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How to Read a Sundial Compass in Either Hemisphere

The sundial engraved on your card is not a decoration, and it is not a watch either. On most days of the year it will disagree with your phone, and depending on where you are standing, it can disagree by more than two hours.

That is not a manufacturing defect. It is what happens when you measure the actual sun instead of the fiction we all agreed to live by.

This is the part of the manual that does not fit on a card.

01What you are actually measuring

There are two kinds of noon.

Apparent solar noon is the moment the sun crosses your meridian — the highest point it reaches in your sky, on this specific day, at this specific longitude. Your shadow is at its shortest. This is a physical event, and a sundial measures it directly.

Clock noon is 12:00 in a time zone, and a time zone is an administrative object roughly 15 degrees of longitude wide, drawn around borders and railway timetables rather than around the sun.

Boston and Detroit are both on Eastern Time. Solar noon in Boston arrives at 11:44. In Detroit it arrives at 12:32. Forty-eight minutes apart, because the sun does not know about the state line.

Your card reads the first kind of noon. Your phone reads the second. Everything below is about the distance between them.

02Reading it: the method

The engraved sundial works the same way sundials have worked for four thousand years, which is to say a shadow falls across a graduated scale and you read where it lands.

See every cutout on the card in 3D, sundial fan included, before you take it outside — Tool Explorer →

FIELD SKILLS · SUNDIAL CLOCK Move the sun yourself and watch the shadow swing Drag the hour across the dial: the gnomon shadow tracks it, and the three corrections show up as the gap between dial time and clock time. TRY THE TRAINER →

One. Find true north — not magnetic north. The two differ by anything from a couple of degrees to more than fifteen, depending on where you are. If you have no compass, the sun itself will do: in the northern hemisphere the shadow at solar noon points due north, and the sun sits due south at its highest. No compass and no clock either? Then the classic arc breaks the circle: mid-morning, mark the shadow tip and scribe an arc through it around the gnomon; when the afternoon shadow tip touches that arc again, the halfway line between the two marks is your true north–south meridian. Patience is the price — the navigation guide walks it with drawings.

Two. Lay the card flat and level. A pocket, a rock or a rucksack lid is not level, and neither is a car bonnet — the place everyone photographs a sundial and nobody should read one. Ten degrees of tilt will cost you more accuracy than any of the corrections in the next section will buy back. The field has a free level reference: standing water — a puddle, a pot lid filled to the brim. Sight the card against it and the tilt is gone.

Three. Set the gnomon — the shadow-caster — upright in the card. A matchstick, a twig, a pen tip, whatever seats in the hole. It must stand vertical to the card face, not leaning.

One line of dial-maker honesty before the corrections: a flat card with a vertical gnomon is a horizontal dial, and the geometry of its hour fan shifts with latitude. Near local noon the reading barely cares; at the far morning and evening ends of the day it drifts further the farther you stand from the latitudes the fan was drawn for — one more reason the honest claim below is ±10–15 minutes, not ±2.

Four. Read where the shadow lands on the scale. That number is apparent solar time.

Five. Correct it. This is the step everyone skips, and it is the difference between an approximate reading and a wrong one.

One honest number before the other honest number: uncorrected, expect up to about ±40 minutes against zone time — the figure on the product page. With the three corrections below, that tightens to the ±10–15 minutes this article works with throughout.

03Which hemisphere you are standing in

This is the part that catches people who travel across the equator, and it is not a subtlety — it inverts the whole instrument.

HEMISPHERES

The same shadow, two directions

NORTHERN HEMISPHERE

The sun tracks across the southern sky.

SIDE VIEW

TOP VIEW — CARD ON THE GROUND

SHADOW TURNS CLOCKWISE

SOUTHERN HEMISPHERE

The sun tracks across the northern sky.

SIDE VIEW

TOP VIEW — CARD ON THE GROUND

SHADOW TURNS ANTICLOCKWISE

Between the tropic lines the sun crosses overhead twice a year — there, check which side of you it is on before you read anything.

In the northern hemisphere the sun rises in the east, arcs to the south, sets in the west. The shadow therefore sweeps from west, through north, to east — the same direction as clock hands, which is not a coincidence: mechanical clocks were built to imitate the shadows people already knew.

In the southern hemisphere the shadow sweeps from west, through south, to east — anticlockwise. A sundial scale read the northern way in Melbourne will give you a reading that is wrong by hours, in a direction that changes through the day.

Which is why the scale on the card is marked for both, and why the first thing to establish is not the time but which side of the equator you are on. In the tropics a card sundial is a rough instrument and you should treat it as one.

04The three corrections

Apparent solar time becomes clock time through three separate adjustments. None of them is optional and only one of them is constant.

CORRECTIONS

From what the shadow says to what the clock says

What the shadow reads Apparent solar time — a physical measurement of where the sun actually is.

