Equinox – Me on the net

Seasons (again)

This is a bit of a repeat, since I almost forgot about writing this week. I decided to revisit the seasons thing.

English: Kukulkan at its finest during the Spr... — English: Kukulkan at its finest during the Spring Equinox. Chichen Itza Equinox March 2009. The famous descent of the snake at the temple. (Photo credit: Wikipedia)

We have just begun the season of Southern Hemisphere spring. This officially starts on 1st September and runs through to 1st December. Then summer starts and runs through to 1st March, then autumn runs through until 1st June, and winter extend to 1st September and the cycle repeats.

The reason that the seasons are defined like this goes back to 1780 when an organisation called “Societas Meteorologica Palatina” defined them as above. The organisation chose those dates because the seasons pretty much aligned with those dates in terms of temperature and rainfall and so on. The coldest three months in the Northern Hemisphere tended to be December, January and February, the warmest tended to be June, July and August, and so on.

Since the seasons are reversed in the Southern Hemisphere, the southern cycle is as described above. We have Christmas on the beach and spend July wrapped up and close to any source of heat!

Astronomers do it differently. They divide the year into four seasons, but the seasons are not aligned climatically, but are defined relative to the Earth’s position in its orbit around the Sun.

Because the Earth’s axis is tilted relative to its orbit around the sun, the axis is be tilted towards the sun at one time of the year and away from it six months later. When the axis is tilted towards the sun, the sun is at its highest in the sky and more energy is received on Earth per square metre than at any other time of the year. It’s summer and warmer. When it is tilted away, the sun is at its lowest and we receive less energy than at any other time of the year. It’s winter and colder. (But read on).

On Earth, when the sun is high it is in the sky longer than when it is lower. The day is therefore longest and the night is the shortest in the yearly cycle. When the sun is midway between its highest and its lowest, the day and the night are of equal length.

English: Midnight Sun in Tromsø, seen from the... — English: Midnight Sun in Tromsø, seen from the old port. (Photo credit: Wikipedia)

The time when the sun is highest or lowest in the sky is called a “solstice“, either a winter solstice, or a summer solstice. The times when it is half way are called “equinoxes“, either an autumnal equinox or a vernal equinox, and the night and day are equal in length. These are the four main signposts of the seasons, as used by astronomers.

Strictly speaking, to say “Today is the summer solstice” or “Today is the autumnal equinox” are incorrect. Since the day and night lengths are changing all the time, the solstices and equinoxes are points in time, not whole days.

English: Two equinoxes are shown as the inters... — English: Two equinoxes are shown as the intersection of the ecliptic and celestial Ecuador, and the solstice’s times of the year in which the Sun reaches its maximum southern or northern position. Español: Se muestran los dos equinoccios como la intersección del ecuador celeste y la eclíptica, y los solsticios momentos del año en los que el Sol alcanza su máxima posición meridional o boreal. (Photo credit: Wikipedia)

There are four lesser known and less important signposts of the seasons, they are Beltane, Lughnasadh, Samhain and Imbolc. I’ve used the Gaelic names, but they correspond, in order, to the Christian festivals of May Day, Lammas, Halloween, and St Brigid’s Day. These all fall more or less halfway between the four main seasonal signposts.

Astronomically the Winter Solstice, which occurs around 21st December in the Northern Hemisphere. Many sources identify the date of the solstice as the beginning of winter. Similarly the Summer Solstice is identified as the start of summer, and the equinoxes are identified as the start of their respective seasons.

This is odd, as the climatic seasons are usually considered to start three weeks earlier, with Northern Hemisphere winter climatically starting around the 1st December, and similarly for the other seasons. Starting the astronomical seasons on the 21st (or sometimes 22nd) of the month misses out 3 weeks or nearly a quarter of the season!

It’s also odd for another reason. The Northern Hemisphere winter solstice is when the sun is at its lowest point in its apparent position in the sky, so it is at its turning point in the cycle of the season and indeed the word “solstice” means “the point where the sun stands still”. It seems to me that this should be considered the mid point of the season, not the beginning of it.

