Sunrise, sunset and the Equation of Time

The days are finally getting longer. It is always a slow, subtle and most welcome change from the days of leaving for work in the dark and arriving home again in the dark.sunset-sun-abendstimmung-setting-sun-122443

Since the winter solstice on December 21, 2016 when the North Pole is tilted furthest from the sun the amount of daylight we receive has been increasing each day. The December solstice is the shortest day of the year (in terms of daylight) for us in the Northern Hemisphere and the longest for those in the Southern Hemisphere. Interestingly, this does not mean that the sun sets the earliest on the solstice. The earliest sunset was back on Dec 8, 2016 at 4:20 pm. On the December solstice though, the day length is shortest with only 9 hours, 8 minutes and 30 seconds of daylight – even though sunset was at 4:23 pm.

The earliest sunset actually occurs a few days before the solstice and the latest sunrise is a few days after due to two factors: The Equation of Time and the latitude of a particular location. The Equation of Time is the difference between the Apparent (or True) Solar Time minus the Mean Solar Time.

Apparent solar time is measured by direct observation of the sun (as with a sun dial). Mean solar time is measured with a clock and assumes a day is exactly 24 hours in length. A “true day” goes from solar noon (when the sun is at its highest point in the sky) to the next solar noon and this number varies slightly due to the tilt of the Earth’s axis and to the elliptical shape of its orbit around the sun.

As explained on the website,

On most days, solar noon does not occur at the same time as noon on your watch. Around the solstices, solar noon occurs a few minutes later than the previous day. For example, on December 21, 2016, the day of the Northern Hemisphere winter solstice, solar noon in New York will be at 11:54 AM EST. On January 3, the day of the latest sunrise, solar noon will take place 7 minutes later at 12:01 PM EST.

As solar noons increasingly occur later, sunrises and sunsets also steadily occur later each day after the winter solstice. This is why, a location’s earliest sunset occurs before, and its latest sunrise occurs after, the winter solstice.

I must admit that before researching this topic, I had never heard of the “Equation of Time.” The TimeandDate website offers much more detailed explanations on the topic as well as sunrise and sunset times around the world.

And just when I’ve come to understand the Equation of Time, Massachusetts is considering using daylight savings time year round. Connecticut has even proposed legislation for the 2017 session to this effect. This would mean that we would not “fall back” or “spring forward.” While we would gain more daylight in the latter part of the day, mornings would be dark. Such a proposal only makes sense if the entire New England region were to collectively do so. Such bills and proposals have come up in the past and will continue to be discussed.

For now, we’ve crossed the hump and the days are getting noticeably longer. While I do appreciate all that winter nights have to offer: walks with the dog by moonlight, crisp, cold air, beautiful skies filled with stars and planets (particularly Venus), I’ll be happy to leave the flashlight and reflectors at home…. And there still are not enough hours in a day!