Monday, January 24, 2022

Catch Mercury if You Can!



By Mike P. Usher

As the too bright Moon in the east is nearly full, we might as well look westward to a slightly darker sky. Venus is unmistakable as it floats high in the west; it is in fact the brightest object in the night sky other than the Moon.

Below Venus, and a little to the right and just above the horizon lies the planet Mercury. Mercury is much more difficult to locate than Venus; at the time and date of the chart it’s only a little brighter than Polaris. During the next several days it will rise higher above the horizon each night and grow a little brighter. Mercury is hard to spot as it is rarely or never seen when the sky is completely dark. It orbits quite close to the Sun and so from our point of view Mercury is never too far from our home star. Because of the tilt of its orbit Mercury will rise higher when viewed from southern hemisphere than it does here north of the equator.

Due to a series of mistaken observations and misinterpretations Mercury was long thought to have a day as long as its year (88 Earth days). In other words, one side always faced the Sun while the other side lay in perpetual darkness. About 50 years ago astronomers bounced radar off its surface and discovered to their surprise that Mercury rotates after all, revolving exactly three times for every two orbits around the Sun. When combined with its high orbital speed and elliptical orbit this gives rise to the strangest looking day in the Solar System.

At certain spots on the Mercurian surface an astronaut standing there would see the Sun rise in the normal fashion. Then the Sun changes its mind and slips back below the horizon! Changing direction again it rises once more and proceeds across the sky. Upon reaching the other horizon the Sun sets, then rises once more for a final goodbye before it sets for good.

You could see the Sun dance while standing anywhere on Mercury where the Sun is shining, but you would only see the double sunrises and sunsets from certain narrow bands of longitude.

See you next time!


Mr. Usher is a director of the Everglades Astronomical Society, which meets the second Tuesday each month at 7 PM in the Norris Center at Cambier Park in Naples.

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