How is solar time calculated ?
Solar time is calculated based on the position of the Sun in the sky. At true solar noon, the Sun is exactly at the meridian. This means that solar time is the same for all locations on the same meridian.
True solar time is obtained from legal time as follows:
Legal 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
It is important to note that mean solar time is based on a day of 24 hours, but the length of days varies throughout the year due to the eccentricity of the Earth's orbit and the inclination of its axis. Sundials generally indicate true solar time.
What is the accuracy of the time displayed on this website?
This depends on several factors, starting with the accuracy of your computer clock, which provides the legal time necessary to calculate your solar time.
Therefore, it is imperative that your clock be accurately set if you want to obtain the most accurate result.
The second factor influencing the precision of the result is the precision of your geolocation.
For example, there is a difference of more than 5 minutes between London and Oxford.
If geolocation is disabled in your browser or if the GPS signal is not available, you can still use the map to accurately determine your geographical location.
Finally, the calculation of the equation of time is the last factor to take into account in the precision of the solar time displayed on this site.
This is obtained using astronomical calculations aimed at determining the orbital parameters of the Earth and its position on its orbit.
We use:
- the semi-analytical theory VSOP2013 of the IMCCE to determine the Earth's orbital parameters,
- the P03 theory to determine precession,
- the IAU2000 theory to determine nutation,
- and finally the 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 precision obtained using all these methods is probably less than a second. But to maintain a safety margin, we can say with a fairly high degree of confidence that the inaccuracy of our calculations does not exceed a handful of seconds.
What can you do with this page?
You can, of course, check the accuracy of a sundial. Some dials are accurate to the minute! Will you be able to find one?
If you have a sundial that is not fixed to a support (for example, a horizontal dial), you can use solar time to refine its orientation. In this case, we recommend taking several measurements throughout the day.
You can also compare solar time in different locations by clicking on the map, which will give you an idea of the corresponding differences. For example, there is a difference of about three quarters of an hour between Alsace and Brittany.
The site can also be used to teach children or students about how sundials work and how solar time differs from legal time. They can also learn how to use the map to find the location of different places and discover the corresponding differences.
What about my data?
We store your display settings and your location on your device using technical cookies.
This information is not sent to our server.
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.