Setting the Sundial

Analogous to setting the correct time on a clock or wristwatch (by moving the hands of the timepiece into proper position), properly aligning an equatorial sundial will move the style shadow into position so that the dial face displays the correct time.
• The gnomon should be parallel to the Earth's axis, inclined at an angle equal to the latitude of the observer (see A Tale of Two Sundials).
• The dial plate should be parallel to the Earth's Equator (perpendicular to the gnomon), inclined at an angle equal to the complement of the observer's latitude (see A Tale of Two Sundials) -- the angle that when added to the angle of the observer's latitude equals a right angle in measure (90°).
• Place the sundial on a horizontal surface and align the upper dial face so that 12 Noon points toward true north, that is, so that the 12 Noon hour line is aligned with your local meridian.

The sundial may be used to align itself with your local meridian. Obtain the exact time of "Sun transit" from the U.S. Naval Observatory Complete Sun and Moon Data for One Day Web page. Set your wristwatch for the exact Time of Day. At the precise moment of Sun transit, align the sundial so that it reads exactly 12 Noon (regardless of the time shown by your wristwatch); the sundial is now aligned with your local meridian. A slightly more complicated but more accurate procedure would be to use the shadow cast by a plumb bob to strike a north-south line at the time of Sun transit.

To correct Solar Time for Standard Time, one must compensate for both the difference in longitude (between the location of the observer and the Standard Time Meridian) and the Equation of Time. The Solar Noon Calendar calculates tables showing either the exact time of Solar Noon for your location for each day of the year, or the Standard Time Correction (the amount you have to add to, or to subtract from, solar time on your sundial to get the time shown on your watch). Add one hour for Daylight Saving Time.

Earth's Rotation and Solar Time-Keeping

Planet Earth is a magnificent timepiece! The Earth rotates counterclockwise once every 24 hours. One complete rotation equals 360 degrees. The rate of the Earth's rotation equals 15 degrees per hour:

360°/24 hr = 15°/hr or 15°/60 min., which reduces to 1°/4 min.

Therefore, all of the hour lines on the dial face of an equatorial sundial are spaced exactly 15 degrees apart. Similarly, there are 24 time zones; each time zone is one hour or 15 degrees of longitude wide.

Because the Earth rotates counterclockwise (as viewed from above the Northern Hemisphere), shadows cast by the Sun move in a clockwise direction. Therefore, morning times are located on the right side of the upper dial face; afternoon times are on the left. The reverse is true for the Southern Hemisphere, which is modeled by the lower dial face.

Earth's Revolution Around the Sun and the Annual Cycle of Change in the Sun's Apparent Path Across the Sky

Long-term investigation using an equatorial sundial enables one to indirectly observe the annual cycle of change in the Sun's apparent path across the sky (caused by the tilt of the Earth's axis and the revolution of the Earth around the Sun).

Theoretically, the gnomon (or style) of a properly oriented equatorial sundial will not cast a shadow on the dial plate during the Equinoxes. Because the dial plate of an equatorial sundial is parallel to the Earth's Equator, the Sun is directly over the edge of the dial plate on the Equinoxes, when the Sun is at the zenith along the Equator. From the March Equinox to the September Equinox, when the Sun is at the zenith in the Northern Hemisphere (between 0° and 23.5°N latitude), the gnomon shadow falls on the upper dial face; from the September Equinox to the March Equinox, when the Sun is at the zenith in the Southern Hemisphere (between 0° and 23.5°S latitude), the gnomon shadow falls on the lower dial face (see example).

Assemble a simple equatorial sundial (courtesy NASA Liftoff to Space Exploration) and empirically observe where the gnomon shadow falls as the seasons change. From the March Equinox to the September Equinox, use the Northern Hemisphere equatorial sundial template. From the September Equinox to the March Equinox, use the Southern Hemisphere template. Be sure to fold the Southern Hemisphere template so that the dial face and gnomon point downward rather than upward (as directed).

Gain valuable insight by checking the NASA J-Track Web page to see where the Sun is currently directly overhead (see small Sun icon, correctly oriented with respect to latitude and longitude).