What Does “Mercury Retrograde” Mean and What Really Happens: The Astronomical Explanation

These days, there is a lot of talk among those who deal with astrology and horoscopes about “Mercury Retrograde” and its supposed effects for the various zodiac signs, because from 5th to 27th August 2024 the planet closest to the Sun appears to “go backwards” in its apparent motion in the Earth’s sky with respect to the background of the fixed stars. The term “retrograde” comes from the Latin retrogradethat is, “that walks backwards”. I would like to point out that astrology is a pseudoscientific belief devoid of any predictive capacity; here we are interested in explaining in a simple way everything there is to know about apparent retrograde motion of a planet from an astronomical point of view. According to science, what is happening is not a real “reversal” of the planet around the Sun: it is aoptical illusion due to the shifting perspective from which we are observing Mercury’s motion. It’s not a rare phenomenon, by the way: Mercury does it three times a year, and the other planets in the Solar System regularly show retrograde motion as well.

What Does “Mercury Retrograde” Mean: The Optical Illusion Explained

The planets move around the Sun along orbits that always maintain the same direction of rotation: in particular, they orbit counterclockwise if observed from above the North Pole of the Sun. This direction of rotation is called I am progressingwhile the reverse direction of rotation is called retrograde. So technically no planet in the Solar System, including Mercury, is retrograde. Its retrograde motion is merely apparent. But then why does the trajectory of Mercury – but also of the other planets – sometimes seem to go backwards?

It is a question of point of view, or perspective: in fact, we do not observe the trajectories of the planets from a “fixed” reference system, but from a planet that in turn is in orbit around the Sun. This means that in our sky we see a combination of the motion of the planet and the motion of the Earth. If we take into account the fact that we too are in revolution with a distance from the Sun and with a speed different from those of the other planets, the result is that the apparent trajectories of the planets in our sky are “deformed” and can even “bend” to the point of temporarily going backwards.

If this sounds complicated, don’t worry: this animation of Mercury’s apparent motion relative to Earth clearly shows why the planet sometimes appears retrograde even though it is always prograde:

As you can see, Mercury is in retrograde phase when it is closest to Earth. This is because, being closer to the Sun, by Kepler’s second law it has a higher angular velocity than Earth: in simple terms, it takes less time to complete its orbit. From our point of view, when Mercury is closest to us it appears to move faster and in the opposite direction, even though its rotation sense with respect to the Sun is always prograde (i.e. counterclockwise in the animation above).

The retrograde phases of the planets of the Solar System

A completely similar argument also applies to the planets that are further from the Sun than the Earth. In the animation below, for example, you can notice that Mars also has retrograde phases at the moments of maximum proximity to the Earth:

In the case of Mercury, taking into account its revolution period (about 88 days) and ours (about 365 days), the moments of maximum proximity between the two planets occur three times every Earth year. Mercury is therefore retrograde three times a yearEverything is fine 116 daysfor a period of approximately 21 days. It is by far the planet in the Solar System that appears retrograde most frequently with respect to Earth. Venus and Mars are the ones that appear retrograde least frequently, every 584 days and 780 days respectively, because they are the closest to Earth and therefore have orbital periods quite comparable to ours. Jupiter enters a retrograde phase every 399 days, Saturn every 378 days, Uranus every 370 days and Neptune every 367 days.

Note that the “return time” of a planet’s apparent retrograde motion decreases with its distance from us. To understand why, imagine a planet that is extremely far away and therefore practically “still” with respect to us: it would become retrograde exactly once a year, because, not having its own motion, its apparent motion in our sky would depend only on our motion around the Sun.

For the same reason, this planet would be retrograde for 6 months (182.625 days), or half an Earth orbit.

This is why the outer planets of the Solar System farther away from us remain retrograde the longest: Mars is retrograde for 72 days at a time, Jupiter for 121 days, Saturn for 138 days, Uranus for 151 days, and Neptune for 158 days.

In conclusion, the retrograde motion of the planets is only apparent, it is far from rare, it involves all the planets of the Solar System and… according to science it has no influence on our lives.