This is obviously true for the summer solstice too, and the equinoxes, being halfway between the solstices are add the mid points of the sun’s climb or descent to the solstices. They too also should be the mid points of their seasons, not the beginning points.

If the solstices and equinoxes are the middles of their seasons, where are the start end points then? Well, they would then coincide with the Gaelic or pagan festivals of Beltane, Lughnasadh, Samhain, and Imbolc! For example Beltane is about halfway between the Northern Hemisphere spring equinox and summer solstice on 1st May.

Although Beltane is a Gaelic or pagan festival and has mostly fallen out of favour, some cultures do celebrate the festival and some of the customs persist, such as the custom of dancing around a Maypole. Beltane and the other three similar festivals coincide with important agricultural events, such as sowing seeds and gathering in of harvests, so were of interest in earlier times.

However, if the astronomical seasons starts and ends were to be moved to coincide with the Gaelic festivals they would not coincide with the climatic seasons. The reason for this is that there is a seasonal shift because of the time that the seas and land take to warm up in spring and to cool down in winter. This pushes the climatic seasons back a few weeks and the start of climatic spring in the Northern Hemisphere is pushed back to about the 1st March and the same for all the other seasons.

That’s why I think that the current idea of the astronomical seasons starting at the solstices and equinoxes is wrong! They should coincide with the Gaelic festivals instead, and then the astronomical and climatic seasons are related by the seasonal shift, instead of not being related properly at all.

Illumination of the earth during various seasons (Photo credit: Wikipedia)

Trust

English: Feathers and wedges are being used to... — English: Feathers and wedges are being used to split a large slab of sandstone. A three pound sledge hammer is being used to drive the wedges into holes drilled in the stone. The crack is just visible as a ragged line connecting the holes. (Photo credit: Wikipedia)

“Trust me, I know what I’m doing”. Sledge Hammer’s famous line encapsulates many things about trust in its seven words. The ironic twist is that the first iconic series ends with Hammer saying the words as he tries to dismantle an atomic bomb. He is not successful!

Trust is a belief that the person or thing that is trusted can be relied upon to do what is promised. There is trust between you and the bank. You trust them to look after the money that you hand over to them to invest and maybe pay you some interest. You also trust them to give you the money back when you request it. There may be conditions on the investment, such as minimum deposit periods or maximum withdrawals, interest rates and so on, but fundamentally you can get you money back.

California Bank & Trust Building in LA (Photo credit: Wikipedia)

Similarly the bank may loan you money, under conditions, which you can use to purchase a house, or a boat, or for any other reasons. They trust you to pay back the loan sooner or later, together with interest, and have the right to pursue you through the law if you don’t repay it.

The money in your pocket requires you to trust in it. After all the value of ordinary coins and notes in terms of the metal and paper is negligible, although gold sovereigns are nowadays worth much more than their nominal one pound sterling. Every coin or note represents something much more nebulous than the distinct coins and notes. Early notes had a “promise to pay” written on them, with the signature of a financial authority to encourage people to trust in them as money.

Hammer’s exhortation implies that his companions don’t trust him, which is ironic because, in a back-handed, gun-related way, he usually did. As is evidenced by the way that he encouraged a suicidal jumper to abandon his intents by shooting chunks out of the ledge that the jumper was standing on. His companions’ distrust was related to the non-standard way that he approached problems and their prior knowledge of his previous actions in such circumstances.

As in Hammer’s case, when two or more people interact, they need to trust each other in many ways. Threats are promises of harm, and there may be promises of benefits. Two people may form an alliance against a joint threat, and in such a case they need to trust each other. Each one trusts the other to back them up.

Often conditions are written down in the form of a contract. All the things that are expected by both parties, that are promised by both parties, or as many of them as can be, are written down, and both parties make their mark or sign the document. The contract can be authorised by a third-party or each party may merely carry away a copy of the document.

A contract strengthens the trust between two parties. If a contract in place, goes the reasoning, then all parties know exactly what is required of them, and what the consequences are if one party or another doesn’t do what is required. If there is complete trust between two parties, then no contract would be required, of course, but there never is complete trust.



