question: Can sundials also give information about month?
comment: An [Armillary Sphere]( URL_0 ) does what you’re thinking of. It’s an oriented sundial that will display both clock time and months and seasons. [the various rings and their use]( URL_1 ) are explained in this link.
passages: In some sundials, the equation of time correction is provided as an informational plaque affixed to the sundial, for the observer to calculate. In more sophisticated sundials the equation can be incorporated automatically. For example, some equatorial bow sundials are supplied with a small wheel that sets the time of year; this wheel in turn rotates the equatorial bow, offsetting its time measurement. In other cases, the hour lines may be curved, or the equatorial bow may be shaped like a vase, which exploits the changing altitude of the sun over the year to effect the proper offset in time.
   Some sundials use both a style and a nodus to determine the time and date.
   Analemmatic sundials are a type of horizontal sundial that has a vertical gnomon and hour markers positioned in an elliptical pattern. There are no hour lines on the dial and the time of day is read on the ellipse. The gnomon is not fixed and must change position daily to accurately indicate time of day.
   There is an early reference to sundials from 104 BCE in an assembly of calendar experts.
   The time is indicated where a shadow or light falls on the dial face, which is usually inscribed with hour lines. Although usually straight, these hour lines may also be curved, depending on the design of the sundial (see below). In some designs, it is possible to determine the date of the year, or it may be required to know the date to find the correct time. In such cases, there may be multiple sets of hour lines for different months, or there may be mechanisms for setting/calculating the month. In addition to the hour lines, the dial face may offer other data—such as the horizon, the equator and the tropics—which are referred to collectively as the dial furniture.
   Analemmatic sundial
  
  Analemmatic sundials are a type of horizontal sundial that has a vertical gnomon and hour markers positioned in an elliptical pattern. The gnomon is not fixed and must change position daily to accurately indicate time of day. Hence there are no hour lines on the dial and the time of day is read only on the ellipse. As with most sundials, analemmatic sundials mark solar time rather than clock time. An analemmatic sundial is completely defined by
  
  BULLET::::1. The size of its ellipse (chosen by the designer).
   When the solar time is exactly noon, or known from another clock, the instrument can be used to determine the time of year.
   Solar calendars assign a "date" to each solar day. A day may consist of the period between sunrise and sunset, with a following period of night, or it may be a period between successive events such as two sunsets. The length of the interval between two such successive events may be allowed to vary slightly during the year, or it may be averaged into a mean solar day. Other types of calendar may also use a solar day.
  
  Section::::Calendar systems.:Lunar calendars.
   The simplest sundials do not give the hours, but rather note the exact moment of 12:00 noon. In centuries past, such dials were used to correct mechanical clocks, which were sometimes so inaccurate as to lose or gain significant time in a single day.
   In some U.S. colonial-era houses, a noon-mark can often be found carved into a floor or windowsill. Such marks indicate local noon, and provide a simple and accurate time reference for households that do not possess accurate clocks. In modern times, in some Asian countries, post offices set their clocks from a precision noon-mark. These in turn provide the times for the rest of the society. The typical noon-mark sundial was a lens set above an analemmatic plate. The plate has an engraved figure-eight shape, which corresponds to plotting the equation of time (described above) versus the solar declination. When the edge of the Sun's image touches the part of the shape for the current month, this indicates that it is 12:00 noon.
   The Greek almanac, known as parapegma, has existed in the form of an inscribed stone on which the days of the month were indicated by movable pegs inserted into bored holes, hence the name. There were also written texts and according to Diogenes Laërtius, "Parapegma" was the title of a book by Democritus. Ptolemy, the Alexandrian astronomer (2nd century) wrote a treatise, "Phaseis"—"phases of fixed stars and collection of weather-changes" is the translation of its full title—the core of which is a "parapegma", a list of dates of seasonally regular weather changes, first appearances and last appearances of stars or constellations at sunrise or sunset, and solar events such as solstices, all organized according to the solar year. With the astronomical computations were expected weather phenomena, composed as a digest of observations made by various authorities of the past. "Parapegmata" had been composed for centuries.
   Many methods have been used to simulate mean solar time. The earliest were clepsydras or water clocks, used for almost four millennia from as early as the middle of the 2nd millennium BC until the early 2nd millennium. Before the middle of the 1st millennium BC, the water clocks were only adjusted to agree with the apparent solar day, thus were no better than the shadow cast by a gnomon (a vertical pole), except that they could be used at night.
   The formulas shown in the paragraphs below allow the positions of the hour-lines to be calculated for various types of sundial. In some cases, the calculations are simple; in others they are extremely complicated. There is an alternative, simple method of finding the positions of the hour-lines which can be used for many types of sundial, and saves a lot of work in cases where the calculations are complex. This is an empirical procedure in which the position of the shadow of the gnomon of a real sundial is marked at hourly intervals. The equation of time must be taken into account to ensure that the positions of the hour-lines are independent of the time of year when they are marked. An easy way to do this is to set a clock or watch so it shows "sundial time" which is standard time, plus the equation of time on the day in question. The hour-lines on the sundial are marked to show the positions of the shadow of the style when this clock shows whole numbers of hours, and are labelled with these numbers of hours. For example, when the clock reads 5:00, the shadow of the style is marked, and labelled "5" (or "V" in Roman numerals). If the hour-lines are not all marked in a single day, the clock must be adjusted every day or two to take account of the variation of the equation of time.
   The Babylonians, Mayans, ancient Egyptians, ancient Indians, and Chinese measured all the basics of their respective time and calendar systems by naked eye:
  
  BULLET::::- the length of a year and a month to ±0.1 hour or to better than 1 minute (0.001%)
  
  BULLET::::- the 24 hours of a day, and the equinoxes
  
  BULLET::::- the periods of the planets were calculated by Mayan astronomers, to within 5 to 10 minutes accuracy in the case of Venus and Mars.
   Solar calendar
  
  A solar calendar is a calendar whose dates indicate the season or almost equivalently the apparent position of the Sun relative to the stars. The Gregorian calendar, widely accepted as standard in the world, is an example of a solar calendar.
  
