How to read your Sundial

This Vertical dial use's an analemma on each of the hour lines.

The curved lines of the analemma acts as a graph at each hour line to compensate for changing length of the day over the course of the year. The length of the gnomons shadow show the suns angular position (azimuth) over the entire year,

I've split the analemma in half and colored the Fall / Winter side Gold and the Spring / Summer side White to make choosing the correct side a little easier when your reading the time.




	Here are 5 sketches of the more common types of gnomon. all 5 have a 'Nodus' the 'donut' on the far sketch is the Nodus, the tip of the next two are the Nodus and the notch on the top edge forth and fifth one are both the Nodus of the gnomon.


Its not as hard as it looks to read a Vertical Sundial The time can be read to the minute only on the hour. For the time between the hours you need to estimate. The time and date are indicated by the location of the gnomons shadow at its tip (if gnomon is rod or pin like the one on The own of Orange dial). If the gnomon is the traditional triangle it may have a notch called the Nodus (the Nodus is in the same place as the tip of a rod would be. that Nodus or tip tells you the suns height in the sky ( or azimuth). It will tell you the time of year When the sun is shinning on the sundial, the shadow will move from left to right across the face of the dial as the day progresses. You will have to first, find Nodus date on the Analemma, then look at the gap between the analemma and the straight hour line. that gap is the correction that you either add of subtract from solar time to get clock time. Its not hard to estimate how far the (gap is from an imaginary Half or quarter hour line and get pretty close) The Orange dial has color coded Analemma at each hour to help people figure out the date In the Spring / Summer half of the year, from December 21 through June 21 the White colored edge marks the hours. In the Fall/ Winter half of the year the Gold side of the analemma is used. When estimating the time between the hours, make sure you estimate between the correct edges of the analemma. Gold for spring, White for fall. If the gnomon has a Nodus or is a vertical pin, the shadow at that point will indicate the date as it cross's the analemma position of the the Gnomons shadow. The top "horizontal" line, corresponds to the winter solstice, the shortest day of the year when the sun is lowest in the sky, December 21 approximately. On the day of the winter solstice the tip of the shadow will move along this day line. The bottom day line correspond to the Summer solstice, the longest day of the year when the sun is highest in the sky, June 21 approximately again, On this date the tip of the shadow will follow this line from morning to night. The day lines which sweep from left to right, crossing the analemma are calculated for specific positions of the earth in its orbit around the sun. Since the year is not a whole number of days, we adjust the calendar to keep in step with these astronomical positions or events, in particular the vernal equinox, by means of a leap day, February 29^th. Thus the date of theses astronomical events can vary by a day year to year.

	This is the layout of a 'Noon dial' or meridian dial. It was used for a while to calibrate clocks and watches at around noon. This noon dial has been split in two for clarity, recombined it would have the analemma for 11:00, 12:00, and 1:00.

		SUN TIME? CLOCK TIME?

When the shadow touches the hour line that is the exact time. Make sure you are looking at the right side of the line.

The changing height of the sun during the year (High in the sky in the Summer and low in the Winter) will cast a long shadow in the summer and a short one in the winter.

Where the shadow touches the analemma each hour is also a good approximation of the date. If the dial were big enough to show enough detail you could read the exact date. A dial can include a date line that the shadow will follow all day long.

The Gnomons shadow will follow a curved path for most of the year. the exceptions being around the vernal or autumnal equinoxes. You will need to be familiar with that path to make good estimations of the time.

This drawing shows the path that the shadow will follow on the first day of each month.


	On April, 1 the morning shadow will start here.


	On June, 1 the tip of the shadow will have followed the line over the course of the day.


	When you are estimating the time between the hours, keep in mind the path the shadow will follow for that day. You will be to looking to see, (for example) if tip of the shadow is half way or a quarter of the distance across its path between each hour line. Add that time to the nearest hour line to the right and that's it. Not all sundials will have the Day or Month lines but they will almost always have the the solstice days marked because they mark the boundaries of the dial. The 21st of December is the Winter solstice and June 21 is the Summer solstice. Each of these arches flatten out a little each day until they are a straight line in the spring in March and September in the Fall, from there they start arching in the other direction until they reach their maximum arches at the solstices again.

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