How to Read a Sundial

Jess Hutton | April 22, 2016 | Did You Know?

Time is at once the most valuable and the most perishable of all our possessions.

- John Randolph, U.S. Politician

Before watches and clocks were invented, ancient civilizations used the sun to distinguish time and seasons. The sun is consistent and its light is plentiful, and ancient civilizations discovered creative ways to tell time, including:

  • Constructing walls and buildings with notches, so the sun shone on specific points along the structures to differentiate between seasons
  • Erecting large stone structures that cast shadows indicating the hours of the day
  • Constructing homes facing north and south, telling time based on the position of the sun in relation to their home

Before the Clock

One of the most widespread methods of telling time was reading a sundial. Around since ancient Egypt, sundials use the sun’s altitude, or position in the sky to mark daytime hours.

Historically, sundials used a nodus (a slanted, pointed rod), which cast shadows along certain points of a dial to indicate hours. Present day sundials appeared during the Renaissance period, evolving into many types, shapes, and orientations. Even after the clock was invented, sundials were often referenced to reset clocks because accuracy was poor.

While we all use watches and clocks today, learning to read a sundial can be fascinating.

Henry Zbyszynski/Flickr

How to Read a Sundial

Reading a sundial requires knowing your local latitude, your vertical direction, and True North. You’ll be able to indicate local solar time (the time measured by the Sun) this way. Remember, sundial time is not necessarily the same as clock time, as variation can be as much as 15 minutes! The orbit of our planet is not perfectly circular, and our rotation on our axis is not perfectly perpendicular to our orbit, so calculations must be made to obtain the official clock time.

Reading a sundial requires knowing your local latitude, your vertical direction, and True North. You’ll be able to indicate local solar time (the time measured by the Sun) this way. Remember, sundial time is not necessarily the same as clock time, as variation can be as much as 15 minutes! The orbit of our planet is not perfectly circular, and our rotation on our axis is not perfectly perpendicular to our orbit, so calculations must be made to obtain the official clock time.

The most common sundial is referred to as a “gnomon”, which can distinguish the time of day based on the shadow cast by the above-mentioned nodus.

To read a sundial from the northern hemisphere:

  • Place sundial on a flat horizontal surface.
  • The top of the gnomon should point to True North or towards the North Star. In simpler terms, align the dial face so that 12:00 noon points north and the 12:00 noon hour line is aligned with your local meridian. The sun’s shadow falls in the opposite direction as the sun, and because Earth rotates counter clockwise from the northern hemisphere, shadows cast by the sun move in a clockwise direction.
  • The shadow moves past each of the lines marked with hours in a similar way that a clock hand moves around its face. The shadow falls along one side of the dial in the morning, moves past 12 noon in the middle of the day, and continues to the other side in the afternoon – much like our modern-day clocks!

It might sound a bit complicated, but with a little practice, you can be a skilled sundial reader in no time.

liz west/Flickr

Want to try it for yourself? Here are a few sundial activities:

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Featured photo by Peter Alfred Hess/Flickr

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