What’s Up With the Sundial?

Have you noticed that our sundial outside doesn’t read the same time as your watch? That’s because the sundial and your watch are marking different kinds of time. The sundial measures “apparent solar time,” which is determined by the position of the Sun in our sky. Your watch measures “civil time.” This is a human convention that is based on a day of exactly 24 hours in length and accounts for things like time zones and Daylight Savings Time.

Do the following to get the two to match up:

1) Is it Daylight Saving Time (roughly mid-March through early-November)? If yes, subtract one hour from your watch.

2) Consult the chart below and add/subtract the appropriate amount to/from the sundial to account for the Equation of Time. (See reverse for more info.)

January Add 8 minutes February Add 13 minutes March Add 8 minutes April No Adjustment* May No Adjustment June No Adjustment* July Add 5 minutes August Add 3 minutes September Subtract 5 minutes* October Subtract 13 minutes November Subtract 13 minutes December 1-23 Subtract 6 minutes December 24-31 No Adjustment*

Your watch should agree with the sundial to an accuracy of five minutes or better!

Read the information on the other side of this page to find out why we have to do all this math to make our watches match the sundial.

*The sundial should match your watch exactly with no Equation of Time adjustment four times throughout the year. These times occur approximately on the following dates: April 15, June 14, September 2 and December 25.

Here’s why the sundial disagrees with your watch:

Daylight Saving Time Daylight Saving Time is a convention designed to meet the needs of our society, and the Sun doesn’t care about any of that. Subtracting an hour from your watch during Daylight Saving Time undoes the time change.

The Equation of Time Sundials work by measuring the position of the Sun in the sky. The Sun appears to move across the sky from east to west every day, but not at the same rate from day to day. When the Sun’s apparent east-west motion is faster or slower than average, the sundial will appear to run fast or slow compared to your watch. Two factors affect this rate of apparent motion, and the Equation of Time keeps track of them.

Earth’s Tilted Axis – Obliquity Earth’s rotation on its axis results in the Sun’s daily apparent motion across our sky. If Earth’s axis were exactly perpendicular to its orbit, the Sun would rise and set in the same places and reach the same height in the sky each day, moving across the sky at a uniform rate throughout the year. However, Earth’s orientation is tilted. So, the Sun reaches varying heights at noon depending on the season, taking longer to traverse the sky when the Sun is highest.

Earth’s Elliptical Orbit – Eccentricity Earth’s orbit around the Sun is elliptical in shape. If the orbit were a perfect circle, Earth’s orbital speed would be constant, and the Sun would move across our sky at a uniform rate. Because the orbit is elliptical, Earth’s orbital speed will vary depending on its distance from the Sun; Earth moves fastest when it is nearest the Sun (perihelion), so the Sun will appear to move fastest across the sky when Earth is closest to it.

Obliquity and Eccentricity can combine or cancel each other out. Together, they can cause up to 16 minutes difference between your watch and the sundial time!

One more thing… Time Zones Did you notice that the marker for one o’clock is where the marker for noon should be? That’s because of our location in the extreme western part of the Central Time Zone. Time zones are large geographic areas where the time is defined based on the Sun’s average position in the sky as seen from a standard reference meridian (or longitude) near the center of the zone. The time anywhere in the entire zone is defined to be the same time as at the reference meridian. At any given moment, however, the position of the Sun in the sky will be different depending on your longitude. That means our sundials will report different times at different longitudes, but our watches will read the same time regardless. To correct for that, we have to add four minutes to our sundial time for every degree that we are west of the reference meridian. For McDonald Observatory’s longitude, that’s a 56- minute difference! We shifted the hour markers to account for this difference.