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However we trust other people all the time without contracts or other documentation. In fact we are sometimes too trusting. Sometimes nefarious characters arrive on our doorsteps and we let them in if they, for example, claim to be from the Gas Board. It is recommended that we always ask for proof of identity if someone who we don’t know knocks on the door. Of course we have to trust the proof of identification if any is proffered, and it could conceivably be faked.

This brings up and issue about trust – we can never be absolutely sure that we can trust someone. We could know someone very very well and still not be absolutely sure that we can completely trust them. The extent to which we cannot completely trust them may be very very small of course.

We cannot even completely trust someone when we have a contract with them. Unexpected occurrences may occur which are not covered by the contract, but relate the the matter that the contract covers. If one of the parties to the contract dies then what happens to the provisions of the contract? Well, there are laws, of course, that relate to contractual matters and it may be that lawyers are needed to sort such matters out.

There’s another sort of trust, other than trust between people. We trust the laws of science. If we throw something up into the air we expect it to come down again. We expect and trust that the sun will come up tomorrow, and it appears that we are justified in our trust. Through many millennia we have trusted that the whole is a sensible logical place where everything has a cause and cause and effect go hand in hand.



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There is a dissenting voice and that voice is the voice of religion. Religions espouse the concert of miracles, that is occasions when the laws of nature are violated, as for instance, water is changed to wine, or a flood covers and destroys the whole earth.

We may trust that the world is a logical place, but we cannot prove that it is. If we keep throwing stones into the air, it is conceivable that one might not come down again. While we can verify that throwing stones into the continues to work, we may for some reason experience a case where the stone does not fall to the ground again.



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If the stone doesn’t come down, our instinct is to look for a reason why it did not, rather than suspect that the law of gravity has been repealed. We trust the law of gravity. The stone may have lodged on a roof of course, or been caught by a passing bird. After we have considered all the possibilities then we might suspect that the law of gravity as we know it has failed.

So we pass it over to the physicists to look into the matter, and they would ponder and experiment, and eventually, we hope come up with a modification to the law of gravity to cover our “special case”. And we can trust the law of gravity again. For now.

Animation showing the motion of a small body (... — Animation showing the motion of a small body (green) in an elliptic orbit around a much more massive body (blue). (Photo credit: Wikipedia)

So, the question arises, when we have found out all that there is to know about the Universe and so be able to predict anything with 100% accuracy. Well, suppose our knowledge of the laws of the Universe is 80% accurate. There’s an old adage that says that the first 80% of anything takes 80% of the time, and the remaining 20% also takes 80% of the time. In other words it is feasible that we could know all the laws of the universe and be able to apply them, but there probably isn’t enough time.

In the meantime, I’m going to trust that the sun is going to come up tomorrow, as, after all 80% is still pretty good!

Time waits for no man

English: Text: "You can't stop time... bu... — English: Text: “You can’t stop time… but you can turn it back one hour at 2 a.m. Oct. 28 when daylight-saving time ends and standard time begins.” (Photo credit: Wikipedia)

It’s approaching the equinox, that time of the year when the day and the night are of almost equal length. It’s the vernal or spring equinox here in the Southern Hemisphere, and the autumnal equinox in the Northern Hemisphere. For a number of reasons, the day and night are not of exactly equal length, and alternative definition is the time when the plane of the earth’s equator passes through the centre of the sun.

At around this time of the year many countries adjust their clocks to take advantage of the increasing daylight in the evening. Most countries who do this change their clocks forward in spring and back in the autumn, hence the mnemonic “spring forward, fall back”.

The reasons for the usage of “Daylight Saving Time” are debatable. The original intent was to align working day more accurately with the daylight hours while leaving more daylight time at the end of the day. Without Daylight Saving Time, people rose in the morning after an hour of usable daylight had occurred. It was during the two World Wars that “Daylight Saving Time” was first practised extensively in many countries.

Nowadays we are accustomed to “Daylight Saving Time”, and naturally there are dissenters who believe that it is unnecessary or counter productive. A farmer may point out that his cows don’t practise “Daylight Saving Time” and so the changes in the clocks are of no benefit to him, and can even cause him inconvenience.

