Apparent Retrograde Motion
Apparent retrograde motion is when a planet seems for a while to be moving backwards against the stars. The planet does not in fact change direction; this is an effect of perspective arising from planets that travel at different speeds overtaking one another.
| In brief | Apparent retrograde motion is when a planet seems for a time to move backwards across the sky because planets travelling at different speeds overtake one another. |
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| Astronomical account | The planets always travel in the same direction along their orbits; but because the Earth and another planet have different angular speeds, the planet's projection against the stars slides backwards during the periods of overtaking. This is a wholly geometric effect of perspective, and the periods are calculated in advance with high precision. |
| Use in astrology | In astrology a retrograde planet is interpreted through themes of turning inward, delay and repetition in the matters it represents; particular meanings are also attributed to retrograde planets in a birth chart. Mercury retrograde is associated in popular culture with disruptions to communication and technology. |
| Evidence status | The perspective explanation of retrograde motion has been confirmed for centuries by precise observation and calculation. Controlled studies have found no supporting evidence for a relationship between retrograde periods and communication problems or life events; that disruptions are noticed more during these periods can be explained by selective attention. |
| Common misconceptions |
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The overtaking effect
When you overtake a slow vehicle on a motorway, that vehicle seems for a moment to slide backwards against the scenery behind it. The same thing happens with the planets: when the Earth overtakes an outer planet such as Mars in its orbit, Mars appears to move backwards against the stars for a few months, and then returns to its usual direction. The inner planets, Mercury and Venus, create the same visual effect during the periods when they overtake the Earth.
An appearance that changed history
Retrograde motion was one of the most fruitful puzzles in the history of the sky: Earth-centred models had to add circles within circles, the epicycles, to explain these reversals. In the Sun-centred model the explanation is elegant and comes of its own accord; the backward motion is a natural consequence of the model. Explaining this appearance was one of the key steps in the birth of modern astronomy.
Retrograde periods in astrology
In astrology, retrograde periods are interpreted through themes of disruption and review in the areas the planet in question represents; Mercury retrograde is the best-known example of this usage. From an astronomical point of view nothing changes in these periods, either in the planet or in its physical effects on the Earth; the layer of interpretation is entirely symbolic. For telling the two views apart, the frame in the Zodiac record applies.
Related terms: The Ecliptic, Zodiac, The Forer or Barnum Effect