Mercury in retrograde

As I mentioned in my introduction, this week I was interviewed about superstitions on the RNZ Nights programme. You can listen to the item here.

What prompted this discussion of superstitions was the fact that the planet Mercury is in retrograde and, according to the internet, this could have a devastating effect on people's lives.

I've not paid too much attention to astrology in the past, apart from the obvious skeptical position of it being complete bunk.

The central claims of astrology are that the position of the planets at the time of one's birth somehow play a central role in determining your personality and life successes. Of course, were one to attempt to rationally explain how this would work, one might posit that the gravitational influences of the relative positions of the planets in our solar system might somehow pull on the water content of the baby's body which might somehow affect its development…. Arrgh… I can't go on making this stuff up!

And, of course, if you do the maths, the gravitational effect of the people in the delivery room would have more effect than that of the far distant planets. And, why is the time of birth somehow special? Why isn't it the time of conception, or the period in the womb when the brain is developing? And, if this is the case, do all people born at the same time (second/minute/day/month) have the same personalities? No, of course not!

Nevertheless, that's never stopped people from trying to explain things happening in their lives by external influences such as planets - those mysterious entities that moved about the sky at night. Back before humans had astronomy, and had viable explanations for exactly what those lights in the sky were, that might have been a reasonable conjecture. But, we live in an age of science now, and have known about the planets for hundreds of years, with the invention of the telescope dating back to the 15th century.

So, what is retrograde motion of planets? Well, when looking up at the night sky, most of the planets are moving in the same direction. But, by making careful observations over multiple nights, you might observe that some planets seem to change direction and go in an opposite direction for a time. That must have been super-confusing for people who didn't have an explanation for how things were behaving in our solar system.

From ancient times, various explanations were given for how this could work, with the assumption being that the planets revolved around the earth - because, of course, we're the centre of things! The height of inventiveness of these explanations came with the theory of epicycles, by Ptolemy which had planets revolving around the earth, but also doing their own circular motions. This complex system managed to sort-of explain the retrograde motions of some of the planets.

Of course, it all became simplified when Copernicus came along and described planets orbiting the sun - the heliocentric model. Epicycles were no longer needed, and the apparent retrograde motion of planets could be explained by their relative positions to the observer - us on Earth and the differences between our orbital periods.

So, modern people, accepting scientific explanations, would agree that Mercury (and other planets) have apparent retrograde motion because of an optical illusion. It's not really moving backwards - it just appears that way.

Mercury has an orbital period of 88 days. Earth has an orbital period of 365.25 days (or a year). As it turns out, Mercury is in retrograde motion several times a year.

During my interview on the RNZ programme, I explained that Mercury would end its retrograde motion on May 14th, but then start again in August, and again in December. It generally happens three or four times a year, so it's nothing special.

I can't figure out why this particular instance of Mercury being in retrograde has suddenly been seen as significant, but the internet is awash with advice on how to navigate the period successfully, mostly by buying gadgets and services.

Mercury is named after the god Mercury - known as the messenger god, and supposedly responsible for travel, communications, and commerce. Accordingly, astrologers think that you should avoid travel during the period, or avoid buying a new car, or watch out for electronic communication mishaps. All without any evidence to support these claims.

Sigh!