“Daylight Saving Time” is around one hundred years old, so it is a fairly recent invention. Indeed the synchronisation of clocks, even in a single country, is a recent phenomenon. Now we have clocks synchronised globally.

Computers have clocks. Indeed the very functioning of a computer requires a very accurate clock, so it should be no surprise that we take advantage of this requisite to extend computer clock usage outside of the computer itself.

In the early days of computers, the clocks were not synchronised between computers. In fact that synchronisation had to wait for the development of networked computers. The people who used these isolated computers had to set the clocks manually, which was acceptable when computers were rare, but became a chore when computer usage started to climb and more desk had computers on them.

In a computer there are two sorts of clocks, a hardware clock and a software clock. The hardware clock is the fundamental clock in a computer system and it ticks thousands of times per second. If you’ve ever browsed the specs of a computer system you will likely have noticed the clock specification, the (these days) gigahertz rating. This is closely related to the clock speed, and the number of operations that the computer can perform in one second.



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The original computers had speeds rated in kilohertz, so today’s computers are of the order of one thousand million time as fast as the old klunkers.

The software clock is related to the hardware and takes the clock information and translates it into a human usable date and time. It can’t do that without reference to the outside world as the hardware clock consists merely of a stream of “ticks” and doesn’t understand the concept of seconds, hours, days, months and days of the week. There is no weekend in the hardware clock’s world.



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The reference to the outside world in the early days of computing meant the operator typing in the time, and the software clock relating that to a tick of the hardware clock. From then on the software clock just counts ticks and works out the human usable date and time from that.

As computers started to be networked together, a problem arose. Computer A’s and computer B’s clocks will have been set by a human to as close as the human can manage, but they may be several seconds apart, a lifetime in computer terms. This can cause issues like money appearing in bank accounts before the money disappears from the sending account when the transaction is automated. All transactions are automated these days.

At the same time as computers got networked, some far seeing people decided to set up a network of atomic clocks. These clocks are much more accurate than computers hardware clocks which can “drift”, because not all computer clocks tick at exactly the same rate. As a service this service is provided on the Internet and this has almost universally been adopted.

Your computer will contact a local time source, which contacts a less local time source, and so on until one of the top tier time sources is connected. Thus they all synchronise with the top tier time source. All the top tier sources synchronise with each other so eventually all computer clocks are synchronised.

A computer synchronises with its time source by basically sending a packet of data to its time source and the time source replies. The computer compares the times and repeats the process a few times to get an average, and then, since the packet has to go out and back, halves the average and estimates the time at the time source as the time sources knows it. Then it sets the hardware clock to match. It continually does this, constantly updating its clock as necessary, which gives a very accurate value for the local time.

One might question the necessity of this accuracy. Isn’t it being a bit pointless to set clocks with such nit-picking accuracy? In a news story which I can’t now track down, a financial organisation lost millions, maybe billions of dollars because they did not handle a “leap second” accurately. Automatic stock market trading programs made thousands of trades in the few milliseconds that the company was out of sync with the rest of the world. But to you or I, doing our “online banking”, it won’t matter.

It’s worth remembering that the world-wide time system is pretty new. In these days we are accustomed to be able to contact someone on the other side of the world and to know what the time is with the contactee. But this is new.

It used to be the case that the local time was a sort of local consensus and did not rely on clocks. Then when clocks became more common the local reference time source was the clock in the spire of the church. Time was still local as the church clocks were not synchronised.

As time keeping became more important, the local time zone might expand to cover a time or a city. Clocks could be synchronised in a small area by use of travelling clocks or watches, and really accurate clocks and watches enabled the explorers from Europe to travel the world.

The advent of long distance travel (by the railways) and of telephonic communications resulted in the need for consistent time information across countries and across continents. However, standard time was only legislated in the United States in 1918, and this subsequently spread to the parts of the rest of the world that were not using Greenwich Mean Time.

