Today, Sunday 17 March 2013, it is the Spring Equilux throughout the UK (and possibly elsewhere too*) meaning that there are almost exactly 12 hours between sunrise and sunset.
This date differs from the Spring, or Vernal, Equinox (1102 GMT on Wednesday 20 March 2013) for a variety of reasons, which I explain in a previous post but here is a list of sunrise / sunset times for a variety of towns and cities throughout the UK:
|Town / City||Sunrise||Sunset|
As you can see the time between sunrise and sunset is not exactly 12 hours everywhere but this is the day of the year when that is closest to being true everywhere*. Yesterday the sun rose a couple of minutes later and set a couple of minutes earlier, and tomorrow the sun will rise a couple of minutes earlier and set a couple of minutes later, as the days lengthen.
Also, the reason that sunrise and sunset do not occur at the same time everywhere* is due mainly to the longitude of the town; the further east a town is the earlier it sees the sun in the morning, and the earlier it loses it again at night.
So happy Equilux everyone*!
* interestingly, the equilux does not occur on the same same day for everyone, it depends on your latitude. The closer you are to the equator the earlier the date of your equilux. For example the equilux in most US cities occurred yesterday, 16 March, and in cities near the equator there is never a day with exactly twelve hours between sunrise and sunset! Take Quito, the capital city of Ecuador (latitude 0 degrees 14 minutes south) for instance. The length of day there only ever varies between 12 hours and 6 minutes long and 12 hours and 8 minutes long!
At around 0500 GMT on 2 January 2012 the Earth was at perihelion, its closest approach to the Sun this year.
If that sounds confusing to you, and has you wondering why it’s so cold given that the Earth is at its closest to the Sun, then this belies (a) a northern-hemisphere-centric attitude (in the Southern Hemisphere it’s summer right now), and (b) a misunderstanding of what causes the seasons.
The Earth orbits the sun in a nearly circular orbit called an ellipse. The degree by which an orbit differs from a perfect circle is called the eccentricity, e. If e = 0 then the orbit is circular; if e = 1 then the orbit is parabolic, and therefore not gravitationally bound to the Sun. The Earth’s orbital eccentricity is 0.0167, meaning that it is very nearly circular, with the short axis of the ellipse being around 96% the length of the long axis. Thus, during perihelion Earth is 0.983AU from the Sun, while during aphelion (its furthest distance from the Sun, occurring this year on 4 July) Earth is 1.017AU from the Sun. (1AU = 1 astronomical unit = the average distance between the Earth and the Sun = 150 million km).
The seasons on Earth have really nothing to do with how close the Earth is to the Sun at different times of year. Indeed how could they, given that the difference in distance between closest and furthest approach is only a few per cent? The seasonal differences we experience are of course caused by the tilt of the Earth’s axis, which is inclined by 23.5 degrees from the vertical.
This tilt means that, as Earth orbits the Sun, for six months of the year one hemisphere tips towards the Sun, so that it experiences longer days than nights, becoming most pronounced at midsummer, at which point the Sun reaches its highest in the sky at noon. Simultaneously the other hemisphere tips away from the Sun, and experiences shorter days than nights, becoming most pronounced at midwinter, on which day the Sun is at its lowest noontime altitude.
The further you are from the equator the more pronounced the seasonal effects. In fact equatorial countries don’t experience seasonal variations, while the poles experience extremes with six-month-long winters and summers. The timing of perihelion and aphelion relative to our seasons is entirely random. The fact the southern hemisphere midsummer (21 Dec) almost coincides with perihelion (2 Jan) is simply that; a coincidence. Given that fact, there is no reason to be surprised that perihelion occurs so close to northern hemisphere midwinter: it has to happen some time and it’s a coincidence that it happens to occur within two weeks of midwinter / midsummer.
The northern hemisphere winter solstice occurs on 21 December 2012, at 1112 GMT. At this point the Earth’s north pole will be tipped away from the Sun. As seen from Earth, the Sun will stop its slow daily decent south in our sky – over the past six months the Sun’s mid-day height above the horizon has been decreasing steadily – and once again turn north, getting higher in the sky at noon each day, until it gets to its highest point in midsummer 2013.
The actual day of the winter solstice – in this case 21 December 2012 – is commonly known as midwinter, the shortest day, and is the day when the Sun spends least time above the horizon. The further north of the equator you are, the more profound the effect. Indeed if you live within the arctic circle the Sun won’t actually rise today.
Today, 22 September 2012, marks the moment of the Autumn Equinox. At 1449 UT (1549 BST) the Sun will cross from the northern hemisphere sky to the southern, and we’ll begin the slow approach to the Winter Solstice on 21 December.
The equinoxes (one in spring and one in autumn) are the two instances every year when the Sun makes that crossing from north to south and vice versa, and they’re commonly thought to be the days when day and night are equal length, but they’re really not, for reasons I’ve outline before:
- astronomers measure the timings of equinoxes, sunrises and sunsets based on the middle point of the Sun’s disk in the sky, so when you read a sunrise time it means the time that the centre of the Sun’s disk rises above the horizon. For a few minutes before that time the top of the Sun’s disk will already have risen, giving “daylight”.
