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Mercury's Orbit
Mercury's Phases


Mercury's orbit and phases
     By David McCann

Mercury's Orbit

Mercury's orbit around the Sun Every planet in the solar system has its own personality, its own features: Earth's biosphere, Jupiter's massive magnetic field, Pluto's eccentric orbit - so it is not surprising that Mercury should have its own distinctive character. Nevertheless, it is in some ways one of the more unusual planets.

Mercury is the least conspicuous of the visible planets. It is reasonably bright - first magnitude on average - but never moves further than 28° from the Sun. Since they need to be at least 12° apart for Mercury to be visible against the Sun's light, its appearance is limited to a short period just before sunrise or just after sunset. In northern Europe, it can be seen on no more than twenty days each year. Copernicus, for example, had to rely on other astronomers' observations, as he had never been able to see it on the days when he needed its position. Living in London, with a bright and obstructed horizon, I have never seen it at all.

Mercury has been a problem for theoretical astronomers, as well as for observers. Ptolemy needed a more complex formula for Mercury than for any other planet. Even Newtonian physics was unable to give a complete explanation of its orbit: the direction of the axis was found to move by rather more than could be explained by precession and the gravitational attraction of the other planets. Eventually Einstein's general relativity theory explained the phenomenon in terms of a distortion of the fabric of space by the Sun's gravity.

The orbit of Mercury has other curious features. Its eccentricity and inclination are greater than those of any other planet except Pluto, but its unique feature is that the year is actually shorter than the solar day! Mercury rotates very slowly on its axis - the sidereal day is 59d - so the Sun moves a considerable distance by proper motion in this time, with the result that the difference between the sidereal and solar days is much greater than the mere four minutes on Earth. The solar day is 176d exactly twice the length of the year. The length of Mercury's sidereal day was only established as recently as 1965, using radar. Before that it had been believed that the sidereal day was equal in length to the year. This was because the day and year of Mercury, and the orbits of Mercury and the Earth, are synchronised in such a way that, when viewing conditions are most favourable, the planet presents the same face to the Earth for a period of several years.

Because of the eccentricity of Mercury's orbit, the variation in the proper motion of the Sun would be noticeable to an observer on the planet. At perihelion, when proper motion reaches its maximum, it exceeds the diurnal motion, and the Sun turns slowly retrograde for a week. At this time, the temperature directly below rises to 800°F (430°C), hot enough to melt lead or zinc. But in the long night, the planet cools dramatically and, just before dawn temperature sinks to -300°F (-180°C). No other planet has such a great range of temperature.

A major advance in our knowledge of Mercury was obtained from the Mariner 10 probe. It made three close approaches to the planet, and provided the first accurate maps and much physical data. The chart for the launch is interesting. One would not normally suggest starting a journey when the Sun is lord of the ascendant and his only major aspect is the square of the Moon. In the event, Mariner 10 experienced many difficulties with its control and communication systems. But the Sun is at the Mercury-Uranus midpoint, showing the technical ingenuity employed to overcome the problems, and Jupiter is at the Sun-Mars midpoint, bringing success.

Mars is lord of the ninth house, and situated within it. Although retrograde, he is also in his own house; the opposition to Uranus shows the problems which arose, but the tine to the Saturn-Uranus midpoint shows the efforts made to overcome them, and the trine of Venus brought success. Mercury himself must be taken as a significator, since he was the destination of probe. He is strongly placed: angular, in square to the ascendant and conjunction with the part of the spirit. Traditionally, he would be said to be peregrine, but I would consider him to be actually exalted, as I shall explain below. It is notable that Mercury has no major aspects: in a nativity this often indicates intermittent loquacity, and one of the problems with the probe was that its radio transmitter kept switching itself on and off!

Mercury's Phases

Mercury's orbit, like that of Venus, can be divided into a complex set of phases, quite distinct from those of the Moon and those of the superior planets. The geocentric motions of Mercury are produced by the combination of its own orbit around the Sun and the Earth's orbital motion. In Figure 1, I have represented the latter by a motion of the Sun around the Earth, as this makes the diagram simpler. The solar motion is relatively constant, but the planetary motion appears to change greatly according to the direction from which it is seen. When the Sun, Earth, and Mercury are aligned, the orbital motion appears fast, because the Mercury is seen broadside on, like a vehicle passing an observer at the roadside. At the inferior conjunction, when Mercury lies between the Sun and the Earth, this motion is in the opposite direction to the solar motion: since the orbital motion is the greater, the result is that Mercury is seen to move in the opposite direction to the Sun -retrograde. When Mercury is on the far side of the Sun, in superior conjunction, the two motions are in the same direction and Mercury has its greatest geocentric speed. When Mercury is at the points GEE and GEW (Greatest Elongation East and West), the orbital motion is directly towards or away from the observer and so does not move the planet with respect to the zodiac: Mercury is then seen to move at the same speed as the Sun. At the points SR and SD the two motions are equal and opposite, so Mercury appears stationary.

