We received a question from a reader named Sherri concerning the operation the Orion-Constellation volvelle in Chapter 5 of the Signs & Seasons workbook. As described in the Signs & Seasons course, volvelles are traditional dial tools that have been used for centuries to simulate the motions of the Sun, Moon and stars. The workbook includes copyable templates for cutting and assembling three different volvelles, to help the student get a “hands on” understanding of the motion of the celestial bodies.
Sherri asked how to use the volvelle to find the sunrise, sunset, and meridian times of the Sun for a particular date. Sherri was not sure where to point the Sun onto a given zodiac constellation for a particular date, for example, January 21.
I scanned my own volvelle to show the Sun wheel aligned with the date for January 21, which automatically aligned the Sun with the position along the constellations. The Sun was aligned at Noon for convenience, which is generally the meridian time, not corrected for standard time.
To find the sunrise, the Sun wheel and constellation wheel are both moved to the eastern horizon, as shown in the next scan. As we can see, at this position, the Sun is very far to the south of due east, as it actually is during the winter. Once the Sun is aligned with the horizon, you can use the Sun as an “hour hand” to read the time of sunrise indicated along the circumference of the vovelle. The volvelle indicates 7:00 AM.
To find the time of sunset, you simply turn the Sun wheel and constellation wheel to the western horizon, and read off the indicated time, again using the Sun as an “hour hand” to point at the time with which it is aligned, which is about 5:00 PM. As the volvelle depicts, the Sun sets south of due west during the winter months.
The whole idea of the volvelle is to simulate the motions of the Sun, Moon and stars. It’s not a perfect simulation for many reasons, but is intended to give a ballpark idea of how these motions transpire. The volvelle must be corrected for the user’s position in the standard time zone, and for daylight savings. There is also considerable variation for latitude, as a very different reading will be obtained in Alaska than in Florida. Additionally, a volvelle is a flat disc with regular rotations, and cannot perfectly simulate the irregular motions of the celestial bodies, as they appear in a spherical sky. If you compare the volvelle sunrise times to the times you’ll find on the US Naval Observatory website, the numbers will probably be off by as much as 15 or 20 minutes.
Volvelles are just a crude (but traditional) simulation, intended to illustrate the general idea of these celestial motions. But if we can learn how to use them, they can help us get a better understanding of how the Sun, Moon and constellations move across the sky over the span of days, months and years.