How to Use a Sundial Compass (and Read the Time Table Inside the Lid)
Quick answer: To use a sundial compass, unfold the brass arm (the gnomon), set the dial on a level surface in sunshine, and turn the whole case until the compass needle's north end lines up with the N mark. The shadow of the arm then shows sundial time. To get clock time, apply three corrections: the table printed inside the lid, your longitude, and daylight saving time.
This guide explains each step in plain English, what the "F" and "S" in the lid table mean, and two entries on the printed table that appear to be misprints.
What you are looking at
A sundial compass combines two old instruments in one case:
- A compass, to find north so the dial can face the right way.
- A horizontal sundial: the hour ring is marked on the dial plate, and the folding brass arm (called the gnomon) casts the shadow that points to the hour.
Pocket "compass dials" like this go back at least to the 18th century; the National Maritime Museum in Greenwich holds an 18th-century brass example with a folding gnomon and a compass needle (Royal Museums Greenwich, object AST1143).
How to set up your sundial compass
Our Anniversary Sundial Compass has short printed instructions and a time-correction table inside its lid. If you own our Brass Sundial Compass in Leather Case or another pocket sundial, the same steps work, and you can use the table below: the equation of time is the same for every sundial. Here are the steps:
- Raise the gnomon. On the Anniversary Sundial, slip the notch in the brass arm over the stud in the lid so the arm stands up over the dial. On a folding pocket sundial, lift the arm until it stands upright.
- Find a level, sunny spot. A table, a flat rock or a deck rail works. If the dial tilts, the shadow will point to the wrong hour.
- Line up north. Turn the whole case, not just the needle, until the north end of the needle sits over the N mark on the compass.
- Read the shadow. The edge of the gnomon's shadow on the hour ring shows the time.
One step the lid leaves out: true north. A sundial's gnomon should point to true (geographic) north, but a compass needle points to magnetic north. The British Sundial Society notes that the difference changes by location and from year to year, so you need to look up the current value and compensate when you read the compass (British Sundial Society: Setting up a sundial). You can look up the magnetic declination for your location with NOAA's free magnetic field calculators, then rotate the dial by that many degrees.
How to read the table inside the lid
A sundial shows apparent solar time: the time according to where the Sun actually is. Clocks keep mean time, which runs at a perfectly even pace all year. Because Earth's orbit is elliptical and its axis is tilted, the real Sun runs ahead of or behind the even clock by as much as about 16 minutes, depending on the date. Astronomers call that difference the equation of time (British Sundial Society: Equation of Time).
The table in the lid is an equation-of-time table. Each entry is a date, a letter and a number of minutes:
- F means the clock is faster than the dial: add the minutes to the sundial reading.
- S means the clock is slower than the dial: subtract the minutes from the sundial reading.
- An entry with no number (for example April 15) means the dial and mean time agree that day.
- The letter is printed only where it changes. Every entry after an F is also F until the next S, and vice versa.
- Values are rounded to 2-minute steps. For a date between two entries, use the nearer one.
The full lid table, written out
We checked each printed value against the equation-of-time formula published by NOAA's Global Monitoring Laboratory (NOAA: General Solar Position Calculations). Every entry agrees within about 2 minutes except the two marked below.
| Month | Date: correction (clock vs. dial) |
|---|---|
| January | 1: F 4 · 6: F 6 · 10: F 8 · 16: F 10* · 22: F 12 |
| February | 1: F 14 |
| March | 5: F 12 · 12: F 10 · 19: F 8 · 25: F 6 |
| April | 1: F 4 · 7: F 2 · 15: 0 · 24: S 2 |
| May | 14: S 4 |
| June | 4: S 2 · 14: 0 · 24: F 2 |
| July | 4: F 4 · 26: F 6 |
| August | 16: F 4 · 25: F 2 · 31: 0 |
| September | 6: S 2 · 12: S 4 · 18: S 6 · 24: S 8 · 30: S 10 |
| October | 6: S 12 · 14: S 14 · 27: S 16 |
| November | 10: S 14 · 27: S 12 |
| December | 2: S 10 · 7: S 8 · 12: S 6 · 16: S 4** · 20: S 2 · 24: 0 · 28: F 2 |
* The lid prints "16 F 16" for January 16. The NOAA formula gives about 9 minutes for that date, and the neighboring entries (8 and 12) point to 10. It appears to be a printing error.
** The lid's last column starts "Dec. 10 S 4". On December 10 the correction is about 7 minutes (between the printed 8 on Dec 7 and 6 on Dec 12). A 4-minute correction matches about December 16, so we read that entry as Dec 16.
From sundial time to clock time: the three corrections
- Equation of time (the lid table). Add F minutes or subtract S minutes, as above. This gives local mean time.
- Longitude. Clock time is set for the middle line (meridian) of your time zone, but your sundial shows the time at exactly where you stand. The Sun moves across the sky 1° of longitude every 4 minutes, so add 4 minutes per degree you are west of your zone's meridian, or subtract 4 minutes per degree east (British Sundial Society; NASA: The Sundial). Each hour of time zone covers 15°, so the U.S. standard-time meridians are 75°W (Eastern, UTC−5), 90°W (Central, UTC−6), 105°W (Mountain, UTC−7) and 120°W (Pacific, UTC−8).
- Daylight saving time. A sundial ignores daylight saving, so add 1 hour while it is in effect. In most of the U.S. that is from 2:00 a.m. on the second Sunday of March to 2:00 a.m. on the first Sunday of November. Hawaii and most of Arizona do not observe it (NIST).
Worked example (illustrative)
Say you are at longitude 80°W, in the Eastern time zone, on October 14, and the shadow falls on 12:00.
- Lid table, October 14: S 14, so subtract 14 minutes → 11:46 local mean time.
- Longitude: 80°W is 5° west of the 75°W Eastern meridian; 5 × 4 = 20 minutes, so add 20 → 12:06 Eastern Standard Time.
- Daylight saving time is in effect on October 14, so add 1 hour → 1:06 p.m. on your clock.
Why your reading may still be a few minutes off
- Magnetic vs. true north. If you aligned the dial to the needle without allowing for declination, the shadow points to the wrong hour.
- Latitude. On a horizontal sundial, the gnomon's angle to the dial should equal your latitude (NASA: The Sundial). A dial whose gnomon is set for one latitude will read less precisely far from that latitude.
- A level surface. Even a slight tilt shifts the shadow.
- Rounding. The lid table moves in 2-minute steps, so expect small differences between dates.
A pocket sundial is a beautiful, working piece of history, not a substitute for a watch. With the three corrections above, though, you can bring its reading much closer to the time on your clock.
Frequently asked questions
Why is my sundial exactly one hour off?
Almost always daylight saving time. Sundials follow the Sun, not the clock change, so add one hour while daylight saving is in effect.
Why is my sundial about 15 minutes fast in October?
That is the equation of time. In late October and early November the Sun runs about 16 minutes ahead of mean time, which is why the lid shows "S 16" on October 27. Subtract those minutes.
What do F and S mean on a sundial table?
F means the clock is faster than the dial (add minutes to the sundial reading). S means the clock is slower than the dial (subtract minutes).
Does the sundial work indoors or on a cloudy day?
No. It needs direct sunlight strong enough to cast a clear shadow. The compass, however, works anywhere.
Explore our sundial compasses
Our compasses & sundials collection includes the Brass Sundial Compass in Leather Case and the Anniversary Sundial Compass in a mango-wood case. If you bought one as a gift, feel free to send this guide along with it.