The phases of Mercury

Figure 2 shows the phases which result from this complicated pas de trois. As Mercury moves from inferior to superior conjunction, it is matutine: a morning star seen rising before sunrise, since it lies behind the Sun in the zodiac. During this period it is waxing (unlike the Moon, which waxes as an evening star), since it is invisible at inferior conjunction and full at superior conjunction. It is also accelerating, since it has maximum retrograde motion near inferior conjunction and maximum direct motion near superior conjunction. Between superior and inferior conjunction, Mercury is vespertine: an evening star seen setting just after sunset; it is also waning and decelerating. Each of these two periods can be divided in two ways. Firstly we have the division according to speed - whether it is faster than the Sun, slower, or retrograde. Secondly, it can be either increasing or decreasing in light: moving away or towards a solar conjunction.

Phases of Mercury

One important distinction in traditional astrology is that between the solar and lunar (or diurnal and nocturnal) planetary sects. Mercury is solar when a morning star, lunar when an evening star. Among the moderns, Marc Edmund Jones and Dane Rudhyar had most to say about the difference between matutine and vespertine. Rudhyar characterised matutine Mercury as Prometheus, eager and progressive, and vespertine Mercury as Epimethius, deliberate and conservative. There is something in this, and it is not inconsistent with the ancient opinion. One notices among communists, for example, the contrast between Marx, Lenin, and Trotsky with Mercury vespertine; Stalin, Mao, and Castro with it matutine: the idealists and the dictators. But it is difficult to recognise Stalin as a progressive vis…vis Trotsky as a conservative! In practice, other factors in the chart usually obscure the effect of Mercury's phase, and persons as disparate as Bach and Wagner, Dostoevsky and Tolstoy, Dryden and Blake may be born under the same phase.

Many astrologers considered phase to be a dignity: Lilly stated that Mercury and Venus were stronger when vestpertine, though Ptolemy and Albiruni had said that they were stronger matutine! Lilly's opinion was presumably based on the idea that the Moon is stronger as a morning star, when she is waxing, and so Mercury and Venus should also be stronger when waxing. Abraham Ibn Ezra considers them stronger when increasing in light, which seems more plausible to me. If the effect exists, it is a minor one; the charts of thirty major philosophers, who should presumably have a strong Mercury, show a random distribution through the phases.

Phase also has implications for dignity with respect to exaltation. The exaltations are of Babylonian origin, but those of Mercury and the Moon do not appear in cuneiform texts and would seem to be Greek innovations. Porphyry pointed out an interesting pattern: all of the exaltations, save that of Mercury, aspect a domicile: by tine if the planet is of the solar sect and by sextile if it is of the lunar one. Since Mercury is solar when matutine and lunar when vespertine, it should surely have different exaltations for the two phases, one in trine to Gemini and the other in sextile to Virgo. I would suggest Aquarius and Scorpio. The outward-looking, matutine Mercury is well placed in honest Aquarius, but can become opinionated in Leo. The reflective, vespertine Mercury gains depth in Scorpio, but becomes earth-bound in Taurus. This idea is not really new: Lilly described Mercury as strong in Aquarius and weak in Taurus, and in our own time Martin Seymour-Smith has claimed that Mercury is exalted in Aquarius.

About one fifth of all charts will have a retrograde Mercury. The ancient idea that retrograde planets are weak may be true for horary charts, but does not seem to apply in nativities; admittedly, I cannot recall having seen any charts of first-rate scholars with Mercury retrograde. Modern astrologers such as Rudhyar, Escobar, and Dobyns have characterised this position as introspective and intuitive. To me, the most obvious result seems to be independence of thought: the native is not trammeled by received wisdom or public opinion, and often espouses controversial ideas or causes. Examples are William Lilly; Alan Leo; Israel Regardie, magician; Christian Hahnemann, homeopath; T.H. Huxley, self-appointed propagandist for Darwin; William Laud, executed for his struggle against Puritanism; Emile Zola, forced into exile over the Dreyfus affair; the journalist W.T. Stead, imprisoned as a result of his campaign against child prostitution; Ludwig Zamenhof, inventor of Esperanto, and also of his own religion; Sir Edwin Durning-Lawrence, the first (but not, alas, the last) to claim that Shakespeare's works were not written by Shakespeare.

Combustion may also be considered among the phases. Like retrogradation, this does not seem to weaken the planet in a nativity. A close conjunction of Mercury and the Sun is found in the charts of Thomas Edison, J.S. Mill, and Ludwig Wittgenstein - all possessed of powerful intellects. It is worth noting that these all have the superior conjunction; the only celebrities I have noticed with the inferior conjunction are Kings James I of England and Louis XVI of France, neither of whom possessed much sense.

David McCann, who lives in London, is an expert on the history and philosophy of astrology. His articles have been published in many international journals of astrology and he was a regular contributor to the Traditional Astrologer magazine, where this article first appeared.

© David McCann, 1999
This article was first published in The Traditional Astrologer Magazine, issue 18, March 1999

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