## How is solar time calculated ?

It is more precisely called the true, or apparent, solar time. It is noon at true solar time when the Sun passes the meridian. Therefore, the solar time is the same for all places located on the same meridian.

Solar time is obtained from civil time as follows:
Civil time - Time zone = UTC (mean solar time on Greenwich meridian)
UTC + Longitude shift = Mean solar time
Mean solar time + Equation of time = True solar time

A distinction is made between the average solar time, for which a day is always 24 hours, and the true solar time, which corresponds to the real position of the Sun in the sky.
The length of the days varies throughout the year due to the eccentricity of the Earth's orbit and its obliquity (the inclination of the Earth's axis). They are sometimes a little more than 24 hours, sometimes a little less.
True solar time is the most commonly indicated time on sundials.

## How accurate is the time given on this site?

It depends on several factors, starting with the accuracy of your computer clock, which provides the calendar time for calculating your solar time.
Your clock has therefore to be accurately adjusted if you want to obtain the most accurate result possible.

The second factor influencing the accuracy of the result is the accuracy of your geolocation. For example, there is a difference of more than 5 minutes between London and Oxford.
If the geolocation is deactivated in your browser or if the GPS signal is lost, you can always use the map to locate yourself precisely.

Finally, the calculation of the equation of time is the last factor that can affect the accuracy of the solar time displayed on this site.
It is obtained using astronomical calculations to determine the orbital parameters of the Earth and its position in its orbit.
We use for this:

• the IMCCE semi-analytical theory VSOP2013 to determine the orbital parameters of the Earth,
• P03 theory to determine precession,
• IAU2000 theory to determine nutation,
• and finally Newton's method as well as some trigonometric calculations to obtain the true anomaly as well as the ecliptic latitude and longitude of the Earth.

The accuracy obtained by using these methods is probably below a second. But, keeping a margin of safety, we can say with a fairly high degree of confidence that the inaccuracy of our calculations does not exceed a handful of seconds.