Multiple Choice (circle correct answer within brackets)

1. The 12 o'clock hour line (also known as the meridian line) that divides the dial face in half represents your line of [latitude, longitude].

2. Viewed from above the North Pole, the Earth appears to rotate [clockwise, counterclockwise]. Viewed from above the South Pole, the Earth appears to rotate [clockwise, counterclockwise]. Hint: If necessary, experiment with a virtual globe before answering these questions.

3. Sun shadows fall in the [same, opposite] direction as the Sun.

4. During the day, the gnomon/nodus shadow appears to move [clockwise, counterclockwise] around the dial face.

5. The Earth's [rotation, revolution] causes the gnomon/nodus shadow to appear to move around the dial face.

6. The Earth's [rotation, revolution] causes the position of the nodus shadow to move north-south between the declination lines (date curves).

7. Daylight Saving Time is Standard Time (wristwatch time) plus one hour. When Daylight Saving Time (DST) is in effect, Solar Time (sundial time) is [earlier, later] than DST. Hint: Remember the mnemonic, "Spring forward; fall back."

Short Answer (write clear and concise answers)

1. For horizontal sundials located in the Northern Hemisphere, why are morning times located on the western side (left side, facing north) of the dial face?
   
   
   
   
2. During the day, when are shadows longest? Shortest? Hint: Refer to a time-lapse animation of ED-7 for the past 24 hours (~2.3 MB).
   
   
   
   
3. Annually, when are shadows longest at midday? Shortest? Hint: See solstice/equinox diagram.
   
   
   
   
4. For an EarthDial located in the northern mid-latitudes, when is the one time of day when the shadows of the gnomon and nodus fall upon the exact same hour line? Hint: Refer to a time-lapse animation of ED-7 for the past 24 hours (~2.3 MB).
   
   
   
   
5. It's 11:00 a.m. Eastern Standard Time (EST). The FCPS/NOVAC EarthDial (ED-7) is visible in the current webcam image, but the gnomon/nodus does not cast a shadow. What information is available on the ED-7 Webcam home page that could help to explain your observations?
   
   
   
   
6. When doesn't a [horizontal] sundial (such as ED-7) work?
   
   
   
   
7. The Sun and Earth are parts of an interconnected system. Use the words "rotation" and "revolution" to summarize your short-term (day-long) and long-term (year-long) observations of the EarthDial.
   
   
   
   

For Further Thought...

1. Are you clock-wise? Why is "clockwise" clockwise? Remember, mechanical clocks were invented in the Northern Hemisphere. Hint: Refer to time-lapse animations of ED-7.
   
   
   
   
2. Why is it incorrect to say that 12 noon is 12 p.m.? Hint: The "m" in p.m. means "meridian" or "midday."
   
   
   
   
3. Why do time zones generally run north-south instead of east-west? Why are time zones nominally 15 degrees of longitude wide? Hint: There are 24 Time Zones around the world.
   
   
   
   
4. Along which line of latitude is the subsolar point currently located? Which online EarthDial is located closest to this line of latitude? For the same time of day (e.g., 12 noon), compare/contrast the gnomon/nodus shadow cast by the EarthDial nearest the latitude of the subsolar point with other EarthDials located at more northerly and southerly latitudes.
   
   
   
   
5. Would your EarthDial read the same time as another sundial 100 miles directly north of you? Would the shadows be the same length?
   
   
   
   
6. Would your EarthDial read the same time as another sundial 100 miles to the east or west of your location? Support your answer with observations of at least two other EarthDials.
   
   
   
   
7. Does an EarthDial work the same north and south of the Equator? Would an EarthDial work at the North and South Poles?
   
   
   
   

Bonus Question

List three reasons that Solar Time (sundial time) may be different from Standard Time (wristwatch time). Hint: Refer to "Solar Time Versus Standard Time," SCSA Educator's Guide to Equatorial Sundials.

Activity Extension

Geography: Location, Location, Location!

On a world map, plot the location (latitude & longitude) of all online EarthDials. Compare your map of EarthDial locations to a world map of Time Zones; see if you can determine the Time Zone in which each EarthDial is located. Determine the time difference (to the nearest hour) between the Valencia, Spain EarthDial (ED-8) and another EarthDial (your choice) that is located in a different Time Zone. Use the map scale to measure the approximate distance between the two EarthDials. Credit: World map courtesy National Geographic Xpeditions.

EarthDial Watch

Keep a daily log of EarthDial observations. Record items such as first/last light, first/last appearance of the gnomon shadow as well as the azimuth of the Sun (as indicated by the gnomon shadow), first/last appearance of the nodus shadow, and the estimated time difference between 12 noon Standard Time (refer to the ED-7 image time-stamped 12:00:00 EST) and Local Apparent Time (sundial time). As the seasons change, carefully observe the annual cycle of change in the path of the nodus shadow across the dial face. Record your observations by creating time-lapse animations of EarthDial imagery. Use the ED-7 image archive to collect a time series of images periodically, e.g., on the 1st and 15th of each month. Keep a daily log of weather observations; correlate past weather with past images (of ED-7).

Print a hardcopy of the EarthDial at a time when the sundial is visible but there are no shadows on the dial face. At the same time each day (refer to the date/time stamp superimposed on images), e.g., 12 noon, plot a point on the dial face showing the position of the nodus shadow. Alternatively, use a small self-adhesive colored dot to mark the position of the nodus shadow. (Note: When Daylight Saving Time is in effect, readings should be taken one hour later. For example, a 12 noon reading should be taken at 1 p.m.) Plot points for the 1st and 15th of each month; label each point with the date & time. If you miss a day (e.g., due to the weather), then collect data on another day soon afterwards. Over the course of a year, you should be able to trace the analemma.

Image Processing: Is it Really Dark at Night?

If you could see the gnomon/nodus shadow cast by the Moon at night, then would an EarthDial work as a "MoonDial?" Use image processing software to enhance night-time EarthDial imagery. For example, see how FCPS Planetarium Teacher Lee Ann Hennig literally turned night into day. Cool, huh? That's the power of image processing! In this case, the shadow cast by the gnomon/nodus is caused by a low-power outdoor light. See if you can "see" actual Moon shadows on the dial face. Hint: It may be helpful to refer to the USNO Phases of the Moon Web page.

The SCSA recommends the following free- or shareware image processing software applications: NIH Image (Macintosh); Scion Image (Windows); Image/J; or Image 2000. All of these software applications (and more) are linked from the Center for Image Processing in Education (CIPE) Web site.

Build Your Own EarthDial and Join The Project

Download Make an EarthDial (right-click, Save Target As...), a free Microsoft® PowerPoint® presentation by Dr. Robert Kellogg, Treasurer, North American Sundial Society, that automatically generates a customizable EarthDial suitable for printing on 8.5" x 11" paper. (Print using cover stock.) Visit the EarthDial Project website to learn more about how you can join the project. Note: The beta version of this program works beautifully using the Microsoft® Windows XP operating system; the dial face may not render correctly using earlier versions of Microsoft® Windows, e.g., Windows 2000 Professional.