Starts With A Bang

Ask Ethan: Why can’t I see Mercury without a telescope? (Synopsis)

The eight planets of our Solar System and our Sun, to scale in size but not in terms of orbital distances. Mercury is the most difficult naked-eye planet to see. Image credit: Wikimedia Commons user WP.

“I long ago abandoned the notion of a life without storms, or a world without dry and killing seasons. Life is too complicated, too constantly changing, to be anything but what it is. And I am, by nature, too mercurial to be anything but deeply wary of the grave unnaturalness involved in any attempt to exert too much control over essentially uncontrollable forces.” -Kay Redfield Jamison

Under ideal conditions, Mercury achieves a maximum elongation, or angular separation, from the Sun of 28 degrees. Total darkness is achieved when the Sun dips 18 degrees below the horizon. So for many of us, why is it that we’ve never been able to see the closest planet to the Sun, even when it appears we have ideal conditions?

It only happens once every 11 years, but occasionally, all five naked-eye planets are visible at once. Mercury is always the toughest to spot. Image credit: Martin Dolan.

As you may have guessed, there’s more to the equation than that. A huge factor is your latitude, and what angle the Sun rises/sets at with respect to the horizon. If you live closer to one of the poles than the equator, there’s a good chance that you’ll never be able to see Mercury, even at this maximum, ideal elongation, as by time darkness sets in, the world is well below the horizon.

The Sun’s apparent path through the sky on the solstice is vastly different at 20 degrees latitude (left) versus 70 degrees latitude (right). Image credit: Wikimedia Commons user Tauʻolunga.

Still, even with that at play, you can still have a chance if you know where/when to look! Find out the tricks on this week’s Ask Ethan!