Join Donate

EarthDial Instructions

Woody Sullivan, the University of Washington Astronomy Department
Bill Nye, The Science Guy® and Nye Labs
Emily Lakdawalla, The Planetary Society

> Download these instructions in a printer-friendly PDF format

EarthDial #1, Seattle, Washington, USA

I. The EarthDial Concept
II. Constructing your EarthDial
A. Some Vocabulary
B. A Sample EarthDial
C. Summary of EarthDial Specifications
D. Materials
E. Lay out the Circular Base
F. Lay out the Hour Lines
G. Draw the Date Curves
H. Mark and Decorate Your EarthDial
I. Add the Gnomon
III. Setting up your EarthDial
A. Location
B. Align Your EarthDial
IV. Set Up Your Web Camera
A. Materials
B. Install your Web Camera and Serve it over the Internet
C. Weatherproof your Web camera
D. Set Up the Camera to View Your EarthDial
V. Appendices
A. Converting Between Sundial Time and Clock Time
B. Equations for Hour Lines and Date Curves

I. The EarthDial Concept

For thousands of years, humans have used shadows cast by sundials to reckon the passage of time. Sundials measure the position of the Sun as it appears to move across the sky.  For observers on different parts of the Earth, the Sun’s apparent motion is different.  So depending on their location on the globe, sundials around the world must have their shadows and hour lines set up slightly differently.  By observing the motion of sundial shadows around the world at the same time, an observer can get fantastic insight into the nature of planetary motion and our place in the Solar System. The Planetary Society is establishing a worldwide network of nearly identical sundials called “EarthDials.” Images from each EarthDial will be continuously updated and posted on the Web.

As your eye sweeps across the images on this page, the Sun’s shadow will sweep across the faces of the dials.  Kids and grownups, classrooms and clubs, around the world will gain a palpable sense of how the Sun illuminates the round globe that is our Earth—and how the Sun’s passage across the sky controls time around the world.

At any moment, EarthDials from half of the world will be in darkness.  The path that the Sun traces—and consequently the lines and curves on the faces of the EarthDials—will be very different from Scandinavia to Seattle to South Africa.  And while the EarthDials will be made to a common pattern, each one will be decorated in the language and cultural motifs of its host.

The MarsDial
The MarsDial
A closeup of one of the flight MarsDials shows the gnomon in the center; it will cast shadows on the face. Credit: Cornell University / JPL / NASA

The EarthDials are designed to remind us of the MarsDials aboard NASA’s Spirit and Opportunity rovers that arrived at Mars in January 2004. The MarsDials and the EarthDials bear the common motto “Two Worlds One Sun” because it’s the same Sun that moves across the skies of Earth and Mars.

The EarthDial project will run at least for the duration of the rovers’ operational missions on the surface of Mars, from January through at least April 2004 (and, we hope, much longer).  Here are plans and specifications so that you can be part of this exciting project by making your own EarthDial.  There are many things in common for all the EarthDials, but also much opportunity to personalize your own.

To set up an EarthDial, there are three main tasks:

Before you begin, you should first read the instructions all the way through, and make up a list of tools and materials you’ll need. As you proceed through the steps, refer often to the illustrations for guidance. When you begin, please also email us about your plans, including when you expect your EarthDial and Webcam to be operating.

If you follow these instructions carefully, your EarthDial will tell accurate time. The central EarthDial website will have links to all the active EarthDial Webcams. It will also host still photos of EarthDials without Webcams. Depending on response, it may be that only a limited number of EarthDial images can be highlighted on a world map on our website each week. Make your Dial well, so that it will be suitable for selection as one of the featured EarthDials.

Be careful when you measure!  Distances should be measured to an accuracy of 2 mm or less.  Angles should be measured to an accuracy of one degree or less.

II. Constructing your EarthDial

A. Some Vocabulary

EarthDial Vocabulary
An EarthDial is one of the oldest and simplest types of sundial, consisting of a vertical post, or gnomon, that casts a shadow on to a horizontal surface. You read the time and the date from the position of the center of the shadow of the marker (or nodus), in this case a sphere that is mounted on the top of the post. A network of lines on the dial face indicates the time of day depending on the position of the shadow. The lines consist of hour lines, which mark the time of day (in local solar time) and date curves, which mark the path of the nodus's shadow on special dates of the year (the solstices and equinoxes). The shape of the hour lines and date curves depends upon your latitude.

B. A Sample EarthDial

Here’s an example of what a EarthDial might look like when it’s done.

A Sample EarthDial
Credit: Bill Nye

The following instructions will take you through the steps of building an EarthDial like this one. But you can build your EarthDial of any materials you want, so long as the final product is sturdy and weatherproof for at least six months, as well as meeting the specifications for the sundial design given below.  You’ll have to lay out the dial, draw the hour lines and date curves, add the gnomon, and then decorate the dial according to your own location and language.  Are you ready?

