Seasonal shift
I was always under the impression that the equinoxes fell on 21 March and 21 September, dividing the year into four equal parts along with the solstices. However, I often read that the equinox will fall on a day other than the 21st. Surely there has to be an equal division of the seasons, relying on the Earth鈥檚 orbit around the sun? What could possibly change this?
鈥� The spring and autumn equinoxes are defined as the point in time when the sun is overhead at midday local time on the equator (in astronomical terms, the time at which the sun crosses the celestial equator). On the equinoxes there is an equal length of day and night everywhere in the world. The precise date of the equinoxes varies slightly; in the northern hemisphere the spring equinox usually falls on either 20 or 21 March and the autumn equinox on either 22 or 23 September (in the southern hemisphere the dates are reversed). This variation is simply because some years are leap years, so there is a shift in the calendar of a day or so relative to the seasons.
The equinoxes occur on exactly opposite sides of the Earth鈥檚 orbit around the sun, but it is interesting that the dates on which they fall do not divide the year into two equal halves. Take the average dates of the equinoxes and the mean length of the year, and the autumn equinox falls 186 days after the spring equinox, whereas the spring equinox is only 179.25 days after the autumn equinox. This is because the Earth鈥檚 orbit is elliptical and the Earth is closest to the sun in early January. In accordance with Kepler鈥檚 second law, which states that a line joining a planet and the sun sweeps out equal areas in equal intervals of time, this is the part of the year when the angular velocity of the Earth in its orbit is greatest. As a result, the half of Earth鈥檚 orbit from the autumn to the spring equinox takes less time to complete than the half between the spring and autumn equinox, when the Earth is further from the sun and moving more slowly. Consequently, spring and summer, during which there are more than 12 hours of daylight, last nearly seven days longer in the northern hemisphere than in the southern.
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Robert Harvey
Swindon, Wiltshire, UK
鈥� The assumption that 鈥渢here has to be an equal division of the seasons鈥� is incorrect. Ptolemy, who flourished in AD 140, tried to account for the unequal lengths of the seasons. As I describe in my book Astronomy: From the Earth to the universe (), current values show that spring lasts for 92 days, 19 hours; summer 93 days, 15 hours; autumn 89 days, 20 hours; and winter 89 days, 0 hours. Ptolemy deduced that the sun鈥檚 orbit around the Earth, assumed to be circular for what were then thought to be obvious reasons, was not centred on the Earth, or that the large circle of the orbit had an epicycle.
Our current explanation, thanks to Johannes Kepler in 1609, is that the Earth鈥檚 orbit around the sun is elliptical, with the speed of the Earth in its orbit varying according to Kepler鈥檚 second law. The Earth is closest to the sun (the perihelion) in early January, so it is moving fastest in autumn and winter, accounting for those seasons being the shortest.
Jay Pasachoff
Williams College,
Massachusetts, US
Nasal attack
The Last Word has told us why chilli burns our mouths, but why does mustard burn our noses?
鈥� While sense of smell is relayed by a small area in the nose called the olfactory area, naked nerve endings of many trigeminal pain fibres (the fifth cranial nerve) are also found in the olfactory mucous membrane. It is these, as well as the olfactory nerve endings, that the irritating substances stimulate. In fact, the irritating component is a part of the characteristic odour of substances such as peppermint, menthol, chlorine and, for some people, mustard. The extent and intensity vary from person to person. Stimulation of these nerve endings is also responsible for sneezing, watery eyes and other reflex responses to nasal irritants.
Vivek Jain
Baroda, Gujarat, India
鈥� Mustard contains a volatile oil (allyl isothiocyanate) that is easily carried into the nasal cavity where the endings of the trigeminal nerve are concentrated. This nerve is sensitive to the chemical, and its signals are relayed to the brain, producing the characteristic burning, nasal sensation that comes from eating mustard.
It is not the only nerve to be stimulated by mustard. Nerves throughout the skin respond to mustard oil, and mustard has been used in herbal baths at least since Roman times. Usually, mustard is mixed with other oils before being added to bath water. A variety of recipes and products are available.
Ed Hoare
Washington DC, US
This week鈥檚 questions
Fresher for longer
On a recent visit to Paris, I bought some milk that had not been pasteurised, but micro-filtered. The label claimed that filtering out the bacteria kept the milk fresh for longer than by pasteurisation. Does it? How does the process work and what stops the filter from getting clogged with milk fat? And would it get around European Union and other regulations that prevent, for example, unpasteurised milk being sold in Scotland?
Paul Keating
London, UK
Spare parts
How many organs can be removed from the human body before it stops functioning well 鈥� in other words, without the assistance of medical devices? I know we can live with one kidney, half a brain, no gall bladder, no tonsils, no appendix. Any more?
Jason McAllister
Sunbury, Victoria, Australia
Don鈥檛 think about it
As a secretary, my job involves a lot of typing. If I am not concentrating on what I am doing, I can type very quickly and accurately, but as soon as I think about it, where the keys are, for example, I type like a fool, extremely slowly and with numerous errors. The same applies to plenty of other activities, such as playing the piano, driving a car, even reading and talking. If you think about what you are doing you do it less efficiently. Why?
Lucy Kaye
London, UK