We know that solar time differs from legal time and that this difference varies throughout the year.
Are there any days when the difference is a round number: zero, one hour, or two hours?
If so, where and when?
The Papophoplon (see description here: theory and practice) highlights the days when the red curve (the adjustment value between the sundial and the watch) intersects one of the blue circles (round number). Example for Trouville:
For the 4 days identified above, the difference between the sundial and the watch is precisely:
- One hour, if this occurs during standard time, and
- Two hours, if this occurs during daylight saving time
But Trouville is a special location, practically on the Greenwich meridian.
Let’s take the Papophoplon calculated for Paris. We see that there are now only two days where the difference is a round number:
What is special about those days is that the equation of time is exactly the opposite of the longitude adjustment (which is zero for Trouville, but nearly 10 minutes for Paris).
To determine where and when the difference is a round number, we must therefore compare the equation of time with longitude.
The relationship is as follows: one degree of longitude corresponds to a 240-second time difference. Indeed, since the Earth completes one rotation (360 degrees) in 24 hours, one degree corresponds to 240 seconds.
We can deduce from the graph above that for locations beyond 3.5°, the longitude difference exceeds the maximum variation of the equation of time. Therefore, there will be no day in the year when the difference between the watch and the clock face is a whole number.
The same applies to locations beyond 4.1° west.
Between 3.5° east and 4.1° west, there are two further cases:
- Locations on the Greenwich meridian, or close enough to it (between 1.6° east and 0.9° west), where the difference between the watch and the dial is a whole number four times a year.
- Other locations, where this occurs twice a year.
We therefore have the following map:
If the difference is a full hour while standard time is in effect, this means that solar noon occurs at 1:00 PM. But if this happens while daylight saving time is in effect, noon will be at 2:00 PM!
You can see the potential!
Check out our special app to find out where and when to look for noon at 2 p.m. … and find it!