Gnomon: Solstice Compass

Gnomon is a field instrument for alignments. Stand at a stone, sight along the top edge of your phone at a notch in the skyline, and Ley turns that direction into a declination: the one number that means the same thing at every latitude. Then it tells you which solstice, equinox or lunar standstill it faces, and in which span of centuries.

It is built for people who walk to stones: archaeoastronomy readers, landscape photographers planning a sunrise, hillwalkers, and anyone who has read that a monument is "aligned" and wants to check.

THE HORIZON
• Where the sun rises and sets today at your place, and where the solstices, the equinox and the Moon's major and minor standstills rise and set
• A dial of every target direction, over a flat horizon or a skyline of the height you set
• The next solstice or equinox, and the next lunar standstill in the 18.6-year cycle

THE INSTRUMENT
• Azimuth and skyline altitude from the phone's fused orientation sensor, or typed from a map or a survey
• True north from the magnetic model built into the phone, and a better north from the sun: sight the sun or the shadow of a stick, and the app corrects its own compass. It always says which north it is using
• Works without a magnetometer
• Declination for the sun and the moon, the targets within your tolerance, and the span of epochs each one allows. Never a single year, never a score

LINES
• Bearing from A to B and back, which on a sphere is not the same line turned round
• Lines through three or more of your points, and beside every result how many lines 200 random layouts of the same points in the same area give

SITES AND ARTICLES
• 377 stone circles, standing stones, dolmens, passage tombs, henges and cairns, with published alignments marked by level: measured, hypothesis or modern
• Written articles: what Watkins actually said about leys, Lockyer's Stonehenge recomputed, dating a stone by the tilt of the Earth, when exactly the sun rises, and how many lines are luck

CHECKED AGAINST THE BOOK
Lockyer published every input for the axis of Stonehenge in 1906. Ley's formula reproduces his declination, 23° 54′ 30″, to 0.2 seconds of arc, and his obliquity for 1906 to under a second. The same declination on the modern long-period obliquity series gives about 1850 BC, inside the 200-year margin he gave himself. The app's tests run this on every build.

You choose which moment counts as sunrise (first gleam, centre of the disc, or the whole disc clear), because the three differ by nearly a degree. Your choice is printed beside every result.

Everything is computed on your phone. No account, and your position never leaves the device.

Free to install, with ads. A single one-time purchase removes them and unlocks the full journal and lines through many points.