Just what is summertime?
Published 18/06/2015 | 02:30
For thousands of years, people used sundials to tell the time - where the height of the sun and length of the shadows were important markers of the seasons and the days. Today, our time is based on amazingly accurate atomic clocks, but we still make an effort to keep in sync with the sun, using summer time, leap years and even leap seconds!
Atomic clocks work by trapping atoms - the tiny building blocks of the universe - and making them vibrate. When done in the right way, this can result in a clock that is good to one second in millions of years.
In Ireland, the Statutes (Definition of Time) Act in 1880 defined Dublin Mean Time as the legal time for the country, which was based around the local mean time at Dunsink Observatory outside Dublin, and was about 25 minutes 21 seconds behind Greenwich Mean Time (GMT). In 1916 the time difference between Ireland and Britain was considered inconvenient, and Ireland moved to the same time zone as Britain.
The concept of putting clocks forward (for the summer months) first took place during World War I in order to save energy and resources. It made better economic sense to wake and work during daylight hours, eliminating the need to use scarce fuel and power. The idea was first introduced by the Germans during the war and subsequently adopted by Britain in 1916, with the United States following in 1918. At the time Ireland was under British rule, so it was simply implemented here in parallel.
There have been some experiments along the way. Ireland fell out of synchronisation with Britain during World War II, with the introduction of 'Double Summer Time', when the clocks were advanced by two hours across Britain to allow more work to take place during daylight hours. This resulted in droll letters to newspapers, pointing out that when Aer Lingus offered a Dublin-Belfast route, one could leave Belfast at 10, and arrive in Dublin at 9:30, apparently before ever departing.
The energy crisis in the 1970s saw 'Daylight Saving Time' (DST) become widespread across Europe and North America, and in 1980 the EU began the harmonisation of daylight saving across Europe, which runs from the last Sunday in March to the last Sunday in October. However, the US continues to introduce its DST three weeks earlier than in Europe.
Every few years the EU re-evaluates the rationale for changing the hour. There are many arguments for and against, ranging from reducing energy and costs, maximising daylight hours during the warmer summer months, reducing traffic accidents and crime, impact on mental health, and that air quality in Mediterranean countries can be better in the evenings.
The arguments change as new scientific research emerges, but the EU is still positive about it, particularly because it gives more daylight in the evenings for our leisure activities.
Why will June 30 be a second longer this year?
We all learn about leap years in school, where sometimes February has 29 days. We're not always told why: we have leap days in our calendar to keep the Spring Equinox (when day and night are of equal length) on March 21.
Though few people know about it, we have leap seconds too. This is because the earth is rotating slightly more slowly than the atomic clocks (see page opposite) are ticking. Consequently, we need an extra second, about every 18 months, to give the earth a chance to catch up on the atomic clocks.
These seconds are introduced at the end of either June or December, and the last day of June 2015 will have a leap second.
There is a rule to tell us that we get a leap year basically every four years (though, there are special rules for 1800, 1900, 2000, 2100…). This is possible because the earth's rotation around the sun is very regular and predictable.
However, rotation around its own axis is more unpredictable, and is influenced by many things, including the moon, the atmosphere, the oceans and even the earth's core. This means there is no rule for leap seconds, and instead we must measure the rotation of the earth and predict when one is needed.