A · Canonical textTutorial 3 • 10 min read

True North Alignment via Solar Gnomon (Śaṅku)

A magnetic compass is thrown off by steel rebar on a modern site and differs from true north by the magnetic declination. Work the classical Indian shadow-circle method (Śaṅku-chhāyā) to find the true north–south line directly from the sun.

Sūrya SiddhāntaChapter 3 (Tripraśnādhikāra), which opens with finding the directions from a gnomon's shadow

A · Canonical text

Cited for: The shadow-circle method below: a vertical gnomon on a levelled surface, a circle drawn round it, and the two points where the shadow's tip meets the circle before and after noon, which lie on the east–west line.

The wording is not quoted here: the locus has not been checked against an edition.

1. Why Magnetic Compasses Fail on Job Sites

The Danger of Magnetic Declination & Steel Interference

A magnetic compass does not point to the True Geographic North (the Earth’s rotational pole); it points to the magnetic pole, and the difference is the magnetic declination. In India it runs from about −1.7° to +2.8°, small beside the second error: buried pipes, cables and steel rebar distort the field on a construction site, and a heading taken indoors near steel can be off by far more than the declination.

Why it matters for a reading: each of the sixteen zones spans 22.5°, so a few degrees of error can move a wall or a door into the next zone on the plan. That changes the reading the studio gives, not the building.

2. The Classical 5-Step Śaṅku Field Protocol

This geometric procedure uses only the sun and a shadow, so a magnetic anomaly does not enter into it:

1Level the Central Survey Platform

Select a cleared 2m × 2m area at the center of the plot. Lay a smooth, horizontal wooden board or cement plaster. Verify absolute levelness using a digital spirit level or water plate level (*Jalasama*).

2Erect the Vertical Gnomon (Śaṅku)

Erect a perfectly straight cylindrical brass or wooden rod of 12 Aṅgulas (9.0 inches / 22.86 cm) height. Fasten it rigidly at the center and verify plumbness with a hanging plumb bob (*Avalambana*).

3Inscribe the Shadow Circle

Using the base of the gnomon as the origin O, inscribe a circle with a radius equal to twice the gnomon’s height (R = 2H = 18.0 inches).

4Capture Morning & Afternoon Intersection Points

Morning (approx 9:00 AM): As the morning shadow shrinks, observe the precise instant the shadow tip touches the circle. Mark this Point A (West).
Afternoon (approx 3:00 PM): As the afternoon shadow lengthens, observe the exact instant the shadow tip exits the circle. Mark this Point B (East).

5Construct the True North Meridian (Matsya Method)

Draw the chord connecting A and B: the true east–west line. With centres at A and B and a radius greater than half of AB, strike two arcs that cross above and below the chord to form the classical fish (matsya). The line through the two crossing points is the true north–south line.

A second check: the shadow at local solar noon

The sun crosses the meridian at the moment its shadow is shortest

Azimuth of the Sun at transit = 180° (sun south of you) or 0° (sun north of you)

At local solar noon, when the Sun is at its highest, it lies on the north–south line, so the gnomon's shadow does too. It points north when the Sun is south of the observer, as it is for most of the year in most of India, and points south when the Sun is north of the observer, as it can be in the summer months in the tropics. Either way it is a second check on the line drawn above.

Parameters & variables

Sun_transit: Solar Culmination: Local astronomical noon instant(HH:MM:SS)