CORRECTION 1 — CONSTANT WHERE YOU LIVE Longitude Four minutes for every degree from your zone's central meridian (15° × UTC offset). Boston −16 min · Detroit +32 min · Madrid +75 min ± 4 minper degree

CORRECTION 2 — CHANGES EVERY DAY Equation of time Elliptical orbit plus axial tilt — the sun does not cross your meridian at a steady rate. peaks ~3 Nov and ~11 Feb · zero four times a year +16 / −14minutes

CORRECTION 3 — THE ONE PEOPLE FORGET, AND THE BIGGEST Daylight saving If it is in effect, add a whole hour — larger than the other two put together. + 60 minor nothing

Longitude — four minutes per degree. Your time zone has a central meridian: 15 degrees times the UTC offset. Every degree you sit east of it, solar noon arrives four minutes earlier; every degree west, four minutes later. Boston is about 4 degrees east of the 75th meridian, so its solar noon runs 16 minutes early. Detroit is 8 degrees west, so its solar noon runs 32 minutes late.

The equation of time — up to a quarter of an hour, either way. The earth's orbit is elliptical and its axis is tilted, and between them those two facts mean the sun does not cross your meridian at the same rate all year. The accumulated difference peaks at +16.4 minutes around 3 November, when the sundial runs ahead of the clock, and at −14.3 minutes around 11 February, when it runs behind.

EQUATION OF TIME

How far the sun runs from your clock, all year

Daylight saving — a whole hour, and the largest of the three. If it is in effect, subtract an hour. This is the correction people forget, and it is bigger than the other two put together.

clock time = solar time + longitude correction + equation of time + daylight saving

And in the other direction, if you want to know when to expect solar noon on the clock where you are standing: start at 12:00, subtract four minutes for every degree you are east of your zone meridian, subtract the equation of time for today's date, add an hour if daylight saving is on.

05How wrong it can get: Madrid

Spain sits geographically where Britain's time zone belongs — Madrid is at 3.7 degrees west, and it keeps Central European Time, whose meridian runs through 15 degrees east.

That is nearly 19 degrees of longitude, worth 75 minutes. Add daylight saving in summer and the equation of time in late June, and apparent solar noon in Madrid on 21 June lands at 14:16 on the clock.

Two hours and sixteen minutes of daylight displaced by administrative decisions. The sundial is not wrong there. Everything else is.

The sundial is on every Universal generation, on the Bushcrafter and on the Adventur. It has never needed a battery and it never will.

06How accurate is it, really

Two limits, and one of them is physics.

The sun is not a point. It subtends about half a degree in the sky, which means the tip of any shadow has a soft penumbral edge roughly half a degree wide — half the width of your little finger at arm's length. The earth turns 15 degrees an hour — a quarter of a degree per minute. So half a degree of blur is two minutes of unavoidable reading error before you have done anything wrong.

Then there is you. A single degree of error in finding true north costs another four minutes. Ten degrees of tilt in how the card is lying costs considerably more than that, and varies through the day rather than staying constant.

Realistically, with care, a card sundial and a straight twig will put you within ten to fifteen minutes of clock time once corrected. Without corrections, expect about ±40 minutes on a good day — the figure on the product page — and up to an hour and a half when the twig leans and north is a guess, which is not a time, it is a rumour.

Fifteen minutes is useless for catching a train. It is entirely sufficient for the question people actually ask outdoors: how much daylight do I have left.

07What it is actually for

Nobody carries a card sundial instead of a watch, and I would be suspicious of anyone selling it that way.

What it does is work when nothing else does. It has no battery, no signal requirement, no firmware and no failure mode short of clouds. It survives the drop that killed your phone, the cold that killed the battery, and the week in the mountains where there was never any signal in the first place. A paper printout has no battery either — until dew wrinkles it, wind folds it mid-reading and the fifth pocket-trip erases the lines. The instrument here is the geometry, and 2 mm of laser-etched metal is what keeps geometry being geometry.

How much daylight is left. The single most useful piece of information in the outdoors, and the one that decides whether you set up camp now or push on for another hour.

Which way is south. Read in reverse: if you know the time, the shadow tells you direction. On a clear day this is a more reliable compass than a magnetised needle near anything ferrous, which in practice includes most of what is in your pack — and it is exactly what the Sundial Compass trainer drills.

Roughly when. For meeting someone back at the trailhead, fifteen minutes of resolution is fine. It has been fine for four thousand years.

There is a fourth thing, and it is not practical. Reading a shadow to find the time is a direct measurement of the rotation of the planet you are standing on, made with a piece of metal and a twig. That is worth knowing how to do even on the days you have a phone in your other pocket.

The sun does this job by day. After dark, the same card switches instruments — a thread, a nut and the pole star do for latitude what the gnomon does for time. Find your latitude with Polaris →

Andrii Gurskyi, founder & engineer, MRF.Tools

FIELD SKILLS · 7 TRAINERS Don't just read it — turn the shadow yourself. →