A plate indicating the Greenwich meridian in S... — A plate indicating the Greenwich meridian in Stidia, Algeria. Photo taken in July 2005 by François Noël. (Photo credit: Wikipedia)

The Solstice Again

Today is the summer solstice in the Southern Hemisphere, the time when the sun is furthest south in the sky and hence at its highest. From here on in, the days get shorter as we slide back towards winter.

In the Northern Hemisphere, it is of course the winter solstice, and those living there can expect the days to lengthen, as they move towards summer. Today is the Northern Hemisphere’s shortest day.

Seasonal lag means that we can look forward to the warmest months of the year after the solstice, and those unfortunate enough to live in the Northern Hemisphere can look forward to a couple of their coldest months before things start to warm up.

I read somewhere that winter months are the months when people tend to put on weight and this was attributed to the fact that in winter, in the coldest weather people tend to exercise less and eat more. The reduced exercise is attributed to the tendency to stay home in the warm, by the fireside to avoid the often hostile weather.

And the eating more is because, well, what else is there to do but eat, when you are trapped by the weather. Our ancestors used to use up all the reserves that they had laid up for just this occasion, the hams and preserves, dried fruit and root vegetables and so on.

When the summer solstice happens, the weather is warmer and better, so people can get out an exercise, and, for our ancestors at least, agriculture kept them on the move, and the aim was to replenish the stores for the winter months, hence an emphasis on growing rather than eating. Besides, most crops would not be ready for harvesting.



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The winter solstice is one candidate for the start of the year. It marks a definite point in the cycle of the year. It’s after the solstice (a few months after the solstice) that things start growing again. The summer solstice is probably not a good choice as things are humming along then, ploughing and planting, growing and nurturing so it doesn’t really fit as the start of the year.

The spring or vernal equinox falls in March, around the 21st in the Northern Hemisphere. This is also a candidate for the start of the year, but to my mind, it is too late. Winter is tailing off at that time, things are starting to grow and because of the seasonal lag, it’s the start of spring. The year, are I see it, is already under way.



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Interestingly our fiscal year ends on 31st March. This is the date used by individuals to account for tax obligations. In many countries using the Gregorian calendar, the fiscal year ends on 31st December and almost aligns with the (winter) solstice based year. Other countries which use other calendars have fiscal years which relate to the local calendar.

As I have said the summer solstice in the Southern Hemisphere falls on 21st December (in most years). It is an astronomical point in time, not a whole day and can happen on 20th December. In the decade from 2010 to 2020 it falls on the 20th on three occasions.



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The summer solstice, Christmas Day, and the official 1st January New Year Day all fall within just over a week of each other. There is good reason to suspect historical links between these days, and there is much debate on the actual historical relationship between these events.

It is often said that early Christians adopted the winter solstice in the Northern Hemisphere, to squeeze out or replace a pagan celebration at that time. This may or may not be the case (or it may be partially true), but what is evident is that many cultures outside of the Tropics of Cancer and Capricorn celebrate a festival at around the time of the solstice.

Between the two Tropics the sun is overhead twice in a year while the sun reaches a southerly point at the time of the southern solstice (winter in the north and summer in the south) and a northerly point at the time of the northern solstice, the hottest time occurs when the sun is overhead. This divides the year into unequal parts in these latitudes.

The climate of these regions is dependant on local conditions, such as whether or not the region is close to an ocean or is in the middle of a continent, and many tropical areas have wet and dry seasons, typically of unequal extents. One example know to many people outside the tropics is the monsoon season when a regions rainfall may predominantly happen.



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On the Arctic and Antarctic circle, at the solstices the sun just grazes the horizon at the summer solstice and the day lasts 24 hours. At the winter solstices the sun just barely reaches the horizon and the night lasts 24 hours. Closer to the poles the number of sunless days or days with the sum always above the horizon increase. At the poles the sun is below the horizon for three months and above it for three months. (I hope this is correct. I did research this a little, but I am not 100% sure).

Interestingly, I learnt recently that the sunset will continue to become later for the next few weeks. The reason for this according to the linked article is because we have tied our clocks to 24 hours exactly and the day is not exactly 24 hours long. Not only is it not exactly 24 hours, but its length varies during the year. In Wellington the sunset goes out to around 3 minutes to 9 and doesn’t dip below that time until 7th January 2015.

