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Mercury In Retrograde

Mercury 1

I picked up that phrase in conversation the other day and it made me curious. I assumed that “retrograde” has its roots in the observable physical behavior of a planet or star as seen from Earth. Beyond that assumption, I did not really know what that meant. Doing a little research, it turns out that the planet Mercury will be entering retrograde on October 11th/12th (depending on your source). The word retrograde applies, in astrology, to the apparent backward motion through the zodiac of a planet. This is an observable phenomenon from Earth, and dates back to the third millennium BCE when the Sumerians made of celestial bodies appearing to move backwards. In reality, they were moving more slowly due to the relationship in their rotational axis to that of the Earth and the other observable celestial bodies, but appeared to be moving backwards. The result of these visual relationships is retrograde motion. The 1947 “” by Nicolas DeVore describes this retrograde motion as:

“like the effect of a slow-moving train as viewed from another train traveling parallel to it but at a more rapid rate, wherein the slower train appears to be moving backwards. However, in the case of the celestial bodies it is not a matter of their actual speed of travel, but of the rate at which they change their angular relationship.”

I do not subscribe to astrology, but I do believe that most of what drives astrological definition is based on the actual physical observations of the relationship between celestial bodies in the sky. All of that to say, the physics of the stars and planets could not initially be explained by humans in scientific terms, so we were left to describe this phenomena in ways that we could understand.

It turns out that exhibit retrograde motion as seen from Earth. The Sun and the Moon do not, but this is due to the rotational relationship of each to the Earth (the Earth revolving around the Sun and the Moon around the Earth). This motion be distinctly different from what is normally observed, it has been ascribed dramatic significance as it relates to our existence on Earth. Mercury has been of particular astrological significance when entering a retrograde period, as the mythology assigns the messenger of the gods influence over our terrestrial communications and commerce. Entering this retrograde period, Mercury has the potential to wreak havoc on our Earthly interactions with each other. Those who believe in astrology portend chaos for us during these periods.

Now, more about . As mentioned above, Mercury has been observed in the sky as long ago as the third millennium BCE. It came to represent the messenger of the gods due to the speed with which it moves across the sky. Mercury has only been visited once by spacecraft when, in 1974 and 1975, NASA’s did three flybys allowing the mapping of about 40-45% of its surface. We do not know a tremendous amount about the planet closest to the sun, but here is a brief survey of what we do know:

  • - It is one of four terrestrial planets in our solar system, meaning it has a rocky surface
  • - Mercury has a higher iron content than any other planet in the solar system
  • - There is an unstable atmosphere made from helium, hydrogen, oxygen, sodium, potassium and calcium
  • - The surface has the greatest temperature difference in the solar system, due to its proximity to the Sun
  • - That difference varies at its extremes by as much as 600° Kelvin
  • - Mercury takes 88 days to orbit the Sun, and has the most extreme orbit of the planets
  • - In its orbit, it will get as close as 46,000,000km and as far as 70,000,000km from the Sun
  • - A rotation of Mercury takes about 58 days
  • - Mercury is the second densest planet, after Earth, but would be first if not for gravitational compression on Earth
  • - It has a large iron core that generates a magnetic field roughly 1.1% the strength of Earth’s
  • - Sunlight on its surface is about 6.5 times that on Earth
  • - Despite the high surface temperatures, there is believed to be ice on Mercury
  • - It is believed this ice is in the deep craters and at the poles, as these are not exposed to direct sunlight.



6 Responses to “Mercury In Retrograde”

  1. Ed Wilms Says:

    Ice on Mercury?
    That’s fascinating about the temperature difference and the ice.

    It’s interesting how little we know about our solar system – I mean I know that the orbits of the planets aren’t perfectly circular -but I wouldn’t have expected that much variation.

    also, guess i’ll need better sunglasses if i ever go there.

  2. John Schneider Says:

    Yes. There is a certain element of rediscovery as we all studied the solar system in grade school… but there is so much more to know now and it is in such a different context. I think we could look more closely at each planet, and the various moons of the planets, and rediscover, and discover for the first time again, so many interesting, odd, and compelling facts. If you’ve not read it already, check out my post on the sun:

    http://schneiderism.com/here-comes-the-sun/

  3. Messenger Beams Back First Image From Mercury - schneiderism Says:

    [...] have posted previously about the planet Mercury, so I was excited to learn that the robotic Mercury research spacecraft [...]

  4. schneiderism » Blog Archive » Mercury: That’s Going to Leave a Mark Says:

    [...] had previously written about Mercury and NASA’s Messenger mission here and here. Posted in astrophysics, robots, science, solar system, space program [...]

  5. Erin Simpson Says:

    I just have a small question, it doesn’t matter if it’s not answered but…
    on the picture above, do you know what that thick blank line is?

  6. John Schneider Says:

    Thanks for the comment. Sure, the blank space in the image above simply represents an area that did not get photographed and subsequently did not make it into the composite that makes up this image. For detail images the robotic probes will make several passes, and from the passes the images are “stitched” together into a composite, or montage.

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