sunny day
A sunny day is the duration between two meridian passages of the sun . The latter is located exactly in the south at the meridian passage (seen from the northern hemisphere of the earth) and has almost its daily high .
A sunny day is not the same length over a year : its length deviates from the annual mean by a maximum of about 30 seconds.
True and mean sunny day
The time span between two successive meridian passages of the sun is of different size because the earth moves at different speeds on its apparent orbit around the sun due to the elliptical shape of the orbit ( second Kepler law ) and because the earth's axis is not perpendicular to the earth's orbit ( inclination of the ecliptic ) their orientation relative to the sun also changes over the course of a year.
The latter means that the apparent annual orbit of the sun cuts the meridian alternately at an angle and the projection of the daily orbit onto the celestial equator varies in size. The time (on average just under 4 minutes) for the small daily additional rotation of the earth beyond 360 ° is determined by the length of this projected section of path. Because of the elliptical orbital movement, the daily orbit on the apparent path of the sun already has changing lengths.
The daily changing sunny day is the true sunny day , its annual mean value is exactly 24 hours long mean sunny day , which is represented by a fictitious mean sun . The real sun is the real sun .
The different lengths of true sunny days add up in such a way that the true solar time temporarily “goes ahead” or “lags behind” the uniformly passing mean solar time. This deviation is given quantitatively using the equation of time .
The longest true days of the sun fall roughly on the solstices or the shortest on the equinox . A rule of thumb states that the true sunny day becomes longer towards the solstices, but shorter towards the equinoxes:
| date | Duration | Remarks |
|---|---|---|
| February 11th | 24 hours | |
| 26th of March | 24 hours - 18.1 seconds | 6 days after the vernal equinox (20.3.) |
| May 14th | 24 hours | |
| June 19th | 24 hours + 13.1 seconds | 2 days before summer solstice (June 21st) |
| July 26th | 24 hours | |
| 16th September | 24 hours - 21.3 seconds | 7 days before the autumn equinox (23.09.) |
| November 3rd | 24 hours | |
| December 22 | 24 hours + 29.9 seconds | for the winter solstice (December 22nd) |
literature
- Bernd Loibl: When is noon? . In: Stars and Space , ISSN 0039-1263 Stars and Space, Spectrum of Science, Heidelberg, 8-9 / 1996. Pp. 643-645
See also
Individual evidence
- ^ Jean Meeus: Mathematical astronomy morsels . Willmann-Bell, Richmond, ISBN 0-943396-51-4 , pp. 346 .
- ↑ Jahreszeiten 1998. In: de-Kalender. Retrieved June 24, 2014 .