- Even before this happens the sky is lit up by the Sun below the horizon, and we experience twilight. Most people would think that the sky is bright enough to call it “daytime” long before the Sun pops above the horizon, during the phase of civil twilight.
So today, even though day and night are said to be equal on the equinox, the “daytime” (i.e the start of civil twilight) started about 0630BST in Glasgow (where I am) and will end this evening around 2000BST, giving me 13.5 hours of “daylight”. (Londoners will have from about 0615 until 1930BST, or approx. 13.25 hours of “daylight”).
The day this year where I have exactly 12 hours of “daylight” (i.e. between the morning start and the evening end of civil twilight) is 11 October and this day is called the equilux. (In London the equilux falls on 12 October).
With summer coming to an end in the British Isles we start the return to the dark skies of autumn and winter. Depending on where you are in the country you will have been without truly dark skies for many weeks, maybe even months, as summer evening twilight lasts throughout the night during the summer.
This all-night-long twilight is almost gone throughout the UK, indeed anywhere on the mainland UK can see astronomically dark skies around 1am at the moment. Only the furthest north outpost of the British Isles still doesn’t have that opportunity.
On the island of Unst, the furthest north of the Shetland islands, lies the UK’s furthest-north town, Skaw, at 60°49′N and 00°47′W. This tiny village will see astronomical darkness return at 0043 on 24 August, lasting only 46 minutes until at 0129 the sun’s light begins to creep into the sky again.
The last time that astronomical darkness was seen at Skaw was on 18 April, over four months ago! Indeed this settlement is so far north that between around 13 and 29 June each year they never get out of civil twilight, meaning that the sky’s bright all night long!
Compare this with the furthest south town in the British Isles, Saint Clement in Jersey, in the Channel Islands. Astronomical darkness returned to Saint Clement on 4 July this year, having been absent since 8 June; only four weeks without true darkness!
Such is the effect of differences in latitude that these two settlements, separated by 1299 km, have such hugely different seasonal swings between summer and winter.
This weekend sees the addition of a Leap Second onto world clocks, happening between 23:59:59 on Saturday 30 June and 00:00:00 on Sunday 1 July 2012. This helps keep our clocks in sync with the real world, since a day on Earth isn’t a regular 86400 seconds long every day. We used to measure time according to the spin of the Earth, but now we use far more accurate atomic clocks. These two methods of time keeping can drift, hence the need for the occasional leap second
Since 1972 25 leap seconds have been added. The last leap second was added at 23:59:59 on 31 December 2008. But leap seconds themselves are irregular, and are decided on by the International Telecommuntications Union whenever the two time signals drift by more than 0.9 seconds.
Enjoy your longer night’s sleep!
The northern hemisphere summer solstice occurs today, 20 June 2012 at 2309 UT (which is actually tomorrow in the UK, 21 June 2012 at 0009 BST).
But surely the summer solstice is just the longest day. How can it “occur” at a specific instant?
That’s because we astronomers define the summer solstice as the instant when the Sun gets to its furthest north above the celestial equator. Or to put it another way, the instant when the north pole of the Earth is tilted towards the Sun as far as it can.
And this happens at exactly 2309 UT on 20 June 2012.
It’s important to remember though that while we are in the midst of summer, the southern hemisphere are experiencing their winter solstice, and their shortest day.
And how much longer is our “longest day”? In Glasgow, my home town, the Sun will be above the horizon for 17h35m13s today and tomorrow (20 and 21 June), a full seven seconds longer than yesterday, and eight seconds longer than 22 June.
Today is the first day of spring in the northern hemisphere! At 0514UT (GMT) this morning, 20 March 2012, the Earth’s axis of rotation went momentarily side-on to the Sun. For the past six months the Earth’s northern hemisphere has be angled away from the Sun, and now we’re angled towards it. Lot’s more info about equinoxes and equiluxes in a previous post.
As most people take delight in the lengthening daylight hours spare a thought for the amateur astronomers who, in a few weeks time, will be packing away their scopes for the summer, eagerly waiting for the darkening nights later in the year.
- Thirty days hath September,
- April, June, and November;
- All the rest have thirty-one,
- Save February, with twenty-eight days clear,
- And twenty-nine each leap year.
This year is a leap year, when the month of February has 29 days in it, rather than the usual 28. The rhyme above is a mnemonic to help us remember the days in each month, but it doesn’t explain why we need leap years, and why they occur once every four years.
To understand the reason for leap years we have to look to astronomy, and in particular to the orbit of the Earth around the Sun. The Earth orbits round the Sun in 365.256363 days. This is a bit awkward as it means that the year cannot easily be divided into a whole number of days. If we round the year down to the nearest whole number of days we get 365 days in a year, which is indeed what we have in most calendar years.
