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.

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.

You can of course check the accuracy of a sundial. Some are precise to the minute! Will you find any?

If you have a sundial which is not fixed to a support (horizontal sundial for example), it is possible to use the solar time in order to adjust its orientation. In this case, we recommend that you take several measurements during the day.

You can also compare the solar time of different places by clicking on the map and get an idea of the corresponding offsets. For example, there is a time shift greater than three quarters of an hour between Detroit and Boston.

Requests are made to the OpenStreetMap servers in order to display the map.
We therefore refer you to their privacy policy if you wish to know more.

These requests are made independently of us and we do not have access to your geolocation data.

In addition, our host, OVH, uses technical cookies. We refer you to their cookie policy.