C. Summary of EarthDial Specifications

Your EarthDial should be constructed to the following specifications.

EarthDial Specifications

D. Materials

1. Consumable Materials

If you are unfamiliar with some of these items, just bring the list to your local hardware store.  The most expensive item is the plywood, which should cost between $10 and $30 depending on its quality.  The rest of the items will cost under $10—and could cost you nothing if you have a workshop to scrounge around in!

2. Tools

E. Lay out the Circular Base

Lay out the circular base
Lay out the circular base

For all of the following diagrams, purple lines and text indicate construction lines; mark them lightly.  Black lines and text indicate lines and text that will appear on your completed EarthDial.

How to install the T-nut
How to install the T-nut

F. Lay out the Hour Lines

Take your time and be careful with your measurements.  Distances should be measured to an accuracy of 2 mm or less.  Angles should be measured to an accuracy of 1 degree or less.

Note: If you are nearer to the Equator than about 3 degrees of latitude, the following construction method will not work; send us an email to find out how to construct the lines.

The hour lines for your EarthDial will be unique to your latitude.  Find the latitude and longitude of the spot where your EarthDial will be set up.  For example, the latitude and longitude of Seattle, USA is 47° 41' (47.68°) North, 122° 21' (122.35°) West.  You can find your latitude and longitude by inspecting a good map, or by visiting one of these websites:

Entire world: http://www.getty.edu/research/conducting_research/vocabularies/tgn/

USA Only: http://www.census.gov/cgi-bin/gazetteer

Here are some examples of what EarthDials look like for different latitudes:

EarthDial layouts at different latitudes

The diagrams for the rest of these instructions show a Northern Hemisphere dial.  If you are instead in the Tropics or in the Southern Hemisphere, refer back to these drawings to remind you of the general appearance of an EarthDial.

Sample EarthDial calculator output
Measuring the hour lines

Finding point
Finding point "C"

The position of the gnomon, or vertical post, is at the center G of the circles. The hour lines will be drawn from a point C directly south (in the Northern Hemisphere) or north (in the Southern Hemisphere) of the gnomon position G.  The distance CG depends on your latitude.  When you set up your EarthDial, the line CG will be aligned exactly north-south.

Lay out the hour lines
Lay out the hour lines

G. Draw the Date Curves

The EarthDial Calculator also gives you the information you need to draw date curves on your EarthDial:

Sample EarthDial calculator output

Date curves show the graceful track of the ball’s shadow across the face of the dial on four special times of the year: the summer and winter solstices, when the Sun is highest and lowest in the sky, and the autumnal and vernal equinoxes, when the Sun tracks a great circle and is, for observers near the Equator, directly overhead at noon. If you wish, you do not need to draw these Date Curves, but the hour lines are essential.

If your latitude is between the Arctic and Antarctic Circles, the date curves are hyperbolae. You can draw these hyperbolae using the information provided in the EarthDial Calculator. The Calculator gives you three distances along each hour line from point C: the first (CS) is for the summer solstice line, the second (CE) for the equinox line, and the third (CW) for the winter solstice line (see the diagram below).

Lay out the date curves
Lay out the date curves

Note: Depending on your latitude, some of these curves will not fall within the 70-cm inner circle of your EarthDial.  If you are farther north or south than 50° latitude, the winter solstice line will fall outside the circle.  If you are inside the Arctic or Antarctic Circles, the summer solstice line (and all date curves) will be an ellipse.

Notice that for any EarthDial anywhere in the world, the equinox line runs due East and West. If the points you plotted for your equinox positions don’t lie in a straight line running exactly East and West, you’ve made a mistake somewhere.

H. Mark and Decorate Your EarthDial

1. Darken the pattern

Darken the pattern
Darken the pattern

On bare wood, you can use a black felt-tipped marker to darken the lines.  Otherwise, you can use black paint.  Making the lines by hand with paint can render the lines distinct and handsome.

In order for your EarthDial to be legible through a Web camera, make sure of the following:

2. Add text to the EarthDial

You can either do this by hand with paint or markers, or print the numbers and words on your printer and glue them to the EarthDial’s surface with white glue or decoupage adhesive.

Add the hour numerals
Add the hour numerals

The hour markings are for local solar time; thus, local solar noon (labeled 12) is the hour line running from point C exactly north (in the Northern Hemisphere) or south (in the Southern Hemisphere); time increases clockwise in the Northern Hemisphere and anti-clockwise in the Southern Hemisphere.  (When clocks were being developed in Europe during the Middle Ages, why do you think they chose to have the hours on the clock face increase in one particular way?)

Add the text
Add the text

47° 41' N
122° 21' W

3. Add Optional Features

Add optional features
Add optional features

4. Final Finishing

When you are done with your artwork and decorating, weatherproof your EarthDial with a coat or two of varnish.