So why not just leave it at that? Isn’t 365 days close enough to 365.256363? After all it’s 99.93% of the actual year, which is nearly 100% right, yes?
Actually; no. In ancient Egypt, where they lived with a calendar year of 365 days, the seasons began to drift at a rate of one day every four years. If we had stuck with the Egyptian calendar of 365 days every year then the longest day, which we take to fall on 21 June in most years would fall on 20 June four years later, then 19 June four years after that, until over the course of 730 years or so the longest day would occur when our calendars said it was the middle of winter.
Obviously something needed to be done to fix this problem. Enter Julius Caesar who, in 46BC, introduced what is known as the Julian Calendar. In this calendar Caeser recognised what Greek astronomers had long known; that the year is closer to 365¼ days long. They didn’t know that the Earth went round the Sun in 365¼ days, but they knew that the seasons repeated themselves on a 365¼ day cycle, and not a 365 day cycle as the Egyptians thought.
To account for this more accurate measure of the changing seasons, and to align the calendar better with the real world, Julius Caeser announced that every fourth year would have an extra day in it, to occur at the end of February. This would allow the calendar to keep in line with the real changing seasons, so that the longest day would always fall on the same day of the calendar.
But by 46BC the seasons had already drifted a lot; in fact the Roman calendar was about 80 days behind the actual seasons, so Caesar proclaimed that 46BC would have extra days in it, and be 445 days long, so that the calendars would be aligned on 1 January 45 BC, at which point the new calendar of leap years would begin.
The Romans didn’t call these leap years though; that name came along about 1400 years later. They were called “leap years” because the occurence of them every four years caused festive days (like Christmas), which usually advanced one weekday per year, to suddenly leap forward by two days. For example, Christmas Day in 2009 fell on a Friday, in 2010 on a Saturday, in 2011 on a Sunday, but this year, in 2012, it will leap forward to a Tuesday.
Not the Whole Story
Of course things are never that simple, are they? In fact the year is not 365¼ days long either, it’s 365.256363 days long if you measure it in terms of how long it takes the Earth to go round the Sun, or 365.242189 if you measure it in terms of how long it takes the Sun to return to the same part of the zodiac (which is indeed what we need to measure if we want to track the seasons).
We no longer have a Julian Calendar of 365 days each year with 366 every fourth leap year. Instead we have adopted the Gregorian Calendar where:
Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100; the centurial years that are exactly divisible by 400 are still leap years.
So 1900CE wasn’t a leap year (nor was 1700 or 1800), event though it was due to be, but 2000CE was. This is to fine-tune our year to fit even better with the changing seasons. Without this slight tweak then even the Julian calendar would drift with the seasons, albeit not as drastically as the Egyptian fixed 365 day year.
Today the International Telecommunications Union are voting on whether to abolish the leap second. This miniscule measure of time is added in to our time-keeping systems every so often to make sure they align more accurately with the time as measured by the spin of the Earth.
The original definition of the second was 1/86400 of a mean solar day which is related to the speed of the Earth’s spin about its axis. We might call this the “Earth second”. However the Earth’s spin in not regular. To begin with the Earth is slowing down by a couple of milliseconds per century due to tidal breaking. This breaking action is as a result of the drag of the Earth spinning beneath the tides created by the Moon. In effect the Moon is “stealing” energy from the Earth, increasing in its orbit about us while our spin slows.
In addition to this discrepancy the Earth is occasionally wobbled off course by major geological events, such as earthquakes. The 2004 Pacific earthquake which resulted in the Boxing Day Tsunami actually caused the Earth to speed up by over 2 milliseconds.
To avoid the problem of an irregular length of day – and therefore an irregular length of second – scientists adopted the much more regular SI second, which is the length of time it takes for 9,192,631,770 cycles of vibration of atomic caesium. This “atomic clock second” is accurate to one part in ten billion, and since 1972 this has been the international standard in timekeeping.
But time kept using the the SI second doesn’t match exactly with time kept based on the spin of the Earth, which after all is the time we experience every day. In order to make these two time signals match leap seconds are added every so often. Since 1972 25 leap seconds have been added. The last leap second was added at 23:59:59 on 31 December 2008, and the next one is due to be added at 23:59:59 on 30 June 2012. But leap seconds themselves are irregular, and are decided on by the ITU whenever the two time signals drift by more than 0.9 seconds.
The argument for abolishing these additional leap seconds is that it creates problems for modern computing and navigation systems that use the atomic clock second. Every time one of these irregular leap seconds is added the world’s hi-tech time keeping devices need to check and adjust by one second. It would be far simpler for us to use only “atomic clock seconds”.
However if we were to ditch the leap second then our civil time keeping would begin to drift with respect to “real” Earth time, so that in thousands of years time our clocks might read 8am just as the Sun is setting. This might seem like a minor concern right now – after all a millennium is a long time – but it’s something that astronomers and scientists do need to consider to avoid future problems. One alternative would be to introduce a “leap hour” to be introduced every few hundred years to keep the clock aligned with the real world.