Adjusting Sundials

How to Make Sundials as Accurate as Possible

Has anyone ever made a working sundial? I made two; and the hour lines seem to be off, both in the summer and winter.

I live in the NE of New York State.

Thank you for your GREAT site and all your help.

Nick

Thank you for the compliments on our site. It's so nice to know we have readers who come to visit frequently and, more importantly, find just what they are looking for!

Adjusting Your Sundial

As to your question regarding a working sundial, you probably know that sun time differs slightly from clock time. A day measured by how long it takes your exact point on the globe to make one rotation with respect to the sun is 24 hours, plus or minus up to 16 minutes.

If your sundial differs from clock time by more than that, the angle of your style (the angled edge which throws the shadow to indicate the hour) may not match the angle of your latitude or you may not have it pointed due north.

Determining True North

Don't use magnetic north to orient the style. You need to find true north, which is the northern point of the earth's polar axis. In other words, the style must point true north which will make it run parallel to the earth's north/south axis.

To find true north, look up tomorrow's sunrise and sunset times for your locale (in the newspaper or on the Internet).

Convert sunset time into military time.

For example, 6:15 p.m. becomes 1815. Add sunrise and sunset times together and divide by 2 to calculate true midday. Again for example, if sunrise is 7:12 and sunset is as above, 712 plus 1815 divided by 2 is 1263.5. This equates to 12 noon plus 63.5 minutes or 1:03:50 p.m.

Place a long straight stick into the ground next to the sundial you want to adjust and make sure it is perpendicular (at a 90 degree angle relative to the ground). In our example, the shadow cast by the stick at exactly 1:03:50 p.m. tomorrow will point true north.

Determining Your Sundial's Style Angle

If your sundial's style is pointed due north and your sundial time still differs from clock time by more than 16 minutes, you may need to adjust or replace your style.

The angle of the style which throws the shadow must equal the latitude. For example, if the sundial is located in Lake Placid NY, where the latitude is north 44.286 degrees, the style angle must be the same.

Find the precise latitude on a map or on the Internet. Get the most accurate protractor you can find (or borrow) and make sure your style has the same angle.

If neither of these suggestions solves your problem, the people at Sundials.org may be able to help.