I. Add the Gnomon

The center of the gnomon should be 8 centimeters above the board
The center of the gnomon should be 8 centimeters above the board

Cut the threaded rod so that it will support the center of the Ping-Pong ball at exactly 10.0 cm above the EarthDial’s surface.  Accuracy is extremely important in this step.  In the sample Seattle EarthDial, the rod is 13.5 cm long.  This allows for the rod to touch the top of the inside of the ball and penetrate the surface of the board by about 1.5 cm.

III. Setting up your EarthDial

A. Location

EarthDial on a flat roof
EarthDial on a flat roof

To help gauge shadowing problems, Table A shows the maximum altitude angle of the Sun (when it is due south in the Northern Hemisphere, or due north in the Southern Hemisphere) for early January and late June. An altitude angle of 90° means that the Sun is straight overhead; 0° means that the sun lies on the horizon. Remember that these are the maximum altitudes (at noon); the Sun will be at lower and lower altitudes as one moves away from noon. You can see that the altitude angle of the Sun depends very much on your latitude!

Table A: Maximum Altitudes of the Sun
LatitudeJanuaryJuneSun Direction
70°N -- 43° S
60°N 53° S
50°N 16° 63° S
40°N 26° 73° S
30°N 36° 83° S
       
30°S 84° 37° N
40°S 74° 27° N
50°S 64° 17° N
60°S 54° N
70°S 44° -- N


B. Align Your EarthDial

Any sundial, if it is to read correctly, must be leveled and aligned north and south.  This must be done very precisely!  Each degree that it is misaligned will lead to about a 4-minute error in telling the time.

There are three main techniques for alignment, listed below. The magnetic compass technique is least accurate and the gnomon technique is most accurate. If you have to shift the EarthDial significantly in order to align it, it may have gotten out of level. Recheck the level when you’re done.

1. Use a Magnetic Compass

http://gsc.nrcan.gc.ca/geomag/field/mdcalc_e.php

2. Use the Sun

http://www.solar-noon.com/

3. Use the Gnomon

IV. Set Up Your Web Camera

EarthDial with no-shadow mount on a deck
EarthDial with no-shadow mount on a deck

In order for all of the EarthDials to have a consistent appearance on the website, the Web camera must be on the south side of the EarthDial, aiming north.  Regardless of which hemisphere you’re in, have your camera point north.  When picking your EarthDial’s location, be sure to make room for a camera mount on the south side.

Design your Web camera mount so that it casts the least shadows possible.  This will not be a problem for Southern Hemisphere cameras, but camera mounts in the Northern Hemisphere will often cast shadows on the EarthDial faces.  The example below shows a Northern Hemisphere mount that does not cast a shadow; the yellow Webcam is pointed at the dial from the south side.

Because the Web camera will be located outside, it must be weatherproof.  The following instructions are one way you can set up a weatherproof Web camera.

Ideally, the camera should be able to run 24 hours a day (or at least from before sunrise to after sunset).

A. Materials

1. Hardware

2. Tools

B. Install your Web Camera and Serve it over the Internet

C. Weatherproof your Web camera

A weatherproof Webcam enclosure
A weatherproof Webcam enclosure

D. Set Up the Camera to View Your EarthDial

Set up the Webcam
Set up the Webcam
No shadow mount for the Webcam
No shadow mount for the Webcam

Let the world see your dial and, if possible, not your tripod or camera mount or its shadow. At right is an example camera mount for the Northern Hemisphere that does not cast a shadow on the face of the Dial.

EarthDial as seen through the Webcam
EarthDial as seen through the Webcam

V. Appendices

A. Converting Between Sundial Time and Clock Time

Many would say that sundial time is truer and provides more insight into the nature of our world than our artificially constructed clock time.  A sundial indicates local solar time, which is time measured by the Sun at a given location; in general, this time differs from clock time.  For example, it is local solar noon when the Sun is highest in the sky. At local solar noon the Sun is generally due south for observers in the Northern Hemisphere, or due north for observers in the Southern Hemisphere. Between the Tropics of Cancer and Capricorn, however, the noontime Sun is to the south on some dates and to the north on others (depending on the latitude).

If you wish to read clock time with your EarthDial (or any sundial), there are three adjustments you need to make: 

1. Adjustment for Location Within Your Time Zone

The Earth’s rotation causes the Sun to appear to move from east to west. So the Sun arrives on the eastern edge of a time zone long before it arrives on the western edge. Consider Seattle, which keeps Pacific Standard Time (PST) on its clocks. At Seattle’s latitude, the two edges of the Pacific Time Zone are over 800 kilometers apart. When sundials on the eastern edge correctly read 12:30 pm, sundials on the western edge (also correctly) read 11:30 am. The sundials differ by an hour, while everyone’s watch in the Pacific Time Zone reads exactly 12:00 noon. Yikes! Imagine two people at the Pacific Time Zone’s edge. They are shaking hands and admiring a sundial correctly telling local solar time, while their watches disagree by one full hour. (In a sundial-admirer’s sense, the sundial is the only trustworthy timepiece available.) We need a rule to compensate for the width of time zones.

