I now want to say something about the stretching of the days in Luleå during my recent stay there. In Cork the shortest day is 7 hours 46 minutes, and the longest 16 hours 43 minutes: hence between midwinter and midsummer the sun has almost 9 hours to chase. In Luleå, the difference is nearly 20 hours; consequently the stretching of the days is more rapid, and I was hoping to be able to observe this effect over the course of the seven days I was there. But I was disappointed, and if you bear with me I shall explore the reasons for this.
My visit was from 18th to 25th February, during which time the days lengthened by 49 minutes. I know this from tables available at timeanddate.com.
Over those seven days, sunrise moved 26 minutes earlier, and sunset 24 minutes later. That adds up to a 49 minute increase (due to rounding). This is almost double the difference in Cork, where during the same week sunrise got 14 minutes earlier and sunset 12 minutes later. You can see all this in the table next to the map.
|Location map for Luleå, and sunrise/sunset table for Luleå and Cork. Minutes are rounded.|
To put it another way, when I arrived in Luleå, the day was 1 hour 20 minutes shorter than in Cork, and when I left, it was 1 hour 6 minutes shorter. Today it was a mere 24 minutes shorter. Come the 21st March (just to remind you) the days in Cork and Luleå will be the same length, namely 12 hours, indeed the same all over the globe. 
I said I was disappointed. I actually noticed no change while in Luleå, no stretching of the mornings or evenings. This was partly because for several days skies were overcast. But only partly. On reflection I realise that two factors work against each other. The further from the equator, the shallower the sun’s trajectory in the sky. This makes sunrise and sunset more gradual, extending the period of twilight; and it means that during the northern spring, even though the daily increment in sunlight becomes greater as you travel north, the stretching of the days becomes harder to mark.
I cast my mind back to Trinidad where I spent a year in 1968. In the tropics the sun never rises and sets far from 6 o’clock. A typical sunset conversation would go: “You noticed the sun’s setting much later now?” ... “Yes, tonight it was 7 minutes past. A few weeks ago it was 4 minutes past.”
Here is a table comparing Luleå, Cork and Trinidad for the week in question. It shows that where the daily increment in daylight is greater, twilight is also longer. 
The paradoxical consequence is that in Trinidad, even a tiny difference in day length can be more noticeable over a short period of time than a very significant increase over the same period in Luleå.
I have a set of three diagrammes which may help to illustrate this. They apply to the winter solstice, 21st December, and show sunrise, sunset and the sun’s altitude at midday, comparing the cases of Luleå (midday sun altitude 1°), Cork (15°) and Trinidad (56°).
Daylength table anomaly
I'll now mention an anomaly I can't get to grips with. This arises from studying the daylength tables in preparation for writing this blog post.
In the spring, as the sun sets further and further north each day, the daily increment in day length ought to peak around the equinox (21st March), and after that gradually lessen. From equinox to midsummer, though the days continue to get longer, the increment from one day to the next becomes less and less marked. When midsummer arrives, the day to day increase is zero; the sun no longer sets further north but stands still and then starts to set further south; so that after 21st June (the solstice) the day length begins to decrease again.
Why is it called the solstice? A digression
Solstice is a bad word. It means sun stands still in Latin - but who knows or cares for Latin? In Swedish it's solstånd (sun-stand) and in German Sonnenwende (sun-turn). Much better.
Thus in Cork, the daily increment in day length peaks in mid March at 4 minutes 1 second, and begins to decline after the 27th. During April the daily increment slows, so that at the end of that month the daily increment is down at 3 minutes 35 seconds. The decline in the daily increment continues during May, down to under 2 minutes. By 12th June it's down to 54 seconds. All that is good and just as it ought to be.
But the day length table for Luleå tells a strange tale.
Throughout February the daily difference is just over 7 minutes - oddly 4 seconds longer at the beginning of February that at the end. For most of March - when you would expect the daily day length increment to be at its maximum – it dips just under 7 minutes. Then, throughout April, when the daily increment ought to be slowing, it again tops 7 minutes. The daily difference is 7 minutes 22 seconds on 30th April and continues to rise, peaking, extraordinarily, in the second half of May, at 7 minutes 40 seconds. And it doesn't dip under 7 minutes till 9th June. Weird! I must find out why this is, something to do with the Earth wobbling on its axis or being a funny shape I suppose.
 In most of the tables I've looked at, it's actually 18th March when the day length hits 12 hours. The leap day on 29th February distorts the picture of course, but even adjusting for that, you get the 19th March. I don't know why this is. And what about the two Poles I ask myself? I haven't found any table for them. My understanding is that they have only one day and one night, sunrise at the North Pole on 21st March and sunset on 21st September. South Pole vice versa. So is “12 hours all over the globe” correct or not?
 About twilight. I've used civil twilight: the limit of which is defined by the sun's centre being 6° below the horizon. Solar illumination is insufficient, even under clear weather conditions, for terrestrial objects to be clearly distinguished, and artificial light is needed to carry on most outdoor activities. There are actually three definitions of twilight, the other two being nautical twilight and astronomical twilight, but civil twilight is most relevant for my purposes today.