English: Sunrise at Winter Solstice (December ... — English: Sunrise at Winter Solstice (December 21, 2006 at 8 a.m.) as viewed through the doorway half way up Maiden Tower (Photo credit: Wikipedia)

(December data here, January data here).

While looking up these numbers I noticed that the day length in Auckland is nearly half an hour shorter up there. Also sunset is about a quarter of an hour later in Wellington meaning that when the summer weather finally arrives we will have an extra 14 minutes to enjoy the balmy evenings. That’s yet another reason to prefer Wellington over Auckland! We have more time to celebrate the solstice.

[Darn! I completed this on Monday but forgot to publish it. Better late than never, I guess!]

The Start of New Year

an old post card (Photo credit: Wikipedia)

According to sources on the Internet, the winter solstice in the Southern Hemisphere occurs at 10:51 am UT on 21 June (this year, 2014). That translates to 10:51 pm in New Zealand. Just as in the Northern Hemisphere the start of the year corresponds roughly to the winter solstice there, I like to think that the winter solstice in the Southern Hemisphere corresponds to the start of the year in the Southern Hemisphere. I don’t think that I would get much support to the start of the year officially changed, though!

The year can be divided into halves by the solstices, the winter solstice marking the sun’s most negative elevation with respect to the South Astronomical Pole since the previous June. From that moment in time the sun starts to move higher into the sky until, at or around 21 December, when the summer solstice occurs.

Midway between the solstices falls a time when the day and night are roughly equal in length. Around this time the sun crosses the celestial equator, and this time is called an equinox. There are two in the year, one when the sun is apparently moving south in the sky (the vernal equinox in the Southern Hemisphere) and one when it is moving north in the sky (the autumnal equinox in the Southern Hemisphere).

The Sun & the ecliptic rotation around the Ear... — The Sun & the ecliptic rotation around the Earth : The green Sun is the one of the vernal equinox (march), it is followed by a summer solstice Sun. Then automn equinox and winter solstice. The ground plane (latitude 50°N) is green, the rotating ecliptic plane is blue. Also represented are the celestial equator, the two tropics and the rotation axis. (Photo credit: Wikipedia)

Each of these quarter points of the year is or was celebrated with a festival of some sort, some of which, particularly the winter solstice were supposedly characterised by “unrestrained revelry“. The summer solstice was comparatively restrained, the vernal equinox was a celebration of new growth, and the autumnal equinox was a harvest festival, a gathering in and celebration of bounty produced by the year’s hard work.

What I wasn’t aware of is that there were other events called “Cross Quarter moments”. These are moments halfway between the equinoxes and solstices, and they are known as Embolc, Beltaine, Lughnasad, and Samhain. The Cross Quarter moments. the solstices and the equinoxes are set out in order for 2014 in the chart referenced here.

Two of the Cross Quarter moments I have heard of, Beltane and Samhain. Beltane falls between the vernal equinox and the summer solstice and is roughly at the beginning of May, so corresponds roughly with May Day. It is astronomically the beginning of summer, but seasonal lag means that the season starts a little later than this.

Children (usually girls) still dance around the maypole or maytree, but few of them, and probably few of the adults have any idea of the origins of this ritual. Although it probably is related to Beltane or the start of summer, the significance and symbolism of the maypole is still debated. Some of the possible suggestions seem dubious and far-fetched, and I don’t think that is wrong to suggest that they reflect the prejudices of the people that make them. In particular it appears that Puritan Christians may have over-emphasised some aspects of the dance and celebration to argue for its banning.

Morris dancing is also associated with a spring festival, usually Whitsun. It may possibly have been associated with Beltrane, but I don’t know the history of morris dancing, Whitsun and Beltrane or spring festivals in general well enough to assert this. There is a long tradition of ancient non-Christian rituals being adopted and given a Christian slant, so this may be possible.

