Out of Time: Time Measurement Oddities You May Not Know About


Last month’s Winter Solstice was the shortest day of the year, with 9 hours, 15 minutes and 16 seconds of daylight. The sun, however, did not set at the earliest time it would set during the year since that event took place earlier in the month. In fact, the earliest sunset in 2014 was at 4:28 pm (December 7, 8, and 9) while sunset on the Solstice was at 4:32 pm. A similar thing will happen in June of 2015, when the longest day of the year will have one of the latest sunsets (8:31 pm) but not one of the earliest sunrises (that would be June 11-18 at 5:24 am). The reason has to do with a concept called the equation of time, which is explained in detail on the website Time and Date. In a nutshell, the sun and earth are not perfect balls travelling in perfect circles so our traditional measures of time on a 24-hour clock do not equate to a nice, precise time for the Solstices to occur. For this reason, they occur on different dates and times each year and thus changes in the seasons are still actually measured by a sundial and not a nifty new Swiss timepiece in some lab.

Which brings us to the seasons. Specifically that most of us in the United States think of summer as the stretch between Memorial Day and Labor Day, which is more accurate in some ways than saying summer starts sometime around June 21st each year and ends during the brisk fall weather of late September. In fact, we actually observe two different sets of seasons in the United States, the traditional astronomical seasons we see on our calendars and the less common meteorological seasons used my many weather outlets in the country. The astronomical seasons are based on the sundial mentioned earlier so that a new season starts on every solstice or equinox. The meteorological seasons, on the other hand, equate more to the climate than to the position of the sun and moon, with summer defined as June-August, fall as September-November, winter as December-February and spring as March-May. The logic behind the climate-based seasons is it makes it easier to compare climate data when you are not mixing a colder month (like September) in with hotter months (July and August) to compare average highs and lows or snowfall. Don Lipman discusses the different types of seasons in more detail in an article he wrote for the Washington Post last September (note: the article also nicely explains how the seasons are different in other countries based on measures they have elected to use).

Returning to the sunset and sunrise times, there is yet another oddity in how these times are defined, depending on if you are observing the sun’s position, usable daylight or an ocean’s horizon. Many people will discuss what is true sunset or sunrise without ever realizing that each day actually has three different light periods that overlap and begin at dawn and end at dusk – civil twilight (should use headlights when driving but can still see enough detail without artificial lights), nautical twilight (when most people turn on their headlights), and astronomical twilight (daybreak to nightfall). I have found myself correcting people when they say they hate that it gets dark at 4:30 during the winter, mostly because it never technically does get completely dark that early. The reason for this is that the sunset we observe and see reported in almanacs (and in the first paragraph above) does not match when we stop seeing what is considered usable sunlight, which occurs at the end of civil twilight. Once the last usable light passes the horizon, when there is not typically enough light that can be used for outdoor activities but there is enough to use to identify silhouettes of objects to navigate, we have entered nautical twilight. The very last band of visible light brings the end of astronomical twilight and the darkness of true nightfall. If we revisit our data on the Winter Solstice of 2014, the official sunset time was 4:32 pm with a Civil Twilight end time of 5:03 pm, Nautical Twilight ending at 5:37 pm, and Astronomical Twilight ending at 6:11 pm. For a more technical definition of these terms, the website SunriseSunset offers a concise outline of each.

One final fact to ponder – while we take for granted the time zones in use today, they really did not exist in any standard form until 1884, when the International Meridian Conference declared Greenwich as the Prime Meridian (see Time and Date for a history of time zones). Even after that declaration, the United States did not adopt standard time zones until 1918 and France did not recognize the Paris time zone as being one hour off the Prime Meridian until 1911 and did not move to Coordinated Universal Time (UTC) until late 1978. For the most part, one can speculate that airline travel and computers, with their need for precise time measurement, were the main reasons we finally standardized time around the globe. But, as we have seen in the way we measure seasons, daylight and sunsets, time really is just relative.

Laura N. 

Comments

  1. very well written and informative

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