We are going to break the usual rule and look at the sky tonight at twilight where we will find the most elusive of all planets, Mercury. There is even a legend that Copernicus, promoter of the sun-centered Solar System, never saw it – though that is hard to believe. Mercury is difficult to find due to its close proximity to the Sun; in fact it is never found more than 28 degrees away from our home star – usually much less. This means that Mercury is never visible in the northern hemisphere when the sky is truly dark, but is seen only at twilight.
To see Mercury you will need a clear view of the western horizon; the beach would be perfect. At 9:00PM Mercury will appear about 10 degrees high, just a little to the left of where the Sun set. It will be about twice as bright as the nearby Regulus, and tinted somewhat orange. If clouds interfere you can try again in the next few days, but don’t wait too long, by the first week of August it will vanish into the twilight as it appears to move closer to the Sun. In reality, Mercury will move between Earth and the Sun and reappear in the morning twilight in a few weeks.
Every planet seems to have a few oddities which serve to distinguish it from all of the others. Mercury has more than its fair share, most of which stem from its extreme proximity to the Sun and it’s very elliptical (egg-shaped) orbit. For example, it has an 88 day year, but spins on its axis just three times for every two orbits. The combination means that an astronaut would experience a day 176 days long – with a Sun up to three times the size as ours and ten times as hot! Noon on Mercury would melt lead and zinc. It gets even stranger: from certain places on Mercury you would see the Sun come up in the east, rise about halfway to the zenith, then turn around and set in the east! Later it would rise once more, cross the sky and set in the west. The Sun will rise once more in the west as if to take a final look, pause and set again for good! This very odd motion is due to Mercury’s combination of slow rotation, fast orbital speed and elliptical orbit.
See you next time!
Mr. Usher is President of the Everglades Astronomical Society which meets every second Tuesday at 7:00PM at the Norris Center, Cambier Park, Naples.