Sundial of the month April

Yet another large sundial

Madrid, Spain

Although not as large as the sundials shown here the two previous months, this sundial in Madrid, Spain, is nevertheless striking because of its dimensions.

And it contains a considerable number of dials:
- a direct south dial
- two polar sundials
- an equatorial band such as in armillary spheres
- and two moon dials.

The south dial is longitude adjusted: to prove it, the hour lines for 6 and 18 hours do not run horizontal.
There are date curves according to the zodiac.

The polar sundials take the equation-of-time into account. To simplify the readout, the EOT loops are split in two halves, one for each six-month period.

The moon dials read time by the shadow cast on moonlit south dials.

These are in fact quite ordinary south dials with pole style, but with a special hour line pattern.
On a night with a full moon, there is a difference of 12 hours between the hour angle of the sun and that of the moon.
If the full moon shines on an ordinary sundial and reads, for example, 10 hours, then it is really 22 hours by the sun.
The next day however, the moon has moved approximately 1.25 hours, and if the sundial reads 10 hours, it is really 22 - 1.25 = 20.75 hours.
The phase of the moon will let you estimate its age in days before of after the full moon.
In this way, and with some computation, you can use an ordinary sundial as a moon dial.

Usually, the age of the moon in days is reckoned from the moment of new moon. But this also gives the number of days before or after full moon.

The computations may be carried out graphically, which is what the Madrid moon dials do.
See below for a drawing ¹⁾ of a different moon dial:

Say the red line shows the pole style shadow, and the moon age is 12 days; then the solar time is approximately 01:00 hours.