SUNDIAL - How To Make / Read

http://www-spof.gsfc.nasa.gov/stargaze/Sundial.htm

http://www-spof.gsfc.nasa.gov/stargaze/Sfigs/Sundial1.gif

(2a)  The Sundial

  Index

 1. Stargazers 

 1a. Celest. Sphere

 1b. Pole Star

 2. The Ecliptic
 2a. The Sundial

 3. The Seasons

 3a. Angle
        of sunlight

 4. The Moon (1)

 4a. The Moon (2)

 4b. Moon Libration

 5.Latitude and
        Longitude

 5a. Navigation

 5b. Cross-Staff
    "Some people can tell what time it is by looking at the sun. But I have never been able to make out the numbers."
            (Attributed to an essay by a student
             in elementary school.)
The simplest sundial is a vertical stick rising from a flat horizontal surface.[IMAGE: A Simple Sundial]
As the Sun rises, passes the highest point in its path (at noon and to the south, in the northern hemisphere) and sets, the shadow rotatesaround the stick in a clockwise direction, and its position can be used to mark time. Indeed, it has been claimed that the "clockwise" direction in which the hands on a clock rotate was chosen for this reason.
A sundial with a vertical pointer ("gnomon") will indicate noon correctly when its shadow points north or south. [North in northern middle latitudes, south in southern ones, while near the equator it can be either way, depending on season.] However, the direction of the shadow at some other time of the day may depend on the season--its value in summer, when the Sun's path is high, may differ from what it is in winter, with Sun low above the horizon.

Such a sundial will however work equally well at all times if the pointer is slanted, to point towards the pole of the celestial sphere (click here for an explanation--but be warned, it is a bit complicated!). The angle between it and the base then equals the geographic latitude of the user.

A Paper Sundial

Ornamental sundials are often found in parks and gardens, with the pointer widened into a triangular fin, which must point northwards. A sundial of this type can be constructed from folded cardboard or stiff paper: click here to see the basic design used around latitude 38 North of the equator, here for a corresponding one in the southern hemisphere. Either can be printed and then photo-copied onto suitable sheets of stiff paper or cardboard [You may want to use the "option" menu to reduce size to 90% before printing--but make sure to return the setting to 100% afterwards!]. It is meant to be used at a latitude of 38 degrees and should work adequately in most of the continental US.

Instructions:

  1. Cut the paper along the marked line: one half will serve as base, the other will be used to construct the gnomon.
  2. In the gnomon part, cut away the two marked corners.
  3. Fold the sheet in its middle, in a way that the two secondary printed lines (leading to the cut-off corners) remain visible. The line of the fold is the gnomon.
      Note: In stiff paper, straight folds are helped by first scoring the paper, by drawing a line along them with a black ballpoint, guided by a ruler and pressed down hard.
  4. With the page folded in its middle, cut out along the curved line, cutting a double thickness of paper in one cut. The cut begins near the top of the gnomon-fold and ends on the secondary line. Do not cut along the secondary line. No pieces come off.
  5. Score the other two secondary lines, then fold the gnomon sheet along them. The fold is opposite to that of the fold in the middle. These two folds should form 90-degree angles, so that the two pieces with the corners not cut in step 2 can be placed flat on the table, and the triangular gnomon rises above them.
  6. In cut (4), the fin of the gnomon was separated from two pieces with curved outlines. Fold those pieces so that they, too, are flat with the table. One goes above the other, and the slots they form near the secondary lines create a place for the fin to fit into.
  7. You are almost done. Take the base sheet, and note the apex where the hour-lines all meet (that is where the bottom corner of the fin will go). Carefully cut the sheet from this point along its middle line, up to the small cross-line marked on it. Do notcut any further!
  8. Slide the fin into the cut you made, so that all horizontal parts of the first sheet arebelow the base sheet; only the fin sticks out. Its bottom corner should be at the apex.
      The sundial is now ready, but you might use tape on the bottom of the base-sheet to hold the two pieces together firmly. For further stability, and to prevent the sundial from being blown away, you may attach its base with thumbtacks to a section of a wooden board or a piece of plywood.
  9. Finally, orient the fin to point north. You may use a magnetic compass; before pocket watches were available, folding pocket sundials were used in Europe, with small magnetic compasses embedded in their bases. If clear sunlight is available, the shadow of the tip of the fin now tells the time.
If you want to make a sundial of more durable materials, draw the pre-noon hour lines at the angles to the fin (given in degrees) given below. These lines are meant for a latitude of 38 degrees; if your latitude is markedly different, see note at the end.

