If your ‘seeing’ is blurry, it may not be your eyes

Greetings stargazers.

It is a good thing you are reading this – it means we entered the next Maya calendar b’ak’tun with fewer disruptions than the Y2K computer glitches that threatened civilization 13 years ago. But we can save that discussion for another tzolk’in (Maya 260-day time span). This month I will talk about seeing.

The nursery rhyme says “Twinkle, twinkle, little star,” but did you ever wonder why stars twinkle? Although many casual stargazers admire a twinkling sky, for serious astronomers, twinkling is not a good thing.

Twinkling, or scintillation, is just the effect of starlight being refracted in a turbulent atmosphere. Just as glass can bend a light ray, so can air, just not as much as glass. The more turbulent the atmosphere is, the more the stars will twinkle. If you take a high-speed video of a twinkling star, it appears to be a point of light quickly bouncing around some average position.

Glass and air can also split the colors of light into a rainbow. With a turbulent atmosphere, the red part of the spectrum might jump differently than the blue part. That is why you sometimes get the effect of a dancing rainbow of colors from a single bright star.

A long-exposure photograph of that scintillating star would show you a blurry disk instead of a nice pinpoint. More twinkling means a blurrier image. Astronomers call this effect “seeing.” They measure how good their seeing is, by measuring the angular size that the star disk appears to be.

In the Astronomer’s Forecast for Durango (see the link), one of the rows is called seeing. This is a prediction of how stable the atmosphere will be. The best seeing usually occurs in the early morning hours after the temperature has had a chance to stabilize.

I have heard (and read) that one way to tell the difference between stars and planets is that stars twinkle and planets don’t. While often true, I assure you that planets can definitely twinkle here on evenings with below-average seeing. Being near high mountains on cool, breezy evenings after warm afternoons, the atmosphere gets about as turbulent as anywhere.

This month

The star Sirius will rise shortly after 8 p.m. That is a great star to check for twinkling and to see how good your seeing is. If you are a Harry Potter fan, you might already know that Sirius is the dog star. It is also the brightest star in the sky. A fun myth is that when Sirius is out in the daytime (during the summer months) the dog days of summer are so called because the brightness of Sirius adds its heat to that of the sun.

The Quadrantids are a usually reliable meteor shower. They should peak early in the morning on Jan. 4. Unfortunately, they rise after midnight about the same time as the waning gibbous moon.


Charles Hakes is an assistant professor in the physics and engineering department at Fort Lewis College and is director of the Fort Lewis Observatory.