Mercury A to Z: The Twinkling Planet

Hello, friends!  We’re in the final week of this year’s A to Z Challenge!  If you don’t know what the A to Z Challenge is, it’s a monthlong blogging event where participants write a full alphabet’s worth of blog posts about a topic of their choice.  My topic this year is the planet Mercury, and in today’s post T is for:


When you look up in the nighttime sky, stars twinkle, and planets don’t.  This is probably the first astronomy lesson most people learn.  It’s one of those basic facts almost everybody seems to know.  However, like most super basic facts that everybody seems to know, there are exceptions to the rule.  Mercury is a planet, and yet Mercury twinkles.

Stars twinkle because they’re very far away, and their starlight is relatively faint.  So when starlight hits Earth’s atmosphere, the atmosphere distorts the light, causing a twinkling effect.  But planets are much closer, and sunlight reflecting off a nearby planet is much brighter and more intense than the light emitted by distant stars.  Earth’s atmosphere still distorts the light reflecting off planets, but the distortion is nowhere near as noticeable.  Usually.

Two factors make Mercury different.  First, Mercury is much smaller than the other planets, so he doesn’t reflect as much sunlight our way as, say, Venus or Jupiter.  It’s probably worth mentioning that Mercury is also a darker colored planet, with much of his surface covered in graphite.  Second, because Mercury is so close to the Sun, we Earthlings usually can’t see him except just after dusk or just before dawn.  This means that whenever we see Mercury, Mercury’s light has to pass through Earth’s atmosphere at an angle.  In other words, Mercury’s light has to travel through more of Earth’s atmosphere in order to reach our eyes.

Less light plus more atmospheric distortion equals a twinkling planet.  As William Sheehan notes in his book Mercury (for the Kosmos series), “[Mercury] is more often seen, no doubt, than recognized.”

I love star gazing.  I love looking for planets in the sky, and I love the moment of realization when I recognize one of them.  I only know for certain that I’ve seen Mercury two times in my life, and I needed help from an astronomy app both times.  However, I may have seen Mercury many times than I know and just assumed he was another star, twinkling near the horizon.

The real lesson here is that there are those basic facts that everyone knows, basic facts like stars twinkle and planets don’t.  But once you learn the basic facts about something, start learning about the exceptions to the rule.  You may miss out on some really neat experiences in life if you ignore the exceptions and stick to knowing only the basic facts.


Here’s an article from about why stars twinkle and planets (usually) don’t.  It’s one of the rare articles I’ve seen that notes how, under the right circumstances, planets can twinkle, too.

And also, I’m going to once again recommend Mercury by William Sheehan.  Chapter One of that book is about how Mercury twinkles, or rather how Mercury “scintillates,” to use a more scientific term.

2 thoughts on “Mercury A to Z: The Twinkling Planet

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