6 -- 90°   9 -- 31.6°
7 -- 66.5°  10 -- 19.6°
8 -- 46.8°  11 -- 9.4°

Accuracy

   The sundial will obviously be one hour off during daylight saving time in the summer, when clocks are reset.   In addition, "clock time" (or "standard time") will differ from sundial time, because it is usually kept uniform across "time zones"; each time zone differs from its neighbors by one full hour (more in China and Alaska). In each such zone, sundial time matches clock time at only one geographical longitude: elsewhere a correction must be added, proportional to the difference in longitude from the locations where sundial time is exact.

    (Up to the second half of the 19th century, local time and sundial time were generally the same, and each city kept its own local time, as is still the case in Saudi Arabia. In the US standard time was introduced by the railroads, to help set up uniform timetables across the nation.)
   Finally, a small periodic variation exists ("equation of time") amounting at most to about 15 minutes and contributed by two factors. First, the Earth's motion around the sun is an ellipse, not a circle, with slightly variable speed in accordance with Kepler's 2nd law (see here as well as the section preceding that page). Secondly, the ecliptic is inclined by 23.5 degrees to the equator, which means the projection of the Sun's apparent motion on it (which determines solar time) is slowed down near the crossing points of the two.

Note on Latitude

The angles listed above are intended for a latitude of 38 degrees. If your latitude is L, denotes "square root of" and K (=cotg2L) is
K = cos2L/ sin2L

then the angle between the fin and the line corresponding to the hour N+6 (N going from 0 to 6) satisfies
sin A = cos(15N) / (1 + Ksin215N)
Here 15N (=15 times N) is an angle in degrees, ranging from 0 to 90, and of course, the afternoon angles are mirror reflections of the morning ones. If your calculator has a button
(sin-1), if you enter (sin A) and press it, you will get the angle A. For an explanation of sines and cosines, look up the math refresher. And don't forget to adjust the angle of your fin to L, too!

And by the way...

The sundial described here, with a gnomon pointing to the celestial pole, is a relatively recent invention, probably of the last 1000 years. Yet sundials were used long before, often with unequal hours at different times of the day. The bible--2nd book of Kings, chapter 20, verses 9-11 (also Isaiah, ch. 38, v. 8) tells of an "accidental" sundial, in which the number of steps covered by the Sun's shadow on a staircase was used to measure the passage of time. In that story, the shadow miraculously retreated ten steps on the staircase built by King Ahaz.

Exploring Further

    The "Sundial Bridge," with a unique design which may well make it the largest sundial anywhere, opened July 4, 2004 in Turtle Bay Park in Redding, California, at the foot of Mt. Shasta. Designed by the innovative Spanish architect Santiago Calatrava, it resembles his stunning 1992 bridgeerected in Seville, Spain. It is a pedestrian bridge, connecting two parts of Turtle Bay Park, and it also operates as a sundial, using plaques set in a semicircular upper plaza.
[IMAGE: Sundial bridge]
For a more detailed article about this bridge, see Sundial Bridge at Turtle Bay

    Before the days of affordable wristwatches, people often carried a folding sundial in their pocket ("poke" below), with a small magnetic compass embedded, to show the north direction. In "As You Like It" by William Shakespeare (act 2, scene 7) one of the characters tells of meeting in the forest a fool (= witty court entertainer) carrying such a "dial":
"Good morrow, fool," quoth I. "No, sir," quoth he,
"Call me not fool till heaven sent me fortune:"
And then he drew a dial from his poke,
And, loking on it with lack-lustre eye
Says very wisely, "It is ten o'clock:
Thus we may see how the world wags:
'T is but an hour ago since it was nine;
And after one hour more 't will be eleven;
                              (and continues)

