Astronomy Answers: Daylight Savings Time

Astronomy Answers
Daylight Savings Time


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1. Introduction ... 2. Solar Time and Time Zones ... 3. Daylight Savings Time ... 4. Abolish Daylight Savings Time? What Then? ... 5. Elsewhere in Europe

1. Introduction

In September 2018 it was in the news that the European Commission intends to propose to the European Parliament that the member states of the European Union decide for themselves how they want to handle Daylight Savings Time from 2019 on. What is Daylight Savings Time again? And what choices for it do we have? And what are the consequences?

2. Solar Time and Time Zones

Long ago, before there were accurate clocks and trains, time was read determined by the Sun. If the Sun was at its highest point in the sky, then it was exactly 12 o'clock noon. The time that is tied to the Sun that way is called solar time.

Because the Earth is round, the Sun cannot be highest in the sky at the same moment everywhere on Earth. The locations on Earth from where the Sun is highest in the sky at the same moment all have the same geographical longitude, so each geographical longitude has its own solar time. The solar time that belongs to a location at a particular geographical longitude is called the true solar time of that location. A well-adjusted sundial shows true solar time. For every degree of longitude that you travel to the east, the solar time is exactly 4 minutes later, and for each degree to the west it is exactly 4 minutes earlier. Venlo is at 6.17° east longitude and Middelburg at 3.62° east longitude, so solar time in Venlo is \( (6.17 − 3.62)×4 ≈ 10 \) minutes later than in Middelburg. If in Middelburg it is precisely 12 o'clock noon (Middelburg) solar time, then in Venlo it is already 12:10 (Venlo) solar time.

When in the course of the 19th century trains started travelling long distances east - west it turned out to be inconvenient that each station along the route had its own time. Suppose that there were a direct train connection between Middelburg and Venlo. If it is announced that the train will arrive in Venlo at 14:33, then is that 14:33 Venlo time? Or is it in the time of the departure station, Middelburg? And how much do they differ again? And do you have to add or subtract that difference?

Every country (or part of a large country) declared a standard time that should be used there for official purposes. That means that a certain longitude was chosen and that the time of that longitude is used in the entire country. All places that use the same standard longitude form a time zone.

For the places that are exactly on the standard longitude, the true solar time is on average exactly equal to the standard time, so then the Sun is highest in the sky at 12 o'clock noon on average. At places east of the standard longitude, the Sun is highest in the sky before 12 o'clock noon on average, and at places west of the standard longitude, the Sun is highest in the sky after 12 o'clock noon on average.

In almost all countries of the world, the standard time is tied to a longitude that is a multiple of 15°, which means that those time zones differ from each other by a whole number of hours. In Europe, the used standard east longitudes are 0° (Western European Time), 15° (Central European Time, Western European Daylight Savings Time), 30° (Eastern European Time, Central European Daylight Savings Time), and 45° (Eastern European Daylight Savings Time).

In the Netherlands and Belgium we use (at least through 2018) the standard time of 15° east longitude in winter and that of 30° east longitude in summer. The east longitude of the most westerly and most easterly locations in the Netherlands and Belgium are shown in Table 1 and it follows that both countries lie far to the west of 15° east longitude, so the Sun is highest in the sky quite a bit after 12:00 in both countries.

Table 1: East Longitude Variation Netherlands - Belgium

Netherlands Belgium
Most westerly 3.37° 2.54°
Most easterly 7.22° 6.41°

3. Daylight Savings Time

With true solar time, there are as many hours of daylight before 12 o'clock noon as after 12 o'clock noon. With time zones, the distribution of daylight across clock hours depend on how far east or west your location is from the standard longitude of your time zone, and you can shift that distribution by choosing a different time zone.

In all European countries (at least through 2018) and also in many countries outside of Europe the official time shifts between two time zones twice a year. In the spring the clock is advanced by one hour (to Daylight Savings Time), and in the autumn the clock is set back by one hour (to Standard Time). In summer the standard longitude of which the time is used is 15° more easterly than in winter. The result is that in summer one hour of daylight is moved from the morning to the afternoon. That is only useful if (in the same time zone) there is at least one more hour of morning sunlight in summer than in winter. At the equator the number of hours of daylight is the same every day, so near the equator it is not useful to switch between Standard and Daylight Savings Time.

Fig. 1: Day-Night Amplitude
Fig. 1: Day-Night Amplitude

Figure 1 shows the difference between the clock times (in the same time zone) of the latest and earliest sunrise and between the clock times of the latest and earliest sunset during a year, because those are practically the same. Both are determined by the geographical latitude \( φ \). At the latitude of the Netherlands and Belgium (49.5° - 53.5°) that difference is 4 - 4.5 hours.

The most often mentioned reason for wanting to keep a different time zone in summer than in winter is energy savings. The argument is that many more people are awake after 18:00 hours than before 06:00 hours, so it is better to have more daylight hours after 18:00 than before 06:00, because then the lights can stay off until later (on the clock) and so consume energy and cost money for fewer hours.

In winter there isn't an hour of daylight to spare to shift from the morning to the evening. In the Netherlands and Belgium in the middle of winter there is less than 8 hours of daylight, so it cannot be avoided that we spend part of our waking hours without daylight, and for the number of waking hours with daylight it doesn't matter if we use Standard Time or Daylight Savings Time then, so there must have been a different reason to move back to Standard Time during the winter.

