What is Mercury retrograde astronomically?
Everyone has heard the phrase "Mercury has gone retrograde". So what actually happens in the sky? The short answer: no planet goes backwards; the only thing that moves backwards is our point of view.
Short answer
Mercury never physically travels backwards. The Earth and Mercury orbit the Sun at different speeds, and during the periods when Mercury "laps" us from the inside track it appears for a while to move backwards against the background stars. This effect of perspective happens about three times a year and lasts around three weeks each time.
Apparent motion and real motion are not the same thing
We track the position of the planets in the sky using the very distant stars as a fixed backdrop. Against that backdrop the planets normally move from west to east. From time to time a planet slows, stops, and for a few weeks appears to travel in the opposite direction, before returning to its usual course. This is apparent backward motion, commonly known as "retrograde". The key point is that nothing changes in the planet's orbit while this happens; what changes is only the direction along which we are looking at it.
The inside-lane analogy
Picture a running track: you are in an outer lane and Mercury is in the inner one. Because Mercury is closer to the Sun it both runs a shorter lap and runs faster: it goes around the Sun in about 88 days, while the Earth takes 365. So Mercury is always the one overtaking; it approaches from the inside, passes you and pulls away. During the passage itself the direction in which you are looking at Mercury changes rapidly, and for a few weeks the planet appears to move against its usual direction relative to the distant stars in the stands. A car overtaking you fast on the inside on a motorway is similar: its place in your window keeps shifting throughout the pass. Nobody goes backwards; only the line of sight changes.
How often, and for how long?
Mercury comes back into the same alignment with the Earth about every 116 days. So periods of apparent backward motion recur roughly three times a year, each lasting about three weeks. The dates can be calculated precisely in advance, because what lies behind them is not a mysterious force but the well-known geometry of two orbits. For the outer planets these periods occur less often and last longer; Mercury's record frequency comes from its high speed in the inside lane. By the same calculation, the entire retrograde calendar of the coming decades can be worked out today.
All planets go retrograde
Apparent backward motion is not peculiar to Mercury; it is observed in Mars, Jupiter, Saturn and every other planet. Indeed it was one of the great puzzles of ancient astronomy: the old models that placed the Earth at the centre had to build elaborate mechanisms of circles within circles to explain retrograde motion. One of the most elegant achievements of the Sun-centred model is that it explains the appearance with no extra assumptions at all, purely through differing orbital speeds.
Catching Mercury in the sky
It is quite possible that you have never seen Mercury, and you are not alone; it is the hardest of the five planets visible to the naked eye. The reason is its orbit: because it circles so close to the Sun it can never stray more than about 28 degrees from it in the sky. So Mercury can be seen only in the evening twilight above the western horizon, or at dawn above the eastern one, close to the horizon and for a short time. There are a few favourable periods each year; the dates when the planet reaches its greatest apparent distance from the Sun are announced in sky calendars. It looks like a bright star, and the practical way to tell it from a star is that planetary light twinkles far less. The labour of the generations who tracked a small planet in these difficult conditions and worked out its orbit is the quiet foundation of our ability to calculate retrograde dates years in advance.
How should claims about "the retrograde period" be assessed?
Beliefs that electronic devices break down and communications go awry during retrograde periods are widespread, and many people feel they have confirmed them from their own experience. Yet when the claim is put into testable form, for instance by comparing records of faults and delays between retrograde periods and other periods, no pattern above chance level has been demonstrated. Devices fail and communication mishaps happen in every period; the ones that happen in retrograde weeks are more memorable. We explain how this selective recall works in our guides to confirmation bias and correlation and causation. This explanation is no disrespect to readers who find retrograde periods a valuable occasion for personal stocktaking; it is simply a record of what is happening in the sky.
Frequently asked questions
Does anything look different in the sky while Mercury is retrograde?
Not to the naked eye. Mercury is already a difficult planet to observe because of its closeness to the Sun; the backward motion becomes apparent only when its position is recorded over a number of days.
Why do retrograde dates vary by a day or two between sources?
The stationary points are the moments when the planet's speed against the backdrop falls to zero, and by definition the transition is gradual. Depending on the method of calculation, the start and end days may be given a day or two differently.
Has the effect of retrograde periods on human life ever been tested?
When claims of this kind are defined in measurable form, comparisons carried out on records have found no consistent effect peculiar to retrograde periods. We take up the logic of testing in our guide to testability.
How long do the retrogrades of other planets last?
The further away the planet, the longer the apparent backward motion lasts: about two to three months for Mars, and around four months for Jupiter and Saturn.
This guide draws on space agencies' educational materials on planetary motion and on the content of standard astronomy textbooks.