Wandering Noon
Created | Updated Feb 27, 2005
If you always thought that noon came at the middle of the day, think again. In fact, the timing of sunrise and sunset can vary independently, causing noon to wander about the day as much as 16 minutes. An explanation of why this is so requires knowledge of a good deal of space and elemental astronomy.
The Earth's Motion
The apparent motion of the sun across the sky is composed of three simple elements which, combined, make things a bit complex. A basic understanding of all these motions is important to an understanding of more complicated astronomical phenomena.
Rotation
It is best to start at the beginning. The apparent 'rise and set' of the sun are due to the earth's constant rotation1 which causes the sun to appear to rotate constantly around one's location. Although you might think the earth rotates once in every 24 hours, in fact it rotates only once every 23 hours and 56 minutes.
Orbit
The other four minutes are due to the fact that the earth is in a slightly different location in its orbit around the sun each day, causing it to be at a slightly different angle toward the Sun, which requires an average of four minutes2 to correct. This four minute figure is only an average, however, because the earth's orbit is not circular but is in the shape of an ellipse with the sun at one of the foci. In other words, at some points in its orbit the Earth is closer to the sun (winter in the northern hemisphere) and at some points it is farther away. The earth goes faster around the sun when it is closer to it, so the sun appears to go faster in the sky when the earth is closest to the end of the ellipse. This happens in late December.
Ecliptic
In addition to not being circular, the earth's orbit is not at right angles to the earth's axis. The earth's axis is tilted with respect to its orbit around the sun3. The effect is that the angle of the sun toward the earth varies over the course of the year, causing the sun to appear more in the sky in summer than winter. This affects the hours of daylight in a day, the apparent height of the sun at noon, and the apparent course of the sun from rise to set. It is this variation that causes the seasons, as the sun shines more on either the northern or southern hemisphere4. The day on which the sun is exactly over the equator is called the equinox (vernal or autumnal) and when is it at its farthest tilt it is called the solstice (summer or winter).
Analemma
Because of the combination of the ecliptic and the earth's elliptical orbit, the sun's midday position appears to move about the sky. If you took a picture of the sun at 12:00 PM each day for a year, as indeed has been done here, you would be left with a figure 8 type image. This phenomenon is called the analemma. The slant of the earth's axis is responsible for the figure of eight, the elliptical orbit is responsible for the figure eight having one bigger and one smaller half.
Wandering noon
Such basics having been established, it is now necessary to clarify what exactly is meant by 'wandering noon'. As already mentioned, the day when the earth's orbit tilts the north pole farthest from the sun is called the winter solstice and is the shortest day of the year in the northern hemisphere5. This occurs on December 21st, 22nd during leap years. Some may have noticed, however, that sunset starts getting later beginning some days before the solstice6. Thus evenings get longer even as mornings continue to get shorter until the 21st. Thus clock noon diverges from astronomical noon as much as 16 minutes. Note that clock noon is the time at which clock show 12:00 PM and is repeated exactly every 24 hours, while astronomical noon is the point directly between sunrise and sunset when the sun is over the meridian, i.e. due south in the northern hemisphere or due north in the southern hempisphere. It is around the solstice, when the length of the day changes very slowly, that this effect is the most noticeable.
What causes noon to wander?
The explanation comes from the analemma. Recall that the analemma is the pattern by which the noon location of the sun varies. What it means is that the sun is not always directly at the meridian when clocks read noon. We measure an occurence: noon, the daily apogee of the sun, which fluctuates subtly throughout the year, by dividing up its average period into twenty-four hours of sixty minutes, each sixty identical seconds long; no wonder noon appears to wander
For more information
To learn more about the analemma, or to see helpful animations, go to www.analemma.com.
1At a speed of 1037.5 mph at the equator and slightly slower as you move to higher or lower latitude, according to the Galaxy Song2Four minutes being 1/365th of 24 hours, although really the orbit is completed every 365.25 days3At an angle of 23.5 degrees4Which is why seasons are opposite in the northern and southern hemispheres5Although the same day is the summer solstice and the longest day in the southern hemisphere.6In 2003 the earliest sunset was December 14th in Britain
