<blockquote>
<p>Is solar insolation always maximum at noon local time in diurnal cycle figure?</p>
</blockquote>
<p>It depends on what one means by &quot;noon local time&quot;. If you mean local apparent solar time, then by definition that is when the top of atmosphere insolation is at its maximum for a plate oriented parallel to the Earth's surface.</p>
<p>However, if by noon local time, you mean time from your watch, computer, smart phone, stove, microwave, etc. (corrected for time zone, daylight savings time, and longitude), then solar insolation does not always reach a maximum at noon local time. The issue is the equation of time.</p>
<p>Your watch, etc., more or less tic in sync with atomic time. Except for sundials, clocks measure what is essentially the equivalent of a now obsolete concept called mean solar time.  The concept of mean solar time has been replaced with atomic clocks, which are much more precise and much more consistent measurements of time than are measurements based on the Earth's rotation with respect to the distant stars.</p>
<p>The difference between mean solar time and apparent (or true) solar time is called the equation of time. This difference is nontrivial, as depicted below as the difference between mean solar time and apparent solar time. (Others use the opposite sign convention, the difference between apparent solar time and mean solar time.)</p>
<p><a href="https://i.sstatic.net/Lh7WbwHd.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Lh7WbwHd.jpg" alt="Equation of time, depicted here as the difference between mean solar time and apparent solar time" /></a></p>
<p>As can be seen in the above graph, mean and apparent solar time differ by as much as a bit over 14 minutes in February and 16.5 minutes in November. The two key factors that contribute to the equation of time are</p>
<ul>
<li>The obliquity of the ecliptic, which results in a sinusoid with a period of two cycles per tropical year (365.24219 days), and</li>
<li>The eccentricity of the Earth's orbit, which results in a sinusoid with a period of one cycle per anomalistic year (365.259636 days).</li>
</ul>
<p>There are other factors that lead to a difference between mean and apparent solar time, but these other factors are very small compared to the huge differences caused by the obliquity and eccentricity.</p>
<p><strong>References:</strong></p>
<p><a href="https://aa.usno.navy.mil/faq/eqtime" rel="nofollow noreferrer">US Naval Observatory</a> page on the equation of time.</p>
<p><a href="https://en.wikipedia.org/wiki/Equation_of_time" rel="nofollow noreferrer">Wikipedia</a> page on the equation of time.</p>
<p>There are many others.</p>

CC BY-SA 4.0
2024-08-28T13:50:38.220Z
2024-08-28T13:50:38.220Z
David Hammen
<p>Maximum insolation occurs at <a href="https://en.wikipedia.org/wiki/Noon#Solar_noon" rel="nofollow noreferrer">solar noon</a>, by definition, in the modern world it is quite rare for solar noon to coincide with clock noon because of two main factors:</p>
<ul>
<li><p>Modern <a href="https://en.wikipedia.org/wiki/Time_zone" rel="nofollow noreferrer">zonal time</a> means that most countries have a clock time that approximates solar time at some point in the country to the country as a whole.</p>
</li>
<li><p><a href="https://en.wikipedia.org/wiki/Daylight_saving_time" rel="nofollow noreferrer">Daylight savings time</a> is present in a number of countries, usually shifting clock time by an hour.</p>
</li>
</ul>
<p>This means that, as a rule, the local clock time that is reported in the data has little or no relationship to the celestial movements that govern insolation and that originally gave rise to modern time keeping.</p>

CC BY-SA 4.0
2024-08-23T07:40:20.430Z
2024-08-23T07:40:20.430Z
Ash
<p>I am comparing different global models for various atmospheric fields. When I plotted diurnal cycle (at different locations) for top of the atmosphere net shortwave radiation for each model, I get maximum peaks at different times (all of them near noon local time which are not very far off). Is this normal? Is solar insolation ALWAYS maximum at noon local time? Or does it depend on various factors, like location, season, etc. ?</p>

CC BY-SA 4.0
2024-08-20T22:27:22.023Z
Solar noon varies with the longitude within the current time zone and by whether or not the area is on daylight savings time.
Brian Borchers
CC BY-SA 4.0
2024-08-22T00:14:56.030Z
So, as long as it is at the same location (longitudinally), every model should have insolation peak at the same local time?
Redshoe
CC BY-SA 4.0
2024-08-22T02:13:44.030Z
My comment was simply that local solar noon occurs at different times depending on one's longitude and that you'd have to take that into account in comparing the time of maximum insolation at different locations.  For example, solar noon was at 1:09 pm today in Albuquerque NM and at 1:17pm in Gallup NM which is about 130 miles west of Albuquerque.
Brian Borchers
<models><diurnal-cycle>
Is solar insolation always maximum at noon local time in diurnal cycle figure?
Redshoe
# Is solar insolation always maximum at noon local time in diurnal cycle figure?

(Asked by: username_0 on 2024-08-20)

I am comparing different global models for various atmospheric fields. When I plotted diurnal cycle (at different locations) for top of the atmosphere net shortwave radiation for each model, I get maximum peaks at different times (all of them near noon local time which are not very far off). Is this normal? Is solar insolation ALWAYS maximum at noon local time? Or does it depend on various factors, like location, season, etc. ?

**Tags:** models, diurnal-cycle

> **Comments:**
> **username_1**: Solar noon varies with the longitude within the current time zone and by whether or not the area is on daylight savings time.  
> **username_0**: So, as long as it is at the same location (longitudinally), every model should have insolation peak at the same local time?  
> **username_1**: My comment was simply that local solar noon occurs at different times depending on one's longitude and that you'd have to take that into account in comparing the time of maximum insolation at different locations.  For example, solar noon was at 1:09 pm today in Albuquerque NM and at 1:17pm in Gallup NM which is about 130 miles west of Albuquerque.  

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## 2 answers

### Answer by username_2 on 2024-08-28. Score: 1 | Accepted answer

> Is solar insolation always maximum at noon local time in diurnal cycle figure?

It depends on what one means by "noon local time". If you mean local apparent solar time, then by definition that is when the top of atmosphere insolation is at its maximum for a plate oriented parallel to the Earth's surface.

However, if by noon local time, you mean time from your watch, computer, smart phone, stove, microwave, etc. (corrected for time zone, daylight savings time, and longitude), then solar insolation does not always reach a maximum at noon local time. The issue is the equation of time.

Your watch, etc., more or less tic in sync with atomic time. Except for sundials, clocks measure what is essentially the equivalent of a now obsolete concept called mean solar time. The concept of mean solar time has been replaced with atomic clocks, which are much more precise and much more consistent measurements of time than are measurements based on the Earth's rotation with respect to the distant stars.

The difference between mean solar time and apparent (or true) solar time is called the equation of time. This difference is nontrivial, as depicted below as the difference between mean solar time and apparent solar time. (Others use the opposite sign convention, the difference between apparent solar time and mean solar time.)

[![Equation of time, depicted here as the difference between mean solar time and apparent solar time](https://i.sstatic.net/Lh7WbwHd.jpg)](https://i.sstatic.net/Lh7WbwHd.jpg)

As can be seen in the above graph, mean and apparent solar time differ by as much as a bit over 14 minutes in February and 16.5 minutes in November. The two key factors that contribute to the equation of time are

* The obliquity of the ecliptic, which results in a sinusoid with a period of two cycles per tropical year (365.24219 days), and
* The eccentricity of the Earth's orbit, which results in a sinusoid with a period of one cycle per anomalistic year (365.259636 days).

There are other factors that lead to a difference between mean and apparent solar time, but these other factors are very small compared to the huge differences caused by the obliquity and eccentricity.

**References:**

[US Naval Observatory](https://aa.usno.navy.mil/faq/eqtime) page on the equation of time.

[Wikipedia](https://en.wikipedia.org/wiki/Equation_of_time) page on the equation of time.

There are many others.


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### Answer by username_3 on 2024-08-23. Score: 3

Maximum insolation occurs at [solar noon](https://en.wikipedia.org/wiki/Noon#Solar_noon), by definition, in the modern world it is quite rare for solar noon to coincide with clock noon because of two main factors:

* Modern [zonal time](https://en.wikipedia.org/wiki/Time_zone) means that most countries have a clock time that approximates solar time at some point in the country to the country as a whole.
* [Daylight savings time](https://en.wikipedia.org/wiki/Daylight_saving_time) is present in a number of countries, usually shifting clock time by an hour.

This means that, as a rule, the local clock time that is reported in the data has little or no relationship to the celestial movements that govern insolation and that originally gave rise to modern time keeping.


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