A Brief Description
of the Nearest Planet to the Sun
J. Timothy Unruh1
J. Timothy Unruh1
Mercury is nearest to the Sun of any of the definitely known planets, wrote Joel Dorman Steel, Ph.D., in his book Descriptive Astronomy, published in 1869, reflecting the fact major new members of the solar system were still being discovered. Swift Mercury, one of the five classic planets known since ancient times, was named after the Roman messenger of the gods. When the sky is clear Mercury may be seen as a bright sparkling star just before sunrise low above the eastern horizon, or just after sunset in a likewise manner above the western horizon. Because Mercury never appears very far from the Sun it rarely is seen above the horizon when the sky is completely dark in the northern hemisphere; generally one must look for it in twilight. Its elevation never rises to exceed 28 degrees, and barely that for merely a week at most before it rapidly drops back down toward the horizon within a few more weeks to become lost in the Sun's rays. Thus, after appearing as an evening star it reappears as a morning star and continues to slowly oscillate like a pendulum, to and fro, from one side of the Sun to the other. The ancients deceived by this puzzling movement, failed to recognize the identity of the two stars, and called the morning star Apollo, the god of day, and the evening star Mercury, the god of thieves who walked about in the nighttime seeking plunder. Due to the nearness of this planet to the Sun, it is difficult to be detected. It is said that Copernicus, an old man of 70 years, lamented in his last moments that, as much as he had tried, he had never been able to see it. Some astronomers even today have never consciously seen it. In the latitudes of the United States it can generally be seen rather easily if it is watched at the time of its greatest elongation or angular distance from the Sun, which ranges from 18 to 28 degrees due to the eccentricity of its orbit, as commonly indicated in an almanac. When it is an evening star, and at its greatest eastern elongation in February and March, its elevation is at the minimum value, although the higher inclination of the ecliptic to the horizon at that time makes such an occasion the most favorable for seeing the planet since at that point it sets latest after the Sun. For similar reasons it is best seen as a morning star in October, favoring southern hemisphere observers. Thus three pairs of eastern and western elongations as such occur within a year.
Viewed through a telescope Mercury shows very little detail even under the best of observing conditions. The planet's angular size is rather small, ranging only from five to twelve arc seconds as compared to Venus' 10 to 64 arc seconds. It also presents all the phases as those of the Moon, from a slender crescent to gibbous. These phases prove that Mercury is spherical and shines by reflected sunlight. As for the actual surface appearance of Mercury very little was really known until Mariner 10, the automated spacecraft that photographed Mercury in 1974. It provided the first close up views of a very Moonlike appearing little world rife with impact craters, scarps and other rough features. With a diameter of 3,029 miles, Mercury is the second smallest planet in the solar system, Pluto being the smallest at about 1,420 miles across. Because of a relatively large iron core Mercury is a rather dense heavy little world, about as dense overall as the Earth, although only about 1/18 as massive or heavy as the Earth. It would also take about 18 Mercury sized volumes to fill a volume the size of the Earth. If a 100 pound Earthian could take his bathroom scales and go weigh himself on Mercury he would weigh 37 pounds there. The surface of Mercury is relatively dark with a low albedo of about .06 which is very similar to that of the Moon.
The sidereal period of Mercury, or the time it takes to make one complete revolution about the Sun with reference to the fixed stars is 88 Earth days, while its synodic period, or the period of time between successive passages of Mercury directly between the Earth and Sun, is 116 days. In other words, if Mercury is an evening star at sunset now it will again be an evening star at sunset after another 116 days. Mercury's average distance from the sun is 36,000,000 miles, however, because of a pronounced eccentricity of its orbit, the actual distance from the Sun varies from 28,600,000 miles at perihelion to 43,400,000 miles at aphelion. If one were to stand on Mercury's hot surface, given that likelihood, the Sun would appear two to three times larger than it does back on Earth, providing searing daytime temperatures sometimes as high as 800 degrees Fahrenheit, which is hot enough to melt lead. Thus, a typical patch of Mercury's surface receives seven times as much solar radiation as does an equal area of Earth. Interesting enough, ice is believed to have been found on Mercury in the basins of polar craters where the Sun's radiation never reaches, based on a more recent scrutiny of photographic data. Mercury's virtually nonexistent atmosphere undoubtedly provides very little insulation from the savage extremes of its environment, hence the night side temperature is estimated to drop to almost -300 degrees Fahrenheit. If space travelers from Earth were to land on Mercury they would probably want to stay within the terminator, that narrow temperate band around the planet between day and night, also referred to as the twilight zone in Erik Van Lhin's 1953 classic science fiction novel Battle On Mercury.
Radar observations of Mercury have disclosed that this planet rotates about an axis virtually perpendicular to its orbital plane in a sidereal period of 59 days of our own time. The manner of its rotation is direct, that is, the planet rotates in the same sense that it revolves around the Sun. When we remember that Mercury's orbital period is 88 days, it transpires that this planet completes three rotations about its own axis while revolving twice around the Sun. The 3-2 ratio is an indication of a spin-orbit tidal coupling with the Sun, but why this period has stabilized in the particular 3-2 ratio has not been satisfactorily explained. As a result of this unearthly circumstance the huge fiery Sun hangs in the Mercurian sky for 176 days at a time. Thus, contrary to previous opinions, Mercury does not always show the same face toward the Sun. For an observer situated anywhere on Mercury, the Sun would rise in the east and set in the west every 176 terrestrial days, but at the time of each perihelion passage, i.e., when Mercury swings around in its closest approach to the Sun the latter would appear to pause in the sky and reverse its direction for about eight days before resuming its westward advance, because at this time Mercury's mean angular velocity of revolution exceeds that of its axial rotation. It was this unusual coupling of its rotation and revolution that accounted for mistaken reports about Mercury early on, which suggested a synchronism of the two motions. Mercury can never be effectively observed from earth around a complete orbit. Because after every two revolutions the planet does present the same face toward the Sun, and this coincides with opportune times to observe Mercury from Earth, it is easy to see how earlier optical observers of this difficult planet could fall into a trap of thinking that one side always faces the Sun. Thus, technically Mercury has a fractionally synchronous motion, but who would have ever thought of that?
