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Sundial noon and clock noon agree in middle April

Equation of Time on Sundial by John Carmichael

Tonight for April 15, 2015

Cool sundial.  Image via Flickr user kingston99.

Cool sundial. Image via Flickr user kingston99.

Every year around the middle of April, time by the sun and time by the clock agree. For instance, when the midday sun climbs highest in the sky in mid-April, the sundial reads 12 o’clock noon and your local clock time says 12 o’clock noon.

Your local clock time is the same as standard clock time, as long as you live on the meridian that governs your time zone. If you live east of the time zone line, then your local time runs ahead of standard time. If you live west of the time zone line, local time lags behind standard time.

For simplicity, let’s refer to places that sit right on the time zone meridian, like Denver or Philadelphia. Midday – noon by the sun – reads 12 o’clock noon standard clock time or 1 p.m. daylight saving time.

Equatorial sundial at Denver's Crammer Park, when it is a little past 11:00 a.m. by the sun. (The shadow moves counterclockwise, with the afternoon hours on the left.) Click here for the equation of time in 2015.

Equatorial sundial at Denver’s Crammer Park, when it is a little past 11:00 a.m. by the sun. (The shadow moves counterclockwise, with the afternoon hours on the left.) Click here for the equation of time in 2015.

At present, the length of the day as measured by successive returns of the midday sun is slightly less than 24 hours long. This slight daily discrepancy between the clock and the sun will accumulate until mid-May. In mid-May, midday – noon by the sundial – will come four minutes earlier by the clock than it does today.

After mid-May, day length as measured by successive middays (sundial noons) will become slightly more than 24 hours long. By around mid-June, noon by the sun and noon by the clock will agree once again.

Photo at top is from Flickr user Carmichael.

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