Comment »Posted on Tuesday 21 April 2009 at 2:58 pm by Colin Stuart
In Space & Astronomy

I have to admit I have a bit of a soft spot for wine; good old grape juice is probably my greatest vice. As an astronomer at The Royal Observatory, Greenwich, the Moon also plays a big part in my life too, whether I’m being asked about how it was formed or looking at it through a telescope. So it will come as no great surprise that a story invoking these two staples of my life grabbed my attention.

The BBC magazine reported yesterday on Maria Thun’s Biodynamic Sowing and Planting Calendar, devised in the 1950′s. The calendar is based on the cycle of the Moon as it orbits around the Earth and its effect on living organisms on it. It particularly talks about how wine tastes better on different days of the month. Now obviously this calendar is nothing new, it’s been around for half a century. However, the interest comes in when you find out that Tesco and Marks & Spencer have adopted Thun’s musings, to the extent that they will only let wine critics sample their wares (or should that be ‘weres’) on days when her Moon calendar suggests their goods to be most palatable. On a day when Tesco have announced ‘credit crunch’ busting profits of £3.13bn who am I to argue with their business strategy? Well let’s take a closer look at this magical moon calendar.

Thun classified five different types of day based on the motion of our nearest neighbour in space. These days are “fruit”, “root”, “leaf”, “flower” and “unfavourable”, and apparently the wine will satisfy your tastebuds most completely on a “fruit” day. Quite how these days are defined is far from clear. However, it seems to form part of growing belief that the Moon has an effect upon human, animal and now even plant behaviour. From what I can tell, the thinking most often behind this notion is that as the Moon has such a significant tidal effect of the world’s water mass that it somehow must have an effect upon water in living things, altering their behaviour. Now I’m not normally one for diving into the maths of things in a blog entry, but this time I will make an exception (Jacob will be pleased!)

Let us turn to our good friend Sir Isaac Newton for a little mathematical inspiration. I want to find the force with which the Moon pulls on a 1 millilitre droplet of water. Newton’s Universal Law of Gravitation states that the gravitational force between two objects is proportional to the product of their masses divided by the distance between them squared. In equation form,

Might look scary but not if you break it down. In the above equation the letters mean the following:

• F is the gravitational attractive force we are after.
• G is a constant, just a number and it’s equal to 6.67 x 10-11 (units unimportant)
• m1 = mass of the Moon at 7.36 x 1022 kg
• m2 = mass of water particle. 1ml of water has a mass of 1 gram or 1 x 10-3 kg
• r = distance between the Earth and the Moon. Let’s use 3.63 x 108 (see below)

NB. If you want to nitpick, then the Moon doesn’t follow a perfectly circular orbit around the Earth. Instead it traverses around our planet in a squashed circle, or ellipse, and so at some points it is closer to the Earth than at others. To account for this I have used the closest distance the Moon gets to the Earth, above. Right, stick with me it’s about to get interesting.

Crunch all these numbers together and you get a force of gravitational attraction between the Moon and a 1ml droplet of water as roughly 4 x 10-8 Newtons. To put this into perspective that is the equivalent force that a speck of dust exerts as it rests on a table! Or put another, perhaps more apt way, the same as the force exerted on a table by about 250 millionths of your average (full!) bottle of wine; a miniscule force and hardly likely to have much of an effect.

Wine may well taste better on different days but a small piece of high school physics tells us that it is highly unlikely that our Moon will have a gravitational effect. It might stabilise our seasons, pull our tides and make our days gradually longer, but I’m afraid to say its gravitational influence doesn’t improve a smooth glass of Merlot. Another piece of lunar lunacy.

In fact, taking a closer look at Thun’s predicted upcoming “good” days for wine tells a truer tale. Join in me in raising a glass between Friday at 6pm and 9am on Sunday and you’ll realise that she probably just wanted an excuse for a good weekend bender!


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