I am back at McDonald Observatory for another observing run, seven nights of staring at the stars. So of course the weather forecast is for unsettled weather, with numerous weather fronts and troughs passing through.
Tonight started cloudy, but it rapidly cleared as trough number 1 passed through (A trough is a weather feature, like a cold front or a warm front. This trough had no marked temperature difference here at ground level). As I went out to marvel at the suddenly star-strewn sky, I noticed that the stars were twinkling furiously, especially those that were setting. The familiar bright stars of winter, including Sirius (the brightest star in the sky), the twins Castor and Pollux, Capella, and the red supergiant star Betelgeuse were flashing, changing colors, and even briefly winking out almost completely before reappearing with a brilliant flash.
Anyone who has looked at the night sky has seen twinkling stars. The twinkling we see is not an actual change in the star, but twinkling due to our atmosphere. Our atmosphere bends light, and if the atmosphere is not perfectly steady, the light from a star will get bent this way and that way. Sometimes it is bent away from the viewer (causing the star to get fainter), sometimes it is bent toward the viewer, causing the star to get brighter. This bending of light also depends on the color of the light, so sometimes the blue light from a star is bent away from us while the red light is bent toward us, and vice-versa. This causes the stars to appear to flash different colors.
This phenomenon is really quite fun to watch. The star Betelgeuse, which normally shines with a ruddy red color, was dancing to and fro, sometimes appearing steely white, and sometimes it was a bright crimson. The normally brilliant white star Sirius was rarely white, flashing all colors of the rainbow.
Amid all these wildly dancing stars, there was a bright orange spot that was steady as a rock. It wasn't changing color, nor was it twinkling. This was the planet Mars. Planets rarely twinkle because they are not points of light, but actually appear as disks. Individual parts of the planet's face do "twinkle", but most of the time the twinkling of different parts of the planet's disk cancel each other out, giving a nice steady glow at a constant color. However, if the planet is small (like Mercury or Mars when it is far from the Earth), and if the atmosphere is especially turbulent, even planets can twinkle. I've seen it!
While twinkling stars are cool to look at, they are bad news for the astronomer at the observatory. We get the best data when the atmosphere is the steadiest, and when the atmosphere is turbulent, we get blurry images. Because of the weather trough that moved through, the atmosphere is quite turbulent tonight. This makes the stars twinkle, but it also means my images are quite blurry. In fact, I've had to skip one of my main targets. It is so faint that the blurry atmosphere blurred the star out to where I couldn't see it well enough to make useful measurements. I have backup targets, and hopefully the weather will be better for faint targets tomorrow!