Elusive Mercury

     Though most people know the name Mercury as a planet, most people have only seen Mercury as a car driving down the road: few people have seen this elusive world. The end of May provides an excellent opportunity to spot Mercury, in fact you can't miss it if you know when and where to look.

  
  For the first part of the last century, Mercury held many claims to fame, perhaps to make up for its small size! It held the title of planet closest to the Sun, planet to make fastest trip around the Sun and it was believed to be the hottest planet. It was also believed that Mercury had one face locked in orbit facing the Sun. Then scientists learned more, we sent probes into space, and one by one Mercury began losing it's claims to fame.

     Today, Mercury retains two titles, closest planet to Sun and fastest orbit around the Sun. In the 1970's Pluto stole the title of smallest planet, and then the probes proved that Venus was hotter.  The final blow came in 1974 when it was discovered that Mercury did rotate and did not have one side constantly facing the Sun.

     Mercury is about one third the diameter of the Earth, but it is a very different world of extremes. Mercury was named after the Roman messenger god with winged sandals. It was given this name because it completes it's trip around the Sun in only eighty eight days! But in contrast to this short year is Mercury's slow rotation. One day on Mercury lasts fifty-nine Earth-days!

     The surface of Mercury resembles the moon so closely that even experts can be fooled by photographs. Even so,  if Mercury were to replace our moon in orbit, we would be in for a surprise.  Our new moon would be 1.5 times larger in diameter and would be twice as bright. But the biggest surprise would be higher tides due to Mercury's larger mass. Mercury induced tides would be a true surfer's delight, over four times higher than our current tides!

     A very thin envelope surrounds Mercury, but it is not a true atmosphere. Whatever atmosphere Mercury did have was lost billions of years ago, lost by Mercury's weak gravity. This thin layer of helium is believed to come from the solar wind and is trapped by Mercury's magnetic field. Even so, the amount of helium is so sparse that all of it in a four mile diameter would not be enough to fill a child's balloon.

     None of the planets have a perfectly circular orbit around the Sun, but rather the orbits are elliptical. Mercury's orbit is so elliptical that it produces major temperature changes as the distance from the Sun varies. When Mercury is farthest from the Sun, a sunny afternoon would see temperatures around 545 degrees F. When Mercury's orbit takes it closest to the Sun, temperatures on the side facing the Sun reach 806 degrees F! Nights on Mercury are just as extreme, reaching a frigid \292 degrees F. This temperature variation did give Mercury an additional title, planet with the greatest temperature ranges! At its hottest, Mercury would melt lead or turn steel red hot in minutes: at its coldest, Mercury would freeze a person solid in minutes.

     Mercury is hard to spot from Earth both because it is so small and because it is so close to the Sun. Only at certain times do we have a chance to see it, the end of May provides such a chance. You will want to look in the western sky at sunset. Locate the spot where the sun sets and then around 8PM look about five degrees above that spot. Five degrees is about half a fist's width (held at arm's length.) Mercury is quite dim, so don't mistake it for the nearby bright stars of Procyon and Capella.  The planet will fade in brightness as we move into June, so now is the best time to spot the elusive Mercury.