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August 2011 Feature: Less than 24 Hours Between Successive Noons

What is Noon?

With people paying more attention to the clock than the Sun nowadays, the old-fashioned concept of noon may be lost upon modern-day humanity. Even the use of so-called "daylight-saving" time removes humanity yet another step from time as measured by the Sun.
When we use daylight saving time, we're pretending that it's one hour later than it is. That way we can get done with our tasks one hour earlier than we would otherwise to enjoy an extra hour of sunlight at the end of the day. It's hard for me to understand why we can't admit to ourselves that we're choosing to get up an hour earlier at this time of year.
Analemma on globeNoon, at least as far as the Sun is concerned, represents that fleeting moment when the Sun reaches its highest point for the day. If you drove a stake perpendicularly into level ground, the stake would cast its shortest shadow at noon. Or if you live in the tropics, it's possible to have no noonday shadow at all at a certain time of the year.

Clock Noon Versus Sun Noon

So there you have it. Noon is when the Sun soars to its highest point for the day, with the shadow of a stake pointing due north at temperate latitudes in the Northern Hemisphere, or due south at temperate latitudes in the Southern Hemisphere.
(Analemma on globe on right tells the difference in minutes between clock noon and sundial noon. Photo coutersy of Wikimedia Commons.)
Although the clock records an even 24 hours between successive noons all year round, it isn't all that often that successive noons as measured by the Sun equal 24 hours exactly.
For instance, let's say that you recorded the Sun's noontime shadow by the clock on August 1. One month later, on September 1, the noonday shadow would come about 6 minutes earlier by the clock. Two months later, on October 1, the noonday shadow would fall 16 minutes earlier. Three months later, on November 1, the noonday shadow would occur 22 minutes earlier.

Earth's Rotation Speeding Up in August, September & October?

Analemma SundialGiven that the Sun is returning to its noontime position in less than 24 hours time for the next three months, does that mean the Earth is rotating more quickly on its axis? Absolutely not! For our purposes, we can suppose the Earth's rotational speed is constant. What it means is that the Earth does not have to rotate as far for the Sun to return to its noontime position.
(Photo of analemma sundial on left courtesy of Alun Salt. For an explanation, click here and scroll down to description.)
In a nutshell, the Earth rotates in less than 24-hours time relative to the noontime Sun for a period of about 3 months, centered on an equinox. On the other hand, the Earth takes more than 24-hours time to rotate full circle relative to the noontime Sun for a pewriod of about 3 months, centered on a solstice.

copyright 2011 by Bruce McClure

July 2011 Feature * Sept 2011 Feature