   You may also be interested to know that a North American Sundial Society (NASS) exists, with its home page at http://sundials.org. From the main page, the visitor can click on "About NASS", and/or access the many other features the site offers. And in case you wonder about the creatures drawn at the bottom of the "About NASS" page, they are toves, the whimsical invention of Lewis Caroll in his poem Jabberwocky. Concerning what toves are, see Humpty Dumpty's explanation, also reachable by clicking the winking sun icon on the top of the NASS home page. The reference is from the 6th chapter of Lewis Carrol's Through the Looking Glass.
The British Sundial Society also has its sundial page.
  A sundial was included as part of the Mars lander mission and is shown in "Astronomy Picture of the Day" for 28 April 1999. It has a thick vertical gnomon, so that its readings may need some extra corrections
.    For those with serious interest in history (and access to a good library): "The Material Culture of Astronomy in Daily Life: Sundials, Science and Social Change" by Sara Schechner (History of Sci. Dept., Harvard) Journal for the History of Astronomy Vol. 32, part 3, August 2001, p. 189-222, with many illustrations.
From the 1.1.2000 book catalog of Willman-Bell in Richmond, Virginia (www.willbell.com):
    Easy-to-make Wooden Sundials, by Stoneman, 38 pp., $4.95
    Sundials: History, Theory and Practice by Rohr, 230 pp, $12.95.
    Sundials: Their Theory and Construction by Waugh, 19 chapt., $8.95

Questions from Users:   What is the Analemma?
                  ***         The equation of time 
                  ***         The "Sundial Bridge" in Redding
  ***         Latest Sunrise, Earliest Sunset 









http://www.planetary.org/explore/projects/earth-dial/how-to-read-a-sundial.html


How to Read a Sundial

Or, 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 more than 800 kilometers (500 miles) apart. When sundials on the eastern edge correctly read 12:30 pm, sundials on the western edge (also correctly) read 11:30 a.m. 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 23.4 degree 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:

How Does a Sundial Work?
The sun shines on the gnomon (the triangle or device set
perpendicular to the base plate - pronounced NO-men) and casts
its shadow on the appropriate hour line, thus displaying the "time."
 
Information contained on this page:
What Goes Into Accurate Sundial Design?

What Can Be Done About the Time Change Of Daylight Saving Time?

Is One Type Of Sundial “Better” Than the Others?

Can You Purchase a Sundial That Displays Accurate Clock Time Or the Plans and Layout To Build One For Your Location?

Is It Difficult To Build Your Own Sundial?

How Does the Equation of Time Affect Sundial Accuracy?

Why Do Garden Centers and Nature Stores Sell Sundials That Cannot Display Accurate Clock Time?

What goes into accurate sundial design?
#1 - The angle on the gnomon has to be parallel to the earth's axis so as not to require manual adjustment for the changing "height" of the sun in the sky during the changing seasons. This angle must be equal to your latitude, if it is to display accurate clock time.

#2 - The position of the hour lines must be calculated and laid out based upon your latitude, also. Generally speaking, the farther south you live, your accurate sundial's hour lines will be closer together at the hours nearest noon and farther apart at hours nearing sunrise and sunset. The hour lines will not be equally spaced like those dials in the garden shop.
North Dakota Drawing


Texas Drawing


(Same longitude, different latitudes. Notice the difference in the height of the gnomons - laying on their sides - and the positions of the hour lines.)
 
Reason #3 – The position of the hour lines must be calculated and laid out based upon your longitude, also. There can be a difference of almost 2 hours, in some time zones, between two identical sundials, one placed at either East-West end of the time zone.
 
Michigan Drawing


Maine Drawing


(Same latitude with identical gnomons, different longitudes. Both dials in Eastern time zone.)
 
What can be done about the change of Daylight Saving Time?
All state’s clocks (except Arizona and part of Indiana) “spring ahead” by one hour in early April and “fall back” by one hour in late October. A sundial does not have “hands”, like a clock, which can be set ahead or back and it would be confusing to label a sundial with two sets of hour numbers.