Cotswold-style morris dancing in the grounds o... — Cotswold-style morris dancing in the grounds of Wells Cathedral, Wells, England — Exeter Morris Men (Photo credit: Wikipedia)

Samhain also has a long history and probably pre-dates Christianity. It is associated with the beginning of winter and marks the point where all crops are gathered and animals prepared for winter. Once again the Christian church has adopted the festival and the roots of “harvest festivals” are to be found in Samhain’s pre-Christian traditions.

The Christian church adopted the festival as All Saints (Hallows) Eve or Halloween. I note from the Wikipedia article that I linked to that some people consider that Halloween has no relationship with Samhain, but considering the similarities of the two traditions which happen at the same time of the year, I think that this seems unlikely.

Bonfires form a great part of the Samhain festival, maybe as an attempt to ward off the coming darkness of winter. I don’t think that it is a coincidence that there are still “celebrations” on 5 November, otherwise known as Guy Fawkes Day. An effigy of Guy Fawkes is burnt on a bonfire, in spite of the fact that Guy Fawkes was actually hanged.

All of the example above refer to the “Gaelic versions” of the various dates and festivals. It’s a bit simplistic to refer to a single “Gaelic version” as the dates and festivals have, naturally, changed over the years. Other cultures of course have their own versions of the various festivals. In the Tropics (between the Tropic of Cancer and the Tropic of Capricorn the sun is overhead at least once in the year, an obvious time for a festival!

Since we have just passed the winter solstice, we can look forward to longer days and shorter nights from now until the summer solstice, which for us in the Southern Hemisphere comes around 21 December. So far this year winter has been fairly mild and a little wet. As we move towards the vernal equinox we still have the bulk of winter to come, as the astronomical year does not match the climatic year because of the seasonal lag.

Nevertheless it is a time to look forward and one can understand why the winter solstice is a such a time. It is a time of feasting, of using up some of the stores put away at the time of the autumnal equinox, the salted beef and cured hams. It is a time to relax, for mending and repairing, and for staying out of the weather as much as possible, as the weather of winter means that essential tasks only will be undertaken and the rush of springtime is still ahead. While the end of winter may bring shortages , it is still near the beginning and the stores are still full.

Summer, Autumn, Winter and Spring

Revolving earth at winter solstice on the nort... — Northern winter solstice

On 21st June we in the Southern Hemisphere get our shortest day of the year. This corresponds to the longest day in the Northern Hemisphere of course, and my wall calendar, which originates from the Northern half of the planet says that 21st June is the start of summer. I believe that the official start of winter, here in New Zealand, is 1st June.

That started me thinking. One would expect that 21st June, the southern winter solstice, would be the middle on winter, since the earth is tilted furthest away from the sun in southern latitudes at that date, and that the seasons would change mid way between the solstices and the equinoxes in both hemispheres for similar reasons. The equinoxes are the days when the night and days are the same length in both the northern and southern hemispheres. (Pedants will notice that I’m not being precisely correct in my explanations of equinoxes and solstices, but that doesn’t matter for my purposes.)

It is obvious to anyone who has reached a sufficient age that the warmest and coldest parts of the year don’t correspond to the solstices and that the change from higher than average temperatures to lower than average temperatures and vice versa don’t happen at the equinoxes, though these latter events are probably not that noticeable. There is obviously some seasonal lag.

So I browsed to Wikipedia, which is a useful place to start, even if some people question its accuracy and veracity. I’ve not found it too bad, myself. Sure enough, there is an article on seasonal lag, and I’ve no reason to doubt the information there. To summarise, the authors of the article attribute the lag to the oceans which, because of the latent heat of their water absorb heat energy and release energy as the seasons change. I’m not sure that I completely understand the reasons for this, but there are undoubtedly deeper analyses on the Internet. The Wikipedia article contains one reference.

Apparently the seasonal lag is different in each half of the year. I believe that means that the four seasons are not all equal in length. Hmm, summer does seem shorter than the other seasons, but that may be only subjective. However, our shortest day is only four weeks away, so we will at least be seeing more daylight each day from then on. We will be on the upwards slide to Spring and Summer, even though Winter will not have bottomed out, and we can look forward to barbecues and a summertime Christmas!

Pohutakawa flowers. They bloom at Christmas, in early summer.