The application developed by the author allows you to compare the hours of sunlight in each municipality with and without summer time.
As every year, on the last Sunday in October, next weekend the time change will arrive on time. If you are one of those who consider the time change an annoying hassle, I have good news for you: this October 25 could be the last time we have to turn our clocks back.
The reason is the decision of the European Parliament to end the practice of time change. This decision has been supported by a public consultation of 4.6 million Europeans, which found an overwhelming 84% in favor of abolishing the time change. Among the Spanish respondents, the vote in favor far exceeded 90%.
The reasons for abandoning the traditional time change are varied. Most of us agree with the most obvious one: the time change is a hassle that causes us a couple of days of discomfort. There is also considerable controversy about the supposed energy savings generated, as well as evidence of a higher incidence of traffic accidents.
Winter or summer hours?
The Member States of the European Union have been called upon to make the decision to adhere to winter or summer time, permanently, before April 2021. Embracing winter or summer time permanently is not a simple decision, and it is not without controversy either.
From a socio-political point of view, this decision presents difficulties. Among them, it stands out the possibility that neighboring countries that currently share a time zone take advantage of different schedules. It would be undesirable for Belgium, the Netherlands and Germany, for example, to choose different times. Effective coordination among member countries will require dialogue, political will, and diplomacy.
An even more fundamental problem is that the time change mitigates the effects of an astronomical phenomenon that will continue to be there after the rule is abolished: the variability of hours of sunshine throughout the year. For greater difficulty, this variability depends not only on the day of the year, but also on the position of the observer.
Solar diagrams and how to read them
The study of the relationship between hours of light, position and date is lost in the mists of time. It is not exaggeration or poetic license: the subject began to be studied in the times of the Babylonians, it already appeared in Claudius Ptolemy’s Almagest and has been part of the daily life of, for example, sailors, from the first editions of the nautical almanacs seventeenth century.
Today, with quick and easy access to databases and software packages, it is easy to simulate the effects of possible scenarios in any location in the world.
To illustrate this article, I have developed a small application (source code here) that generates diagrams of sunlight (without taking into account atmospheric effects) throughout the year for any municipality in the EU with a population of over 100,000 inhabitants. I invite the reader to look for yours or, failing that, the closest one, either in the interactive applet or in the list of Spanish and European municipalities.
In the following diagram you can compare the situation of the city of Barcelona with a change of time (as at present) and with permanent summer time. Note how the time change keeps the sunrise time roughly stable around 8 a.m., while with a permanent daylight saving time, winter sunrises would occur after 9 a.m.
A more compact way to visualize this same information is by representing the three possible schedules in a single graph with different colors.
Exploring different possibilities, we observe how the distribution of daylight hours depends dramatically on latitude (the position on the north-south axis), being more uniform the closer we are to the equator. This is shown by this comparison between Stockholm and Santa Cruz de Tenerife.
Effects of length
Geographic longitude, that is, east-west position, is also crucial. In fact, this is the reason why we have different time zones (the famous “one hour less in the Canary Islands”).
As an example, let’s see the diagram corresponding to A Coruña and Maó (Menorca), two of the westernmost and easternmost towns (respectively) of Spain (we exclude here the Canary Islands towns that, although they are even more western than the Galician ones, are apply to another time zone). We see that, despite being at a similar latitude and belonging to the same time zone, sunrise and sunset occur with a little more than an hour difference between the two.
