Noon

Noon is astronomically the time of the passage of the center of the sun through the meridian of a location.

At locations in Europe the sun is exactly in the south at this moment and reaches with the noon altitude almost exactly its daily peak in upper culmination. The angle of the sun at noon changes with the season from day to day between the solstices and depends on the latitude of the respective location. However, the daily peak of the sun, or more precisely: its upper meridian passage, defines 12 o'clock true local time (WOZ) there. Exactly at this time - true noon - at the location, the sun is again above the same point of the horizon, the south point.

Times such as 12:00 CET or 13:00 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 equation of time. The respective deviation from the true noon 12h WOZ - when the sun is exactly in the south - results from

• a possible changeover 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 periodically fluctuating differences between true local time and mean local time (MOZ), which are indicated by the equation of time and can amount to up to 16 minutes.

In common parlance, midday refers to a period of time that can deviate considerably from the time of true noon. While in Görlitz, due to its location close to the 15th degree of longitude, the sun is in the south on December 25th, at Christmas, quite exactly at 12:00 CET, in Madrid, Spain, on the same day, the daily solar maximum is reached only at 13:15 CET, about 2 minutes later than in Edinburgh, Scotland, at 12:13 GMT.

Astronomical noon

noon high

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

  h = 90° - z = 90° - |B - δ| (90° - B is the cob width or pole distance)   z = 90° - h = |B - δ| with -23.45° ≤ δ ≤ +23.45° 

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

During the course of the day, the position of the sun and thus the shadows cast change depending on the arc of the day. It is shortest at noon, because now the sun reaches almost exactly its daily peak in upper culmination at the meridian passage with the noon altitude. The shadow of a rod as pointer (gnomon) of a sundial points north of the northern tropic exactly to the north, south of the southern tropic exactly to the south. Between the tropics, it depends on the season whether the shadow falls to the north or south at noon.

Time of the true noon

The time of the astronomical noon, the true noon, thus 12h true local time, depends on the one hand on the location and its geographical or astronomical longitude. On the other hand, the equation of time, which indicates the difference between true and mean local time (ZG = WOZ - MOZ), must also be taken into account in the calculation. This difference fluctuates several times in the course of a year due to the ecliptic slope and the elliptical shape of the Earth's orbit. At present it amounts to maximally +16 minutes, minimally -14 minutes. Four times a year the amount of the equation of time is zero and thus MOZ = WOZ; this was the case for the year 2019 on April 15, June 13, September 1, and December 25.

The changes in the equation of time from the previous day are each less than half a minute and are greatest shortly after the date of the winter solstice. At this time of year they are occasionally noticeable in the daily routine, because the light days are short and the changes in the duration of daylight are small. As the times of sunrise and sunset around the solstice receive special attention, it may be noted that they do not show a uniform pattern. In this connection, times referred to MOZ give the earliest sunset several days before, and the latest sunrise several days after, the date of the winter solstice, the day of shortest daylight duration. In all cases, however, the true noon is the middle of a bright day.

Specifically, the true noon can be calculated as follows:

1. Calculate the difference between the longitude of the observer and the longitude of the meridian for which zone time is defined; Central European Time is defined for MOZ at 15° East longitude.
2. Since the earth rotates from west to east, the further west the observation site lies, the later the sun reaches its peak. Because a solar day lasts 24 hours on average and the degree grid of the rotating Earth is divided into 360 degree lengths, there is a difference of 4 minutes per degree of longitude (1°) or 4 seconds per minute of arc (1′).
3. The calculated time difference is to be added to 12:00 if west of the zone time meridian, otherwise subtracted from 12:00.
4. The current value of the equation of time for the respective day can be taken from tables or calculated via a formula and subtracted from the already calculated time.
5. If daylight saving time applies, add one hour.

Calculation example for the city centre of Munich on geographical longitude 11.6° East on 12 July 2008 with equation of time ZG = -5 minutes in Central European Summer Time: 12:00 h + 0:04 h - (15 - 11.6) - (-0:05 h) + 1:00 h = 13:19 h CEST.

Noon in geophysics

In geophysics, magnetic and charge effects of the high atmosphere are strongly related to the time of day - e.g.

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