Today, the Earth’s Northern Hemisphere experiences the summer solstice—the longest day of the year. (South of the equator, of course, it’s the winter solstice). Officially, this moment occurs today at 1:16 p.m. EDT (10:16 a.m. PDT).
Being the longest day of the year—the day when the sun is in the sky the longest—you’d think this would also be the day when the sun rises at its earliest and sets at its latest, no? But, amazingly, it’s not.
Even a quick glance at any sunrise and sunset table will show this “flaw” around both the summer and winter solstices. So what’s going on here?
It's a great question, and one I receive at this time every year. One of the best explanations of this phenomenon is offered by the Royal Greenwich Observatory in their Short Special Information Leaflet No. 5 titled ‘The Apparently Odd Behaviour of Sunrise/set times near the Winter Solstice.’
“The winter solstice is the time when the Sun reaches its southmost distance from the celestial equator and hence, in northern latitudes is the day when the Sun is lowest in the sky at noon. This is, naturally, the shortest day of the year in northern latitudes. To many people it seems odd, therefore, that the time of sunrise continues to get later in the day after the solstice.
“The reason for this is that the Sun does not cross the meridian (when it is highest in the sky) at precisely noon each day. The difference between clock-defined noon and the time when the Sun is on the meridian is called the Equation of Time and represents the correction which must be applied to the time given by a sundial to make it agree with clock time.
“There are two reasons why the Sun is not on the meridian at noon each day. The first is that the path of the Earth around the Sun is an ellipse, and not a circle. The second is that the Earth's equatorial plane and its orbital plane are inclined to one another. The two effects add together to yield the Equation of Time which can amount to some 16 minutes difference between solar and mean time.
“The period when the Equation of Time changing fastest in the whole year is very close to the Winter Solstice. It changes by 10 minutes from December 16 to January 5. This means that the time at which the Sun crosses the meridian changes by 10 minutes in this interval and also that the times of sunrise and sunset will change by the same amount.
“Near the Solstice the Sun's height in the sky changes very slowly and the length of the day also changes slowly. The rapid change due to the Equation of Time dominates the very slow change in day length and leads to the observed sunrise times.”
If you could measure the sun's position at the same time every day throughout the year--say at 12 noon--you'd discover that it would trace out a figure-8. It would be highest, of course, in the summertime and lowest in the winter, but would not appear due south every day at noon. This figure 8 is that strange figure we've all seen on a map or globe at one time or another. It's known as the analemma, and at this link you can see graphically how the “equation of time” affects the sun’s position in our sky throughout the year.
And be sure not to miss some truly spectacular—and quite difficult to take—photos of the analemma by my friend and TWAN colleague Anthony Ayiomamitis.
--Dennis Mammana
21 June 2011
No comments:
Post a Comment