<P> There is a simple verification of hyperbolic declination lines on a sundial: the distance from the origin to the equinox line should be equal to harmonic mean of distances from the origin to summer and winter solstice lines . </P> <P> Nodus - based sundials may use a small hole or mirror to isolate a single ray of light; the former are sometimes called aperture dials . The oldest example is perhaps the antiborean sundial (antiboreum), a spherical nodus - based sundial that faces true North; a ray of sunlight enters from the South through a small hole located at the sphere's pole and falls on the hour and date lines inscribed within the sphere, which resemble lines of longitude and latitude, respectively, on a globe . </P> <P> Isaac Newton developed a convenient and inexpensive sundial, in which a small mirror is placed on the sill of a south - facing window . The mirror acts like a nodus, casting a single spot of light on the ceiling . Depending on the geographical latitude and time of year, the light - spot follows a conic section, such as the hyperbolae of the pelikonon . If the mirror is parallel to the Earth's equator, and the ceiling is horizontal, then the resulting angles are those of a conventional horizontal sundial . Using the ceiling as a sundial surface exploits unused space, and the dial may be large enough to be very accurate . </P> <P> Sundials are sometimes combined into multiple dials . If two or more dials that operate on different principles--say, such as an analemmatic dial and a horizontal or vertical dial--are combined, the resulting multiple dial becomes self - aligning, most of the time . Both dials need to output both time and declination . In other words, the direction of true North need not be determined; the dials are oriented correctly when they read the same time and declination . However, the most common forms combine dials are based on the same principle and the analemmatic does not normally output the declination of the sun, thus are not self - aligning . </P>

In which direction must the sun be located how do you know