Have You Ever Seen Mercury?

If your answer is “no” then this month offers an excellent opportunity to see this elusive little world. Sadly, legend has deterred many from looking for it—including a number of seasoned amateur astronomers.

The story is often repeated that Mercury is so difficult to see that even the great astronomer Copernicus never saw it. This is quite unlikely but it has kept lots of skywatchers from trying to view this innermost planet. Yes, it moves so quickly around the Sun that it makes very brief appearances a few weeks at a time alternately in the evening and morning sky and then is gone.

This month Mercury reaches “greatest eastern elongation” on June 4th, at which time it lies 24 degrees east of our Daytime Star after sunset. It will then be at its highest point above the western horizon as the sky darkens. Many who see it for the first time are surprised at how bright and easy to see it is even in the twilight! Binoculars give an interesting view, often showing a pinkish hue. Those of you who are fortunate enough to own a telescope will be able to see its half-illuminated disk at magnifications of 50X or more, depending on atmospheric turbulence near the horizon. (If you haven’t yet given yourself and your family the gift of a quality telescope, see scientificsonline for a great selection of sizes, types, and prices.)

Mercury is one of the five planets known since antiquity. It is the closest of them to the Sun, circling it every 88 days. At just over 3,000 miles in diameter (roughly the size of the continental United States!), it is also the smallest of the five—actually being smaller than Jupiter’s moon Ganymede and Saturn’s moon Titan! But because it orbits the Sun and not a planet, it is itself a planet rather than being a satellite.

Something else to consider while looking at this peaceful-appearing world. Due to its proximity to our Daytime Star—averaging 36,000 000 miles—its surface temperature on the side facing the Sun is 800 degrees F. But on the night side it drops to -280 degrees F. This is the most extreme range of temperature of any of the planets and is a result of little Mercury not having enough gravity to hold onto an atmosphere, so the daytime heat rapidly escapes into space at night. Wow—who would have guessed just looking at it in the evening sky?

— James Mullaney