Latitude Correction

Perhaps you were given a horizontal sundial as a gift, purchased one or you had designed and constructed one and have moved. Unless the dial that was gifted was designed for your geographical location or you have moved to new home that just happens to be at the same latitude as the one you moved from, it is unlikely that the dial will indicate correctly. The important fact to note is that the gnomon must point directly at the celestial pole.

In the case of the gift or purchased sundial, it is assumed that the dial has been designed for some particular location and built to a reasonable quality standard. There is no doubt that this would be the case for the one that you constructed. It is also assumed that the sundial indicates local apparent time and has not been corrected for longitude. If it has it will only work if your new home happens to be at the same longitude as your old one. Some sundials can be designed for adjustable longitude correction. Finally, we assume that you are not moving from the Northern to Southern hemisphere or the other way.

If you do not know the latitude for which your sundial was designed, you must first measure the angle of the gnomon. In Figure 1, sundial A is installed at the location it was designed for. The dial plate is level in all directions and the gnomon points directly at the celestial pole. If the sundial is to be located at a higher latitude it would be installed as sundial B. The north-south axis is tilted upward until the gnomon points at the celestial pole. If the sundial is to be located at a lower latitude it would be installed as sundial C. The north-south axis is tilted downward until the gnomon points at the celestial pole. The entire sundial must be tilted, not only the gnomon. The angle that the dial plate must be tilted is the difference between the new latitude and the old latitude.

Never rotate the dial plate to the east or west in an attempt to make it work. It will not and will only make it worse. If after positioning the sundial and correcting the indicated time for latitude and the Equation of Time, it still does not give the correct time you should give up and treat it for what it really is - a garden ornament.

Figure 1: Latitude Correction

This technique can be used on a vertical sundial but remember that the gnomon angle is equal to co-latitude (90° - Latitude).