Shadow-Length Journal: A Seasonal Project for Tracking Solstices and Equinoxes

A stick, a flat patch of ground, and a pencil. That is the entire instrument. Push the stick upright, mark where its shadow ends at local noon, and repeat. Do it through a year and the marks will fan out and draw back again, and somewhere inside that fan is a complete description of the Sun's path over your head.

This is a seasonal project: it rewards patience far more than equipment, and it works in a back garden, on a balcony, or on a school playing field. What you get at the end is a personal almanac, measured by hand, with all of its irregularities left in.

What a Shadow Actually Measures

A vertical rod casts a shadow whose length depends on one thing: how high the Sun stands above the horizon. The relationship is simple geometry. Divide the shadow length by the rod's height and you have the shadow ratio; the Sun's altitude is the angle whose tangent is the inverse of that ratio.

Some anchor points are worth memorising. When the Sun is 45 degrees up, the shadow equals the rod's height. At 30 degrees the shadow is about 1.7 times the height. At 60 degrees it shrinks to roughly 0.6. Low Sun, long shadow; high Sun, short shadow. Everything else in this project follows from that.

At local noon the shadow is at its shortest for the day, and — anywhere north of the tropics — it points essentially due north. Across the year, the noon shadow lengthens and shortens because the Sun's declination swings roughly 23.4 degrees either side of the celestial equator. Recording that swing is the aim.

Building a Station That Stays Put

The measurement is only as good as the setup. A rod that leans by a degree, or a base you cannot find again, will quietly ruin a year of work.

The rod

Use a straight dowel, metal rod or length of bamboo about a metre tall and thick enough not to wobble. Fix it so it cannot be knocked or rotated: driven into firm ground, or clamped to a post, or set into a weighted bucket of gravel. Check it is vertical with a plumb line in two directions at right angles, and re-check it every few weeks.

The ground

A level surface is easier to mark and easier to photograph. A concrete slab, a plywood board, or a few paving stones all work. Mark the rod's base position permanently, so that if the rod is moved you can restore it exactly.

• Straight rod, roughly one metre, with a fine tip
• Level board or firm flat ground, plus chalk or masking tape
• Plumb line or small spirit level
• Tape measure and a ruler for the short winter shadows
• Notebook, pencil, watch, and a camera for backup

Finding True Noon Without a Compass

Forget clock noon. The Sun crosses your meridian at a time that drifts through the year by up to about a quarter of an hour, and your position within a time zone shifts it further. The shadow will tell you when noon is, if you ask it properly.

Draw two or three concentric circles on the surface, centred on the rod's base. In the morning, mark the point where the shadow tip crosses each circle. In the afternoon, mark where it crosses the same circles again. The line joining each pair of marks runs east–west; the midpoint of each pair lies on your north–south line. Bisect the angle and you have true north, drawn on the ground, no magnet required.

The moment of shortest shadow is local apparent noon. That is the reading you keep.

Keeping the Journal

A single notebook page per observing day is plenty, but keep the columns the same every time so the year's data can be compared at a glance.

• Date and, if you like, the Sun's declination for that date
• Clock time of the shortest shadow
• Shadow length, measured from the base to the middle of the shadow tip
• Rod height above the surface, measured on the day
• Ratio of shadow to height
• Notes on cloud, ground wetness, and anything that moved

Two habits matter. First, take three readings a few minutes apart and keep the shortest; the minimum is flat, and eyeballing it once invites error. Second, always mark the middle of the blurred edge at the shadow tip rather than its outer fringe — the penumbra can be a centimetre or two wide and it will bias every measurement in the same direction. A photograph of the board with the tape measure in frame, taken from the same angle each time, is a useful cross-check.

Reading the Year in Your Numbers

Within a month the pattern becomes obvious, and within a season it becomes interesting.

The two extremes

Around the December solstice the noon shadow reaches its yearly maximum; around the June solstice, its minimum. If you are watching from the northern hemisphere, those are the dates when the Sun's noon altitude bottoms out and peaks. Plot shadow length against date and you get a shallow wave, not a straight line.

The two crossings

At the equinoxes the noon shadow sits close to the midpoint between the extremes, and the shadow tip traces something close to a straight east–west line across the board rather than a curve. It is a satisfying thing to see happen on a surface you built yourself.

The pace of change

Here is the detail most people miss. The noon shadow changes fastest around the equinoxes and almost stalls near the solstices. For a week or two either side of late June and late December, consecutive readings can look identical. That is not a mistake in your method; it is the geometry of a turning point, and it is exactly why the solstice dates were hard to pin down for ancient observers.

Your latitude, from a stick

Convert your noon shadow ratio into a solar altitude, then combine it with the Sun's declination for that date: latitude equals 90 degrees minus the altitude, plus the declination. Use a solstice reading, when the declination is near 23.4 degrees, and the arithmetic is easy enough to do on the back of the page. Most observers land within a degree or so of the true value — a decent result for a dowel.

Where the Project Usually Goes Wrong

Almost every failed shadow journal fails for one of a handful of reasons.

1. The rod leans, or gets knocked, and is not re-checked against the plumb line.
2. Readings are taken at clock noon instead of at the shortest shadow.
3. The board is not level, so the shadow is measured along a slope.
4. Measurements are monthly only, which hides the fast equinox change and produces a jagged, misleading graph.
5. The shadow length is measured to the outer blur rather than the middle of the tip.
6. No note is made of which side of the rod the shadow fell, which matters once you start comparing directions, not just lengths.

One safety point, and it is not a small one: never look at the Sun directly, and never point binoculars or a telescope anywhere near it. The shadow is the safe instrument. It does all the looking for you.

Starting This Week

You do not need to begin on a solstice. Begin on any clear morning, because the four hinge dates will arrive soon enough and you will already have a baseline to compare them against. Aim for one reading a week, plus a deliberate effort on the two solstices and two equinoxes, when it is worth measuring daily for a few days either side.

Write the date on the board itself in chalk, photograph it, and transfer the numbers indoors the same evening. Keep the board dry and the rod vertical. After twelve months you will have a row of marks that describes a full circuit of the Sun, made entirely by your own hands, and a much sharper feel for why the seasons turn.

Photo: Pezibear / Pixabay