation, also plays a role in predicting the weather, with twinkling illustrating how winds are moving. On really rough nights in the atmosphere, stars can appear to even jump around, which is a frustration for astronomers. Astronomers try to overcome the twinkling by using adaptive optics, in which many small mirrors on the scope adjust constantly to allow for the atmospheric disturbances. The pros can also use telescopes orbiting Earth above the atmosphere to avoid the problems caused by turbulence.

City lights work on the same theory. The city lights are distorted by twinklifying gasses depending on the temperature, atmospheric densities and moisture (like rain or humidity) in the area. Some city lights don’t twinkle at all in places that have very low atmospheric disturbances and other cities are known for their pretty sparkling lights. Hobart tends to put in a good showing.

So next time you’re out at night look up and see if you can predict what’s happening in the atmosphere.