ArtsAutosBooksBusinessEducationEntertainmentFamilyFashionFoodGamesGenderHealthHolidaysHomeHubPagesPersonal FinancePetsPoliticsReligionSportsTechnologyTravel

Sundial

Updated on May 5, 2010

A sundial is an instrument that tells time by measuring the apparent passage of the sun through its daily course in the sky. The vast majority of sundials consist of a pointer, called a style or gnomon, and a calibrated plate upon which the pointer casts a shadow.

The calibration of the dial can be understood by imagining a sundial placed so that the style lies on the earth's north-south axis and the plate is secured to a horizontal surface on the north pole. Thus the plate would be perpendicular to the style and parallel to the earth's equator. As the earth rotates about its axis, the shadow cast by the style would sweep out equal periods of time, since the earth rotates at an approximately constant velocity. Therefore the plate of the sundial could be calibrated by dividing it into 24 equal arcs. If the dial were moved from the north pole to any other place on earth and the plate kept parallel to the equator, these calibrations would not need to be changed. This type of sundial is called an equatorial or equinoctial sundial.

If the sundial is to have a plate in any position other than one parallel to the equatorial plane, such as horizontal, vertical, or in another plane, equal arcs on the plate will not correspond to equal periods of time. However, the correct calibrations can be determined by means of either geometry or trigonometry.

Photo by Jolene 00
Photo by Jolene 00

Time Measurement

Sundial time, or solar time, does not correspond to clock time, or mean solar time. Clock time corresponds to a period measured by the sun's apparent movement from zenith to zenith, assuming fictitiously a sun rotating at constant velocity around the earth in the equatorial plane. Actually, the earth revolves around the sun in an elliptical orbit, and the plane of the orbit is inclined about 23° from the plane of the earth's equator. As a result of these two factors the sun's apparent velocity fluctuates. Solar time, therefore, varies from clock time according to the day of the year, except at the equinoxes and solstices. The difference between solar time and clock time (mean solar time) is called the equation of time.

Types of Sundials

There are three major types of sundials. The dials discussed above depend basically only upon the rotation of the earth about its axis. These dials are often given names corresponding to the placement of the calibrated surface—horizontal, vertical, inclined, or equatorial. All sundials of this class are called directional sundials because their styles must always be aligned with the north-south axis of the earth.

Another type of sundial depends not only upon the rotation of the earth about its axis but also upon the changes in position of the sun above and below the equator in the course of a year. The advantage of this type of dial, called an altitude sundial, is that it does not require a compass for orientation. Typical of altitude sundials is the pillar, or shepherd's sundial. The calibrations of the instrument reflect the change in the altitude of the sun in the sky with the succeeding months. The months are marked off by the vertical lines and the hours by the diagonal lines. When the dial is suspended from its rings, with the pointer, or gnomon, erected over the line of the proper month, the instrument is rotated until the shadow falls directly beneath the gnomon, indicating the time.

A third group of sundials measures the sun's azimuth (horizontal angle between the sun and the south point of the horizon), rather than its altitude. Some dials incorporate the principles of two types. For example, the universal ring dial is an equatorial dial with a time-indicating device that must be changed from month to month.

History of the Sundial

The most ancient of scientific instruments is probably the sundial. A stick placed in the ground gave early man a simple way to tell time. The most important early developments in the construction of sundials occurred in Egypt, Babylonia, and Greece. Most sundials from these civilizations divide daylight into equal hours, called temporary hours, taking no account of the variation in the period of daylight during the year.

By the beginning of the Christian era, the principles of construction of most of the sundials discussed above were known. Before the invention of the compass, portable sundials were mainly of the self-orienting altitude type because of the difficulty in aligning a sundial of the directional class.

The introduction of trigonometry into mathematics by the Greeks by about 150 A.D. supplied the tool for plotting the hour lines with simple arithmetical calculations instead of the more cumbersome geometric constructions. The method was exploited by the Arabs and subsequently by European Renaissance sundial makers.

The great age of the European sundial lasted from the 16th to the 19th century. The development of the mechanical clock spurred the search for more accurate sundials for their regulation. With the increasing accuracy of the watch and the wide adoption of standard time in the 19th century, the sundial lost its practical value. Nevertheless, sundials have not gone completely out of use.

