Celestial Information

The sun now

Azimuth is the compass direction to it, measured clockwise from north: 90° is east, 180° south. Altitude is its height above the horizon: 0° on the horizon, 90° overhead, below zero after it has set.

Right ascension and declination place it on the sky’s own grid, the celestial longitude and latitude, fixed to the stars rather than to the ground. Transit is when it crosses the meridian, due south of here, at its highest for the day.

The sundial

A horizontal sundial cut for 37.43° N — the gnomon's shadow sweeps the true Palo Alto hour lines, east to the right. After sunset there is no shadow to cast.

Sunshine and UV

Sunshine is the hours of it so far today against the hours possible, estimated from the radiation sensor by the World Meteorological Organization’s pyranometric method, ten minutes at a time; nothing counts while the sun is under 5.7°, so a cloudless day reads short of sunrise to sunset. Energy is the sun’s energy that has reached the sensor today, a square meter at a time, against what a clear sky would have given by now.

UV dose is today’s UV index added up through the day, in standard erythemal doses (1 SED is 100 joules a square meter of sunburning ultraviolet), with the day’s peak index and when it was read. UV tomorrow is the EPA’s forecast peak for ZIP code 94303, made by NOAA’s Climate Prediction Center the day before.

The Solar radiation and UV index gauges’ cards on the Gauges page have each of these through the day and over the year.

Seasons and eclipses

The eclipse is the next one visible from Palo Alto, and its type is as seen from here — a station that catches only the penumbra of a total eclipse sees a partial.

The equation of time

Sundial time minus clock time at local standard noon, week by week — above the zero line the sundial runs ahead. The curve's two bulges are Earth's tilt and its elliptical orbit, the same pair that draws the analemma's figure-eight; the brass point is today.