The simple solution is for you to select having your dial labeled for either Standard Time or Daylight Saving Time, depending upon when your dial will experience its heaviest use. The 12-noon hour during Standard Time should be labeled as 1 PM for Daylight Saving Time, and all the other hour labels should be shifted ahead one hour also.

We, being in the northern part of the Central Time Zone, label all our local dials for Daylight Saving Time. (We recommend this option except for Arizona and a portion of Indiana.) We’re out in the yard viewing our dials often in the spring, summer and fall.

You can select either option for a sundial made for you by Accurate Sundials.

 
Is one type of sundial "better" than the others?
Each type of sundial has its advantages:
 
Horizontal Drawing-2


Horizontal Dial – Mounted parallel to earth’s surface
+ Easy to set up and read
+ Familiar to most people
+ Displays time from sunrise to sunset

 
Vertical South Drawing


Vertical South Dial - Mounted on a south-facing wall, tree, pole or structure
+ Mounted up and “out-of-the-way”
+ More unusual, a good conversation piece
- The sun can never shine on this dial before 6AM or after 6PM

 
Analemmatic Dial – Mounted parallel to earth’s surface. Gnomon is a “rod,” which must be moved into different "date-holes" in the base plate's Scale of Dates
+ Easy to set up and read
+ This sundial can be laid out in your yard to have rocks, flower beds or bushes as your hour points. Your Scale of Dates can be flagstones laid out in the proper positions that you stand on. You would become the gnomon, and your shadow would cast the correct time on your flowerbed hour-points.
+ Displays time from sunrise to sunset
+ More unusual, a good conversation piece
- Not quite as accurate as the other dials unless it’s made quite large (a smaller dial can’t accommodate 365 holes to move the gnomon into – one for each day of the year).

 
Analemmatic Drawing



Can you buy a sundial that displays accurate clock time or the plans and layout to build one for your location?Yes, but…

Caution #1 – Make sure your gnomon and hour lines will be calculated and designed for your specific latitude and longitude.
Caution #2 – Make sure you are asked for your specific latitude and longitude or your address, if the maker will find your latitude and longitude for you. The use of your ZIP code to determine latitude and longitude is not precise enough because many ZIP code areas are too geographically large to insure an accurate sundial at your specific location.

 
Is it difficult to build your own sundial?No:
+ If you have good instructions and an expert makes the calculations and prepares the layout for your specific latitude and longitude
+ It can be done in a couple of hours using common tools that most people have in their home
+ In fact, it makes an excellent project for school classrooms, Scouting, 4H and other youth organizations
+ It’s a good learning experience for anyone who undertakes it

 
How does the Equation of Time affect sundial accuracy?
It is important that your sundial display as accurate clock time as is possible because there is one correction that can’t be built into these dials, and this correction is for The Equation of Time. All stationary sundials will need to be mentally “corrected” for this factor to achieve accurate clock time.

This variation from clock time changes each day of the year causing the sundial’s time to appear to be “fast” by as much as 16 minutes or “slow” by as much as 14 minutes throughout the year. This variation has two main causes; (1) The earth moves more quickly in it’s elliptical orbit around our sun when closest to it and more slowly when it’s farthest away, and, (2) The sun “travels” across our sky on the ecliptic, as opposed to the equator.

Don’t let this phenomenon alarm you. It has its least effect in the months of April, May, June, July and August when a sundial in North America will likely experience its heaviest use. The maximum deviations during these months are from 4 minutes “fast” to 6 minutes “slow.” Accurate Sundials includes an ‘Adjustment for The Equation of Time Table and Explanation’ with your sundial purchase.

 
Why do garden centers and nature stores sell sundials that are not accurate?
Reason #1 – It is inexpensive to mass-produce sundials which are identical with no concern as to whether they will display accurate clock time.

Reason #2 – The seller doesn’t know that their dials do not display accurate clock time.

Reason #3 – The buyer doesn’t know that these dials do not display accurate clock time.

Reason #4 – Some buyers are looking for a lawn ornament and have no concern as to whether their sundial will display accurate clock time and no desire to properly set up their dial.

 
Please call us at (262)365-3610 for more information.

http://www.accuratesundials.com/site/591582/page/140934