Comments

    0 of 8192 characters used
    Post Comment

    • Deborah Demander profile image

      Deborah Demander 

      8 years ago from First Wyoming, then THE WORLD

      Wow, that's a lot of information. Fascinating.

    working

    This website uses cookies

    As a user in the EEA, your approval is needed on a few things. To provide a better website experience, hubpages.com uses cookies (and other similar technologies) and may collect, process, and share personal data. Please choose which areas of our service you consent to our doing so.

    For more information on managing or withdrawing consents and how we handle data, visit our Privacy Policy at: https://hubpages.com/privacy-policy#gdpr

    Show Details
    Necessary
    HubPages Device IDThis is used to identify particular browsers or devices when the access the service, and is used for security reasons.
    LoginThis is necessary to sign in to the HubPages Service.
    Google RecaptchaThis is used to prevent bots and spam. (Privacy Policy)
    AkismetThis is used to detect comment spam. (Privacy Policy)
    HubPages Google AnalyticsThis is used to provide data on traffic to our website, all personally identifyable data is anonymized. (Privacy Policy)
    HubPages Traffic PixelThis is used to collect data on traffic to articles and other pages on our site. Unless you are signed in to a HubPages account, all personally identifiable information is anonymized.
    Amazon Web ServicesThis is a cloud services platform that we used to host our service. (Privacy Policy)
    CloudflareThis is a cloud CDN service that we use to efficiently deliver files required for our service to operate such as javascript, cascading style sheets, images, and videos. (Privacy Policy)
    Google Hosted LibrariesJavascript software libraries such as jQuery are loaded at endpoints on the googleapis.com or gstatic.com domains, for performance and efficiency reasons. (Privacy Policy)
    Features
    Google Custom SearchThis is feature allows you to search the site. (Privacy Policy)
    Google MapsSome articles have Google Maps embedded in them. (Privacy Policy)
    Google ChartsThis is used to display charts and graphs on articles and the author center. (Privacy Policy)
    Google AdSense Host APIThis service allows you to sign up for or associate a Google AdSense account with HubPages, so that you can earn money from ads on your articles. No data is shared unless you engage with this feature. (Privacy Policy)
    Google YouTubeSome articles have YouTube videos embedded in them. (Privacy Policy)
    VimeoSome articles have Vimeo videos embedded in them. (Privacy Policy)
    PaypalThis is used for a registered author who enrolls in the HubPages Earnings program and requests to be paid via PayPal. No data is shared with Paypal unless you engage with this feature. (Privacy Policy)
    Facebook LoginYou can use this to streamline signing up for, or signing in to your Hubpages account. No data is shared with Facebook unless you engage with this feature. (Privacy Policy)
    MavenThis supports the Maven widget and search functionality. (Privacy Policy)
    Marketing
    Google AdSenseThis is an ad network. (Privacy Policy)
    Google DoubleClickGoogle provides ad serving technology and runs an ad network. (Privacy Policy)
    Index ExchangeThis is an ad network. (Privacy Policy)
    SovrnThis is an ad network. (Privacy Policy)
    Facebook AdsThis is an ad network. (Privacy Policy)
    Amazon Unified Ad MarketplaceThis is an ad network. (Privacy Policy)
    AppNexusThis is an ad network. (Privacy Policy)
    OpenxThis is an ad network. (Privacy Policy)
    Rubicon ProjectThis is an ad network. (Privacy Policy)
    TripleLiftThis is an ad network. (Privacy Policy)
    Say MediaWe partner with Say Media to deliver ad campaigns on our sites. (Privacy Policy)
    Remarketing PixelsWe may use remarketing pixels from advertising networks such as Google AdWords, Bing Ads, and Facebook in order to advertise the HubPages Service to people that have visited our sites.
    Conversion Tracking PixelsWe may use conversion tracking pixels from advertising networks such as Google AdWords, Bing Ads, and Facebook in order to identify when an advertisement has successfully resulted in the desired action, such as signing up for the HubPages Service or publishing an article on the HubPages Service.
    Statistics
    Author Google AnalyticsThis is used to provide traffic data and reports to the authors of articles on the HubPages Service. (Privacy Policy)
    ComscoreComScore is a media measurement and analytics company providing marketing data and analytics to enterprises, media and advertising agencies, and publishers. Non-consent will result in ComScore only processing obfuscated personal data. (Privacy Policy)
    Amazon Tracking PixelSome articles display amazon products as part of the Amazon Affiliate program, this pixel provides traffic statistics for those products (Privacy Policy)