The Sidereal day is the length of the rotation period of a planet around its axis. It is determined by measuring the time it takes to return the planet to the same position with respect to the stars.
The average solar day, on the other hand, is calculated by measuring the necessary time to return the planet after a turn on itself in the same position with respect to the sun.
Since during the day the planet moves beyond itself also around the sun, the point of the planet taken as reference will have to travel an angle slightly higher than 360° to return to the same position as the previous day with respect to the sun, so the solar day is longer than the sidereal day by about 1st degree.
Consequently, in a sidereal year there is exactly one more sidereal day than the average 365 solar days.
While the sidereal day is invariant (apart from the secular slowdown of the earth's rotation due to the gravitational interaction with the other planets), the solar day varies throughout the year, since, being the Earth's orbit slightly elliptical, the speed varies in the various points of the orbit, being minimum at the aphelion and maximum at the perihelion (this is a consequence of Kepler's second law).
The difference between sidereal day and average solar day consist in 236 seconds. Having 236 seconds less in the sidereal day becomes of fundamental importance in astronomical observations.
For astronomical measurements, especially those carried out using the equatorial or galactic reference system, it is more convenient to refer to the sidereal day instead of the solar one.
For the measurement of time currently in use, we consider the length of the average calendar day consisting of 86,400 seconds.
For the definition and size of the second, see Wikipedia. The considerations and calculations below are all based on the length of the average calendar day.
Let's try to seriously consider changing the current convention for measuring the time of an average solar day to:
We will call this conventional measurement of the average solar day with: "decimal measurement of the day".
Let's now see how the Unit of Measurement of time to which we have been accustomed for many generations varies by measuring the time of an average solar day with the base 10 system.
For the moment we leave the names of subdivision of the daily time without changes, that is: Day, Hour, Minute, Second. Then we will see if it is appropriate to change them by defining them with suitable names for world internationalization.
If we divide the average calendar day into:
we get that in an average solar day there are 100.000 seconds, while in a day with current conventional calculation (sexagesimal) there are 86.400 seconds making the relationship between the two values it is obtained that the new value of the "second" calculated with the new convention "decimal measurement of the day" is 0.84600 times less than one current (sexagesimal) second.
As already mentioned the average calendar year cannot be modified for natural astronomical causes, and with the current conventional numbering (sexagesimal) it is composed of:
Now let's calculate the number of seconds in a year by referring to the two different ways of representing time:
365 days equals
(to these we must add)
the 5 hours that correspond to
the 48 minutes that correspond to
the 46 seconds that correspond to
5×60×60 = 18.000,0000 sec+
48×60 = 2.880,0000 sec.+
46 = 46.0000 sec.=
÷0,864 = 20.833,3333 sec.+
÷0,864 = 3.333,3333 sec.+
÷0,864 = 53,2407 sec.=
For facilitate the calculation, the number of annual seconds 36.524.219.9073 is rounded up to:
With the new "day decimal measurement" the average calendar year becomes composed of:
As you can already see the two numbers are the same, in the second a comma has been inserted to better distinguish the days from the hours, minutes and seconds (positional numbering).
One year (1 year) with the sexagesimal measurement system is composed of 31.556.926 sec.
One year (1 year) with the decimal measurement system is composed of 36.524.220 sec.
Both values obtained represent the duration of an average calendar year, the difference between them shows that the duration or size of the "Second" has changed in its absolute value.
The new "second" calculated with "decimal measurement of the day" gets a lower or shorter value, and equivalent to 1 × 0.8640 compared to the current second. Certainly the mathematicians and physicists members of the Conférence générale des poids et mesures (CGPM) will find a new definition, new conventions and new references more appropriate to the new SECOND base 10.
In fact, if we divide the first value relative to the number of annual seconds 31.556.926 (sexagesimal system), by 0.8640 we obtain exactly the second number equivalent to the number of annual seconds 36.524.220 (decimal system).