BEHIND THE SCIENCE OF SUMMERTIME

E all learn about and the last day of June leap years in 2015 will have a leap Wschool, where second. sometimes February has There is a rule to tell 29 days. We’re not always us that we get a leap What is the told why: we have leap year basically every four days in our calendar to years (though, there are keep the Spring Equinox special rules for 1800, Just what is (when day and night are of 1900, 2000, 2100…). This Summer equal length) on March 21. is possible because the Though few people know earth’s rotation around about it, we have leap the sun is very regular and seconds too. This is because predictable. the earth is rotating However, rotation Triangle? slightly more slowly than around its own axis is the atomic clocks (see more unpredictable, and is Lorraine Hanlon summertime? page opposite) are ticking. influenced by many things, Consequently, we need an including the moon, the extra second, about every atmosphere, the oceans and David Malone 18 months, to give the earth even the earth’s core. This NE of the celestial astronomers, who use a chance to catch up on the means there is no rule for highlights of our it as a reference point atomic clocks. leap seconds, and instead Oshort summer for the ‘magnitude’ These seconds are we must measure the nights is the so-called brightness scale. An OR thousands of years, In Ireland, the Statutes The world ever departing. changing the hour. There introduced at the end of rotation of the earth and ‘Summer Triangle’, asteroid belt around people used sundials (Definition of Time) Act in clock in The energy crisis in are many arguments for either June or December, predict when one is needed. made up of three bright Vega has recently been Fto tell the time – where 1880 defined Dublin Mean Berlin the 1970s saw ‘Daylight and against, ranging from VEGA stars called Vega, discovered and it may the height of the sun and Time as the legal time for shows the Saving Time’ (DST) become reducing energy and costs, Deneb and Altair, in harbour a solar system length of the shadows the country, which was Earth’s maximising daylight hours the constellations Lyra, with rocky planets. were important markers of based around the local rotation during the warmer summer the seasons and the days. mean time at Dunsink months, reducing traffic Why will Cygnus and Aquila, Altair is a little around respectively. closer to us than Vega, Today, our time is based on Observatory outside the sun. widespread accidents and crime, impact amazingly accurate atomic Dublin, and was about 25 across on mental health, and that Although they while Deneb has been clocks, but we still make minutes 21 seconds behind Europe air quality in Mediterranean are associated with estimated to be about June 30 be a different constellations 1,550 light years away. an effort to keep in sync Greenwich Mean Time and North countries can be better in with the sun, using summer (GMT). In 1916 the time America, and the evenings. in the night sky, their It is a blue super-giant time, leap years and even difference between Ireland in 1980 the The arguments change brightness, even against and about 20 times leap seconds! and Britain was considered EU began the as new scientific research second longer the backdrop of the the mass of our sun. If Atomic clocks work by inconvenient, and Ireland harmonisation emerges, but the EU is still DENEB relatively bright Deneb was in our solar trapping atoms – the tiny moved to the same time of daylight saving positive about it, particularly summer night sky, system it would extend building blocks of the zone as Britain. across Europe, because it gives more makes this stellar past the radius of Earth’s universe – and making The concept of putting which runs from the daylight in the evenings for this year? arrangement, or orbit. them vibrate. When done clocks forward (for the last Sunday in March our leisure activities. ‘asterism’, an easy spot. All three stars have a in the right way, this can summer months) first took to the last Sunday Looking to the east blue-white appearance, result in a clock that is good place during World War in October. However, Dr David Malone is a lecturer in after sunset at this time indicative of similar to one second in millions of I in order to save energy the US continues to the Department of Maths and of year, the first star surface temperatures years. and resources. It made introduce its DST three Statistics, Maynooth University to reveal itself is the of between 7,200 and better economic sense weeks earlier than in dazzling Vega, the apex 9,200 oC. Of course, their to wake and Europe. of the triangle. Altair nuclear cores are much, work during eliminating the need to use the clocks were advanced Every few years is to the lower right, much hotter – unlike our daylight scarce fuel and power. The by two hours across Britain the EU re- with Deneb to the left. Irish summer. hours, idea was first introduced by to allow more work to evaluates the Far away from light In the western sky the Germans during the war take place during daylight rationale pollution, you might see at dusk watch out for and subsequently adopted hours. This resulted in for the band of our galaxy, the spectacular sight by Britain in 1916, with the droll letters to newspapers, ALTAIR whose ‘Via Lactea’ ( Milky of Venus and Jupiter as United States following in pointing out that when Aer Way) runs through the they draw closer together The sundial uses the 1918. At the time Ireland Lingus offered a Dublin- Summer Triangle. through June, with shadows from the sun was under British rule, so Belfast route, one could Vega, at a distance of their closest approach to tell what time it is. it was simply implemented leave Belfast at 10, about 25 light years, is happening on June 30/ here in parallel. and arrive in an important star for July 1. There have been some Dublin at 9:30, experiments along the apparently way. Ireland fell out of before synchronisation with Britain during World War II, with the introduction of Why is the longest day ‘Double Summer Time’, when in the middle of June? Lorraine Hanlon

OST of us probably hemispheres is reversed at the never think of winter solstice. Mourselves as The exact moment of the being passengers on-board summer solstice, which occurs a spinning planet that whips between June 20 and June around on its axis of rotation 22, varies from year to year once every 24 hours, or that because of the influence of travels 970 million kilometres other planets on earth’s orbit on its 365-day journey around and the slight wobble of its the sun. axis. This year, the summer If the earth’s axis of rotation solstice is on June 21. In was perpendicular to the Dublin, sunrise is at 04:57 and sun-earth line, we would have sunset at 21.57, giving just over no solstices and no strong 17 hours of daylight. seasonal effects. However, Be glad we don’t live on the 23.5o tilt of the earth’s Venus, where the very small axis gives rise to different axial tilt of 3o provides no orientations of earth relative to seasonal respite from the the sun during the year, giving 462oC temperature caused by us both. its proximity to the sun and its At the summer solstice, the as reaching its maximum southern hemisphere, which runaway greenhouse effect. northern hemisphere is tipped height in the sky and the day of is tipped away from the sun, most directly towards the sun, the solstice has the greatest the sun’s rays at noon are less Professor Lorraine Hanlon, leading to more concentrated number of daylight hours in the concentrated and shadows is an associate professor sunlight and shorter shadows year. are longer – it is their mid- at the School of Physics, at noon. We observe the sun On the same day in the winter. The situation in the two University College Dublin

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