It has never been shown conclusively that the introduction of Daylight Savings Time has saved any energy. I think that means that the energy savings is very small (compared to the total energy use) or non-existent.

4. Abolish Daylight Savings Time? What Then?

There are two "knobs" and one "switch" to adjust the amount of daylight after the working day:

  1. The switch: Should we stop shifting between Standard Time and Daylight Savings Time?
  2. Knob 1: What time do we want to have on the clock when the Sun is highest in the sky? Long ago this was 12 o'clock noon throughout the year. More recently (1977 - 2018) in the Netherlands and Belgium it was at about 12:40 during winter and about 13:40 during summer.
  3. Knob 2: What time do we want to have on the clock in the middle of the working day? With the typical office day from 09:00 to 17:00 that is 13:00, but it could be different.

If you want more hours of daylight after the working day, then you can get that by turning knob 1 to a later hour or by turning knob 2 to an earlier hour, or both.

What happens if we throw the switch or turn knob 1?

Fig. 2: 15-30° Fig. 3: True Solar Time Fig. 4: 0° Fig. 5: 15° Fig. 6: 30°

The above figures and Table 2 show the clock times of the earliest and latest sunrise and sunset in Rotterdam (the differences for other places in the Netherlands or Belgium aren't large) for some time zone scenarios:

The figures show the clock hours from 00 through 23 along the outer edge of the disk. During the clock hours of the black sector it is nighttime in all seasons. During the clock hours of the yellow sector it is daytime in all seasons. During the clock hours of the green segments, it is daytime during some days and nighttime during some other days, depending on the season. The arrow points at the clock time at which the Sun is highest in the sky on average. For the "15-30°" scenario there are two arrows: The earliest one applies in winter and the latest one in summer.

Table 2: Summary of Time Zone Scenarios

15-30 L 0 15 30
earliest rise 05:20 03:38 03:20 04:20 05:20
latest set 22:04 20:22 20:04 21:04 22:04
highest (av.) 13:17 12:00 11:42 12:42 13:42
latest rise 08:48 08:06 07:48 08:48 09:48
earliest set 16:29 15:47 15:29 16:29 17:29

From 1977 through at least 2018, the Netherlands and Belgium had the case of Figure 2: In the middle of winter, the Sun rises around a quarter to nine in the morning and sets around half past four in the afternoon. In the middle of summer, the Sun rises around a quarter past five in the morning and sets around ten o'clock at night. In winter the Sun is highest in the sky around 12:40, and in the summer around 13:40.

Notice that in Figure 2 the green sector of the morning is considerably smaller than the green sector of the evening: That is the result of the shift from Standard Time to Daylight Savings Time, which moves an hour of daylight from morning to evening in the summer. The other figures have no shifting between time zones. There the green sectors are equally wide. That is the effect of the "switch". Figures 3 - 6 are all the same except that the disk (but not the hour markers) has rotated. That is the effect of knob 1.

To get more daylight hours after the working day, you can also keep the clock time of the highest Sun the same but move the clock times of the school day and working day to an earlier time. If we turn only knob 2 and thus move the working day from 09:00 - 17:00 to 08:00 - 16:00, then that provides an extra hour of daylight after the working day, too.

I suspect that it is too much to ask to use knob 2, because people are very used to the current school hours, work hours, meal hours, sleep hours, and shop opening hours. Also, some laws and regulations mention specific clock times that are based on the average working day, and those laws and regulations would also have to be adjusted then. I expect that only the switch and knob 1 are realistic choices.

The great advantage of abolishing the shift between Standard Time and Daylight Savings Time is that then you don't have to adjust your clocks twice a year, and don't have to physically adjust to the sudden move of daylight from the morning to the evening (like one hour of jet lag) ― and people who work with animals don't have to adjust the clock times of their work with animals (because animals don't look at the clock).

If the shifting between Standard Time and Daylight Savings Time is abolished, then the question remains of which longitude we should use the time, i.e., to where do we turn knob 1? It stands to reason that in the Netherlands and Belgium we'd choose between always keeping the time of 15° east longitude (the current "winter hours") or always the time of 30° east longitude (the current "summer hours").

I think that at the relatively high latitude of the Netherlands and Belgium the effects of that choice during winter are more important than the effects during summer, because there is much less daylight in winter than in summer.

If we choose 30° (Figure 6), then in the middle of winter the Sun rises only around 10 o'clock in the morning. The morning (before 12 o'clock noon) then has only 2 hours of daylight, compared to over 5 hours for the afternoon. Then the morning commute is in complete darkness and the evening commute is near sunset. Then school children have to go to school well before dawn.

If we choose 15° (Figure 5), then in the middle of winter there are 3 hours of daylight in the morning and over 4 hours in the afternoon. Then the morning commute ends at sunrise and the evening commute begins at sunset. That is balanced better. Then school children go to school at dawn.

I would choose to always use the time of 15° east longitude in the Netherlands and Belgium.

5. Elsewhere in Europe

Below are figures that show the old situation in some other European cities: Huelva in Spain, Oslo in Norway, and Chelm in Poland. Huelva is very far west of its time zone longitude, so most of its daytime hours are well beyond 12 o'clock noon. Chelm is east of its time zone longitude in winter, and west of its time zone longitude in summer. Oslo is at high latitude so its green sectors are very large.

Fig. 7: Huelva Fig. 8: Oslo Fig. 9: Chelm


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Last updated: 2018-10-25