How to find the sunrise and sunset times from a Sun clock and calendar

摘要

The purpose of this article is to describe the construction of a Sun clock and calendar (SCandC) that will allow an observer to not only see the time but also the symmetry properties of the Sun-Earth relative movement. A set of circles drawn on the SCandC will allow the observer to see their associated dates as well as to perform a visual interpolation between any pair of consecutive circles to estimate an intermediate date. By introducing the sunrise and sunset horizon lines in the SCandC the observer will be able to find the sunrise and sunset times during most of the year. The observer will also be able to appreciate the difference in time duration between the spring-summer and autumn-winter periods, as a consequence of Kepler's second law, as well as to observe that there is a small difference between the circle radii of equidistant dates from the solstices in the spring-summer versus autumn-winter periods as a consequence also of Kepler's second law. The equations derived in the present article will be tested against data from a particular lighthouse.