The sky facing west about 40 minutes after sunset in mid-May when Mercury will be just shy of one outstretched fist above the northwestern horizon. It shines brightly at magnitude -0.3 this week. Use higher, brighter Jupiter to make a sight line to the planet. Mercury’s making its best evening appearance of the year for northern hemisphere sky watchers. Stellarium
Now through the end of May is the prime time to look for Mercury in the evening sky. Like the rock star Prince, this small, speedy planet is elusive, making only a few brief appearances a year. Consider this a personal invite to the show.
To find Mercury, pick out a place with a wide open view to the west-northwest in the direction of sunset. Start looking a half hour after sundown about a fist to the left of the brightest glow left on the horizon by the setting sun. Mercury will be some 8-10 degrees (about one outstretched fist) above the horizon. It looks like a solitary diamond in twilight’s pink glow.
Mercury shows phases as it revolves around the sun as seen from Earth’s perspective outside looking in. Mercury, like Venus, appears largest when nearly lined up between Earth and sun at inferior conjunction. Planets not to scale and phases shown are approximate. Illustration: Bob King
Mercury gets easier to see as the sky darkens … to a point. Once it’s within a few degrees of the horizon, the thicker, dustier air in that direction quenches its light and the planet fades.
The one-day old evening crescent moon with Mercury (upper left) on Jan. 31 this year. Credit: Bob King
It’s amazing that Mercury’s rates as a planet considering how small it is – just 3,021 miles (4,880 km) in diameter. At 2,160 miles across, our own moon is 71% as large. Jupiter’s moon Ganymede is even bigger at 3,275 miles (5,270 km). If it were orbiting the sun instead of Jupiter, Ganymede would easily be considered a planet. Pluto, demoted to dwarf planet status in 2006, spans just 1,430 miles (2,302 km).
Despite Mercury’s diminutive dimensions, its self-gravity easily crushed it into a sphere long ago. That plus the fact that it revolves around the sun and has cleared its orbit of competition from other smaller bodies places it firmly within the realm of the planets.
And there’s no planet quite like it. Mercury hovers near the sun too close to see and a few weeks later leaps into the morning sky. Drifts back down toward the sun in a few weeks and then leaps into the evening sky. So it goes, back and forth like that a half dozen times a year. Northern hemisphere observers see it best at dusk during late winter and spring ‘elongations’ and at dawn in the fall.
It’s easy to guess the reason for its swift maneuvering – a tight orbit around the sun lasting only 88 days keeps Mercury on the move.
Mercury looks like a tiny gibbous moon this week through a small telescope. Use at least 75x to make out its shape. Illustration: Bob King
Like Venus and the moon, Mercury shows phases. Right now, if you’re lucky enough to train a telescope on it before it atmospheric turbulence near the horizon mushes up the view, the planet would look like a very tiny gibbous moon 66% illuminated.
Its phase changes quickly too. Within a few weeks, as it moves closer to Earth and grows in apparent size, the planet will morph from gibbous to half to a dim crescent. Yes, dim! Mercury is brightest when at ‘full moon’ phase, being nearly as brilliant as Sirius, but fades to 3rd magnitude when a thin crescent. This week we’ll see it brightest; next week the planet will start to fade noticeably.
Orbiting between 28 and 43 million miles (46 and 70 million km) from the sun and possessing no atmosphere, Mercury’s temperature ranges from an extremely hot 800 F (430 C) on the dayside to marrow-chilling -280 F (-170 C) on the nightside.
To the eye, Mercury would appear as shades of dark brown. NASA enhanced the subtle colors to in this photo of the planet, a mosaic of images taken by MESSENGER. Younger craters with their bright rays appear blue. Plains formed form ancient lava eruptions are tan or orange. Credit: NASA
Because the planet’s axis is tilted only .01 degree – it essentially rotates straight up and down perpendicular to the sun – sunlight never reaches into craters in its polar regions. Locked in permanent shadow, NASA’s MESSENGER spacecraft has found strong evidence for abundant water ice and other volatile materials stored there for millions of years.
We could go on and on about this strange little planet, but I’d be holding you back from getting outside to see it for yourself. For more information, check out NASA’s quick-facts summary and a wonderful gallery of photos from MESSENGER.