An irregularity in the orbit of Mercury contrary to any motion which could be explained by the law of gravity caused much speculation in the latter half of the nineteenth century about the possibility of an undiscovered intra-Mercurian planet believed to be causing the perturbations. The perihelion point of Mercury's orbit, was revolving more rapidly than predicted. The French astronomer Urbain Le Verrier theorized that this departure was caused by the gravitational influence of a planet between Mercury and the Sun. This hypothetical planet received the name Vulcan. (See Vulcan by author, Volume 5, No. 72, Spring 1995, page 12, of this publication.) No such planet was ever found, however, and Mercury's eccentricity continued unexplained until the time of Albert Einstein. In his General Theory of Relativity, Einstein declared that, at high relative velocities, orbital motions will deviate from the Newtonian standard. In this context Mercury's deviation neatly fitted the amount of excess. The advance of Mercury's perihelion thus occurs when the acceleration that takes place as the planet approaches nearest the Sun gives it an impetus that carries it slightly beyond the last perihelion turning point as such in its orbit. Mercury's large eccentricity enhances this process in a manner that was not accounted for by Newton or Kepler. In short, we have come, in the last 20 years alone, from knowing hardly more than Copernicus about the Sun's moth-like companion with a complex path, to an understanding of a Moonlike heavy ball with a powdery surface, spinning on its axis three times for every two revolutions around the Sun in an orbit that wobbles like a hula hoop.
There is also something important to be realized from the impact craters revealed on Mercury. First, we are told by many astronomers that the planets of the solar system formed by the accretion of innumerable small bodies called planetesimals in a nebular cloud. The impact craters found on the Moon and other planets are regarded as the marks left by the last of these planetesimals when they fell to form the planets. At the present time there no longer appears to be the quantity and size of objects which could have caused such cratering, assuming that by now they have gone into the making of the planets. Yet the apparent rapid formation of so many craters at one or more times in the past is difficult to account for if we assume their random collisions by planetesimals. There would have to be so many of them that they could not possibly be all gone by now. Remarkably, the collection of space probe photographs of the planets we now have show that they are typically two faced, or unsymmetrical in their crater distribution. Mercury, as well as Mars, Venus, and the Moon is considerably rougher and more cratered on one hemisphere than the other. This unexpected result remains a mystery to the astronomers. It is a solar-system-wide effect that seems to indicate a cause both within and outside the planets themselves and that all the craters in the solar system were caused sometime after the planets themselves were already formed. One explanation is that a large cloud of asteroid-sized objects swept through the solar system and collided with the planets producing craters on the sides of the planets facing the wave of objects, given the cloud passed by in less time than one rotation of the planet involved. There is also evidence for a solar-system-wide cataclysm associated with the explosion or disruption of a planetary member within the solar system. (See Phaeton: The Lost Planet, by this author, Vol. 5, Number 74, Fall 1995, page 18, of this publication.) There is strong circumstantial evidence of a once existent planet in the space between Mars and Jupiter. The orbital gap where this planet should be is littered with fragments and debris. Most of the original mass of material from the parent body can be accounted for as having fallen into the other planets or having been lost into deep space. Those remaining pieces occupying that orbit constitute the present asteroid belt. There is abundant evidence that other planetary bodies in the solar system have barely escaped destruction. Among them Mercury displays its own battle scars of a bygone role in the cosmic clash of the titans. The idea of a catastrophic solution is not new although it now has been largely ignored. Preconceived notions and the philosophical biases of a long held belief system in the scientific community have often caused scientists to ignore or overlook the best and most obvious explanations.
There is yet another piece of interesting information about Mercury worth our attention, and that is, that the planet has no natural satellite or moon. In fact, the planet Mercury is one of two planets without a moon, Venus being the other. Why Venus is alone is a mystery, for this planet is certainly capable enough to sustain a significant companion. What is so profound about Mercury's circumstance is that it is virtually impossible for Mercury to have a moon. In the last century the French astronomer Roche discovered that any satellite too close to its planet or primary (within 1.22 times the diameter of the planet) would be torn to pieces by that planet's gravity, unless the satellite was made of unrealistically strong materials, or very tiny. In the case of Mercury the Roche limit is about 3,700 miles from the planet. But Mercury is so small, and so close to the Sun, that any satellite more than 3,000 miles from Mercury would be pulled away by the Sun. In light of this there is no way for Mercury to have a moon.
To a backyard observer Mercury is an elusive planetary object. Through even a large portable telescope Mercury appears only as a fuzzy object at best, whose shape is barely discernible. Like Venus, Mercury can be seen to go through phases over time. At its brightest Mercury appears as a star of magnitude -1.9 in the twilight sky, comparable to the brightest star Sirius.
Exhibit A shows the comparative sizes of the Moon, Earth, and Mercury (Wagner). Exhibit B shows a diagram of the apparent path of the Sun, as seen from Mercury, showing the loops produced at perihelion during Mercury's year (Murray and Burgess). Exhibit C shows the phases of Mercury as an evening star as viewed from above the Earth and sky from space (Mc Laughlin).
There are many devices in a man's heart; nevertheless the counsel of the Lord,
that shall stand. Proverbs 19:21
1Copyright ã 1994 by J. Timothy Unruh, Back Yard Astronomers, P.O. Box 1034, Rocklin, California 95677-1034. Item No. 29101994.