Astronomical & visual time
Solar noon & true solar time
Read the sun's own clock. See true solar time ticking now, when solar noon actually falls, and the equation of time — the gap between the sundial and the wall clock — for any city.
The equation of time is the gap between apparent solar time and mean (clock) time, driven by Earth's tilt and elliptical orbit. It swings up to roughly ±16 minutes across the year.
How to use the solar noon & true solar time
Pick a location
True solar time depends on longitude, so the calculation uses the place's actual coordinates rather than its time zone.
Read solar noon
This is when the sun is genuinely highest — crossing the local meridian. It is rarely 12:00 on the clock, and in some places it is more than an hour away from it.
Compare with clock time
The gap between solar noon and clock noon is the combined effect of your longitude within the zone and the equation of time.
Watch it drift
Come back in a few weeks and solar noon will have moved. The equation of time swings by roughly half an hour across the year.
True solar time, mean time, and the equation of time
Before railways, every town kept its own time, set so that noon was when the sun crossed the local meridian. That is apparent solar time, and it is what a sundial reads. It has one inconvenient property: solar days are not all the same length, so a clock cannot keep it.
Two effects cause the variation. Earth's orbit is elliptical, so it moves faster when closer to the sun in January and slower in July. And its axis is tilted, so the sun's apparent motion is not parallel to the celestial equator through the year. Together these produce the equation of time — the difference between sundial and clock — which reaches about 16 minutes ahead in early November and 14 minutes behind in mid-February.
Mean solar time smooths this out by using a fictional sun that moves at a constant rate, which is what clocks keep. Standard time then goes further and applies one offset across a whole zone, so solar noon shifts steadily as you move east or west within it. At the western edge of a wide zone, solar noon can fall well after 1pm.
Add daylight saving and the divergence gets large. In western Spain — geographically aligned with London but on Central European Time — solar noon in summer falls close to 2pm, which is a substantial part of why the Spanish day is shifted so late compared with the rest of Europe.
When you'd reach for it
Sundial calibration
Convert a sundial reading to clock time using the equation of time for the date.
Solar panel orientation
Peak production tracks solar noon, not clock noon.
Architecture and shading
Design overhangs and glazing against the sun's real position through the day.
Navigation practice
Traditional celestial navigation works from local apparent noon.
Understanding your zone
See how far your clock has been shifted from the sun where you live.
Teaching astronomy
A concrete demonstration of orbital eccentricity and axial tilt.
Questions & answers
- What is true solar time?
- It's time measured by the sun's actual position: noon is the moment the sun is due south (its highest point). It differs from clock time because of your longitude within the zone and the equation of time.
- What is the equation of time?
- The difference between apparent solar time and mean (clock) time, caused by Earth's axial tilt and elliptical orbit. It ranges from about −14 to +16 minutes over the year.
- Why isn't solar noon at 12:00?
- Clock noon follows a fixed zone offset, while solar noon depends on your exact longitude and the season. The two can differ by tens of minutes.