Moondials are time pieces similar to a
sundial
A sundial is a horological device that tells the time of day (referred to as civil time in modern usage) when direct sunlight shines by the apparent position of the Sun in the sky. In the narrowest sense of the word, it consists of a flat ...
. The most basic moondial, which is identical to a sundial, is only accurate on the night of the
full moon
The full moon is the lunar phase when the Moon appears fully illuminated from Earth's perspective. This occurs when Earth is located between the Sun and the Moon (when the ecliptic coordinate system, ecliptic longitudes of the Sun and Moon opp ...
. Every night after it becomes an additional (on average)
[The Moon's orbit is not circular, so it does not move around the Earth at a uniform rate. Thus while the average difference between moonrises is 48 minutes, the actual time can vary considerably (roughly 20min to 1hr50min depending on the time of year and the location of the Moon in its orbit). The time read by a moondial will also vary in a similar, though not so drastic manner.
] 48 minutes slow, while every night preceding the full moon it is (again on average)
49 minutes fast, assuming there is even enough
light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...
to take a reading by. Thus, one week to either side of the full moon the moondial will read 5 hours and 36 minutes before or after the proper time.
More advanced moondials can include charts showing the exact calculations to get the correct time, as well as dials designed with latitude and longitude in mind.
Moondials are very closely associated with
lunar gardening (night-blooming plants) and some comprehensive gardening books may mention them.
See also
*
Time
Time is the continued sequence of existence and events that occurs in an apparently irreversible succession from the past, through the present, into the future. It is a component quantity of various measurements used to sequence events, to ...
Notes
Bibliography
* Ralf Kern: Wissenschaftliche Instrumente in ihrer Zeit. Vom 15. – 19. Jahrhundert. Verlag der Buchhandlung Walther König 2010,
{{Commons category, Moondials
Clocks
Timekeeping
Sundials