The Rule.  For every degree of longitude you live west of the center of your time zone, add 4 minutes to the sundial reading to adjust to clock time.  If you live east of the center, then subtract 4 minutes for each degree.

Most time zones are one hour or 15 degrees of longitude wide.  With 60 minutes in an hour, it thus takes 4 minutes for the Sun to sweep through 1 degree.  Seattle is at 122.3° West longitude, so dials in Seattle are 2.3° x 4 = 9.2 minutes behind clocks and watches.  One should add 9.2 minutes to the dial reading. 

You can find the center longitude of your time zone by noting how many hours your clocks are ahead of or behind Greenwich Mean Time GMT (also called Universal Time), where longitude is defined to be 0°.  For each hour that you are ahead of (behind) GMT, the center of your time zone is shifted 15° of longitude to the East (West).  For example, PST is 8 hours behind GMT, and thus its center longitude is 120° West.  For another example, India Standard Time is 4.5 hours ahead of GMT, and thus its center longitude is 67.5° East.

2. Adjustment for Summer Time

If Daylight Saving Time (Summer Time) is in force, add one hour to the sundial time to get clock time.

3. Adjustment for the Non-Uniformity of Solar Time

We have created clock time so that it proceeds absolutely uniformly - a second is a second, an hour is an hour, day after day after day.  But the rate of solar time turns out to be slightly variable.  The interval from one solar noon to the next averages exactly 24 hours over a year, but it can be as much as 30 seconds longer or shorter on any given day.  This behavior of the sun’s apparent motion is caused by the slight non-circularity (1.7%) of the Earth’s orbit and by the famous 23.4° tilt of the Earth’s rotation axis.  The net effect is that dial time can differ as much as 15 minutes from clock time (even if you are at the center of your time zone).  This adjustment is called the Equation of Time.  This is an old usage of the word “equation”; think of it as equating one system of time to another.  The required time to add or subtract for each day of the year is given at this website:

http://ourworld.compuserve.com/homepages/patrick_powers/EoT.htm

Be careful to make this adjustment with the correct sign; “dial slow” or “sun slow” means that sundial time lags behind clock time.

B. Equations for Hour Lines and Date Curves

1. Distance CG

The hour lines will be drawn from a point C directly south (in the Northern Hemisphere) or north (in the Southern Hemisphere) of the gnomon position G.  The distance CG depends on your latitude and the height of the gnomon.  The distance CG is given by

CG = (gnomon height) / tan(latitude)

The EarthDial gnomon is 10 cm tall.

2. Hour Lines

The angle B of an hour line that is H hours before or after noon is given by

tan B = sin(latitude) x tan(H x 15°)

Example: for a latitude of 30°, the angle of the hour line for both 11 am and 1 pm (H = 1 hour), measured with respect to the noon (north-south) line, is B = 7.6°.  For H = 6 hours, the value of B is 90° for all latitudes.

The hours always increase from west to east, regardless of the latitude, with the result that in the Northern Hemisphere, the shadow moves clockwise during the course of the day.  In the Southern Hemisphere, the shadow moves anticlockwise during the day.

3. Date Curves

Date curves are drawn for special dates of the year: the equinoxes, when the solar declination D is 0°, and the solstices, when the solar declination is ±23.4°. 

For each value of H, define F to be

F = tan-1 [ tan(latitude) / cos(H x 15°) ] ,

and define R to be

Definition of R.

For each value of H, calculate R for solar declinations D of –23.4° (December), 0° (March and September), and +23.4° (June).

These values of R are the distances along each hour line, measured from point C, to the intersection points with each date curve.  The summer solstice curve lies closer to point C than the winter solstice curve.  The equinox curve will lie in between and will be a straight line. If you want to draw a date curve for some other special date, first find the solar declination for that date at this website:

http://www.gcstudio.com/

Then use the same formulas above with the appropriate value of D.

For easy conversion between metric, Imperial, and point size units (for fonts), use this chart.

Length Unit Conversions
MetricImperialPoints
2 mm 0.08 in 8 pt
4 mm 0.16 in 16 pt
3 cm 1.18 in 124 pt
3.5 cm 1.38 in 144 pt
10 cm 3.94 in --
70 cm 27.6 in --
80 cm 31.5 in --
astronaut on Phobos
Let's Change the World

Become a member of The Planetary Society and together we will create the future of space exploration.

Join Today

Mars
Advocacy

Our Advocacy Program provides each Society member a voice in the process. Funding is crucial.

Donate

You are here: