Noon

The lunch is astronomically the time of the passage of the sun center through the Meridian one location .

View from the north of Sesto in the Dolomites , behind it in the south the twelve of the Sesto sundial , above which the sun is at noon every day if it is seen from a location on the same meridian , 12 ° 22 ′ east .
(Painting by Konrad Petrides )

In places in Europe, the sun is exactly in the south at this moment and, at midday, it almost exactly reaches its highest point of the day in the upper culmination . The elevation angle of the sun at noon changes with the season from day to day between the solstices and depends on the geographical latitude of the respective location. But the daily highest level of the sun, or more precisely: its upper meridian passage , defines true local time (WOZ) there at 12 o'clock . Exactly at this time - true noon - at the location the sun is again over the same point on the horizon , the south point .

Times such as 12:00 p.m. CET or 1:00 p.m. CEST do not indicate the astronomical time of noon, because they refer to time zones and do not take into account the geographical longitude of the location or the time equation . The respective deviation from the true noon 12 ^h WOZ - when the sun is exactly in the south - results from

a possible change to daylight saving time ,
the distance of the location to the longitude 15 ° East, to which the time zone of the Central European countries refers,
and the differences between true local time and mean local time (MOZ), which fluctuate periodically over the course of the year , which are specified by the equation of time and can be up to 16 minutes.

In common parlance, lunchtime means a period of time that can differ significantly from the time of true noon . While in Görlitz, due to its location near the 15th degree of longitude, on December 25th, at Christmas , the sun is almost exactly at 12:00 CET in the south, in Madrid, Spain, on the same day, the daily highest sun does not reach until 13:15 CET reached about 2 minutes later than in Edinburgh, Scotland at 12:13 GMT .

Astronomical noon

Sun high at noon

The elevation angle h of the sun at noon or its zenith distance z depends on the geographical latitude B of the location and the seasonally fluctuating declination δ of the sun:

 h = 90° − z = 90° - |B − δ|    (90° − B ist die Kobreite oder Poldistanz)

 z = 90° − h = |B − δ|     mit  −23,45° ≤  δ  ≤ +23,45°

For locations close to the equator between the tropics (| B | <23.45 °) the sun passes precisely through the zenith twice a year , for locations within the polar circles it is below the horizon for days or even months at "noon" .

In the course of the day, the position of the sun and thus the shadow cast changes depending on the day arc . It is shortest at noon, as the sun now almost exactly reaches its high point of the day in the upper culmination when it passes through the Meridian . The shadow of a stick as a pointer ( gnomon ) of a sundial then points north of the tropic north, south of the tropic, south. Between the tropics, it depends on the time of year whether the shadow falls north or south at noon.

Time of the real noon

The time of the astronomical noon, the true noon , i.e. 12 ^h true local time , depends on the one hand on the location and its geographical or astronomical longitude . On the other hand, the time equation must also be taken into account in the calculation, which indicates the difference between true and mean local time (ZG = WOZ - MOZ). This difference fluctuates several times over the course of a year as a result of the ecliptic inclination and the elliptical shape of the earth's orbit . It is currently a maximum of +16 minutes and a minimum of −14 minutes. Four times a year the amount of the equation of time is zero and thus MOZ = WOZ; this was the case for 2019 on April 15, June 13, September 1 and December 25.

The changes in the equation of time to the previous day are each less than half a minute and are strongest shortly after the date of the winter solstice. At this time of year they are occasionally noticeable in everyday life, because the clear days are short and the changes in the duration of daylight are small. Since the times of sunrise and sunset around the solstice receive special attention, it can be noticed that they do not show a uniform course. Time specifications related to MOZ indicate the earliest sunset several days before and the latest sunrise several days after the date of the winter solstice , the day with the shortest daylight duration. In all cases, however, the true noon is the middle of a clear day.

Specifically, the true noon can be calculated as follows:

Calculate the difference between the longitude of the observer and the longitude of the meridian for which the zone time is defined; the Central European Time is defined for MOZ at 15 ° east longitude.
Since the earth rotates from west to east , the further west the observation point is, the later the sun will reach its highest point. Since a sunny day lasts an average of 24 hours, there is a difference of 4 minutes per degree of longitude or 4 seconds per arc minute .
The calculated time difference must be added to 12:00 p.m. if you are west of the meridian of the zone time, otherwise subtracted from 12:00 p.m.
The current value of the equation of time for the respective day can be found in tables or calculated using a formula and subtracted from the time already calculated.
If daylight saving time applies, add an hour.

Calculation example for the city center of Munich at a geographical longitude of 11.6 ° East on July 12, 2008 with the time equation ZG = −5 minutes in Central European summer time: 12:00 p.m. + 0: 04h · (15 - 11.6) - (−0 : 05h) + 1: 00h = 13:19 o'clock CEST .

Noon in geophysics

In geophysics , magnetic and charge effects of the high atmosphere are strongly related to the time of day - for example

the magnetic Sq variation and
the ion or electron density in the ionosphere (see also TEC ), which reaches its maximum shortly after the local noon and
is also of importance for radio communications and cosmic geodesy (e.g. GPS and VLBI , monitored, among other things, through regular measurement campaigns by the ITS ).

Noon in other contexts

In general, noon or lunchtime is commonly used with an often only roughly defined term for an intuitive time interval, for example between 12 p.m. and 2 p.m. Especially in hot areas, this period includes the midday rest or siesta . But “noon” can also mean an exact time, for example 12:00 o'clock, or a fixed time interval, for example for the lunch break . In this way, noon is used when specifying closing and opening times, similar to noon for noon (open: noon - 7 pm) in English-speaking countries . The meal eaten around noon is called lunch .

The earlier public time measurement was mainly regulated on the basis of astronomical time determinations. For this purpose, reference was mostly made to the current position of the sun, for which u. a. so-called midday cannons existed. In many places it was common to ring the noon .

Individual evidence

↑ ^a ^b The extremal values of the equation of time for 2019 are approximately +987 seconds (November 3rd) and −855 seconds (February 11th); accessed on September 23, 2019.
↑ The rate of change in the equation of time is up to half a minute a day.