  The main other type of calendar is a lunar calendar, whose months correspond to cycles of Moon phases. The months of the Gregorian calendar do not correspond to cycles of Moon phase.
  
  Section::::Examples.
  
  The oldest solar calendars include the Julian calendar and the Coptic calendar. 
   Tablets dating back to the Old Babylonian period document the application of mathematics to the variation in the length of daylight over a solar year. Centuries of Babylonian observations of celestial phenomena are recorded in the series of cuneiform script tablets known as the 'Enūma Anu Enlil'. The oldest significant astronomical text that we possess is Tablet 63 of 'Enūma Anu Enlil', the Venus tablet of Ammi-Saduqa, which lists the first and last visible risings of Venus over a period of about 21 years and is the earliest evidence that the phenomena of a planet were recognized as periodic. The oldest rectangular astrolabe dates back to Babylonia c. 1100 BC. The MUL.APIN, contains catalogues of stars and constellations as well as schemes for predicting heliacal risings and the settings of the planets, lengths of daylight measured by a water clock, gnomon, shadows, and intercalations. The Babylonian GU text arranges stars in 'strings' that lie along declination circles and thus measure right-ascensions or time-intervals, and also employs the stars of the zenith, which are also separated by given right-ascensional differences.
   A "heliochronometer" is a precision sundial first devised in about 1763 by Philipp Hahn and improved by Abbé Guyoux in about 1827.
  
  It corrects apparent solar time to mean solar time or another standard time. Heliochronometers usually indicate the minutes to within 1 minute of Universal Time.
   An analemma may be added to many types of sundials to correct apparent solar time to mean solar time or another standard time. These usually have hour lines shaped like "figure eights" (analemmas) according to the equation of time. This compensates for the slight eccentricity in the Earth's orbit and the tilt of the Earth's axis that causes up to a 15-minute variation from mean solar time. This is a type of dial furniture seen on more complicated horizontal and vertical dials.
   Calendars of the world have often been set by observations of the Sun and Moon (marking the day, month and year), and were important to agricultural societies, in which the harvest depended on planting at the correct time of year, and for which the nearly full moon was the only lighting for night-time travel into city markets.
   Calendars are also used to help people manage their personal schedules, time and activities, particularly when individuals have numerous work, school, and family commitments. People frequently use multiple systems, and may keep both a business and family calendar to help prevent them from overcommitting their time.
  
  Calendars are also used as part of a complete timekeeping system: date and time of day together specify a moment in time. In the modern world, timekeepers can show time, date and weekday. Some may also show lunar phase.
  
  Section::::Calendars in use.:Gregorian calendar.
   A sundial cannon, sometimes called a 'meridian cannon', is a specialized sundial that is designed to create an 'audible noonmark', by automatically igniting a quantity of gunpowder at noon. These were novelties rather than precision sundials, sometimes installed in parks in Europe mainly in the late 18th or early 19th century. They typically consist of a horizontal sundial, which has in addition to a gnomon a suitably mounted lens, set to focus the rays of the sun at exactly noon on the firing pan of a miniature cannon loaded with gunpowder (but no ball). To function properly the position and angle of the lens must be adjusted seasonally.
   BULLET::::3. To adjust for daylight saving time, if applicable, the solar time must additionally be shifted for the official difference (usually one hour). This is also a correction that can be done on the dial, i.e. by numbering the hour-lines with two sets of numbers, or even by swapping the numbering in some designs. More often this is simply ignored, or mentioned on the plaque with the other corrections, if there is one.
  
  Section::::Apparent motion of the Sun.
   This type of sundial is known as an equatorial sundial because the plane of the dial face is parallel to the Earth's equatorial plane. Two primary examples of modified equatorial sundials are bowstring equatorial and armillary dials. Both of these modified equatorial sundials can be read year-round on the same surface, regardless of whether the Sun is above or below the equatorial plane whereas a standard equatorial sundial changes sides with each equinox and is virtually unreadable near the equinoxes when the Sun is located on the equatorial plane.
   The custom of measuring time by one's shadow has persisted since ancient times. In Aristophanes' play, "Assembly of Women", Praxagora asks her husband to return when his shadow reaches . The Venerable Bede is reported to have instructed his followers in the art of telling time by interpreting their shadow lengths. However, Bede's important association with sundials is that he encouraged the use of canonical sundials to fix the times of prayers.
  
  Section::::History.:Medieval sundials.
   Leap years have two dominical letters because the dominical letter changes end February. The first letter covers January and February, the second covers March to December. For leap years, the outer circle will show two dominical letters.
  
  Section::::Description of the dials.:The week.
  
  This dial marks the seven days of the week, represented by ancient gods and their symbol.
  
  Section::::Description of the dials.:The globe.
  
  The rotation of the Earth causes day and night. It is day on the part of the globe that is currently visible.
  