Your clock
—
local time
The oldest clock, still running
Gnomon A stone sundial that keeps your clock's true solar time — carved for the 34th parallel.
‹Drag to move the sun›
—
reading the shadow
How to read a shadow
Nothing moves but the sun
The style — the sloping edge of the blade — is set parallel to the Earth's axis. So the shadow it throws swings at a perfectly even rate, and where it falls names the hour. Not the hour on your phone: the hour the sky actually keeps.
The blade points north
Aligned to the celestial pole, the style ignores the seasons. Only the hour angle turns it.
The shadow marks the hour
At solar noon it lies along the meridian; every 15° of the Sun's travel is one hour of arc.
Length tells the height
Long at dawn and dusk when the Sun is low; clipped to a stub at midday. The shadow breathes with the year.
Apparent solar
—
sundial time
Hour angle
—
from meridian
Sun altitude
—
above horizon
Shadow bearing
—
on the dial
Declination
—
season
Daylight
—
rise to set
Latitude
34.0°N
installed
Why the dial and the clock disagree
The equation of time
A clock ticks a mean, invented day of exactly 24 hours. The real Sun does not: it runs fast in November and slow in February, because the Earth's orbit is an ellipse and its axis is tilted. Trace the noon Sun for a year and it draws this figure-eight — the analemma. The dial reads the true Sun, so we add the difference back.
Today the Sun is — the clock by —.
Move the stone
Set it to your own parallel
Every horizontal dial is cut for one latitude — the hour lines splay differently at the equator than at the pole, and the blade leans back by exactly your latitude. Drag the parallel and watch the face re-carve itself.
34.0°N