EarthDial Activity
Teacher's Answer Key
A. line of longitude (meridian)
A. counterclockwise; clockwise
A. opposite
A. Answers will vary depending upon the location of the EarthDial. In the Northern Hemisphere, the gnomon/nodus shadow appears to move clockwise around the dial face; in the Southern Hemisphere, the gnomon shadow appears to move counterclockwise around the dial face.
A. rotation
A. revolution
A. Solar time is one hour earlier than Daylight Saving Time (sundial is "slow").
A. Shadows fall in the opposite direction as the Sun. When the Sun rises in the east, morning shadows fall toward the western (left) side of the sundial.
A. Longest at sunrise & sunset. Shortest at 12 noon Local Apparent Time.
A. Northern Hemisphere: December Solstice (longest); June Solstice (shortest). Southern Hemisphere: June Solstice (longest); December Solstice (shortest).
A. 12 noon Local Apparent Time.
A. Check The Weather Channel "Weather Magnet" (left sidebar) to see if the condition of the sky is "mostly cloudy" or "cloudy."
A. When the sky is overcast. At night.
A. As the Earth rotates counterclockwise in the Northern Hemisphere, the gnomon/nomon shadow moves clockwise around the dial face (vice-versa in the Southern Hemisphere). As the Earth revolves around the Sun, the line of latitude where the Sun is at the zenith moves north-south between the Tropics of Cancer and Capricorn (in an annual cycle). Similarly, the nodus shadow moves north-south between the declination lines (date curves) on the dial face.
A. For objects in the Northern Hemisphere, shadows cast by the Sun move in a clockwise direction, therefore the hands of analog clocks (and watches) were made to turn in the same direction.
A. "ante" means before; "post" means after; "meridian" means midday. "p.m." literally means, "past midday." Noon is midday--it is neither before nor after 12 o'clock.
A. Lines of longitude run north-south. Longitude is equivalent to time (see Equatorial Sundial Activity Questions, No. 3, For Further Thought), therefore time zones run north-south. The Earth rotates at the rate of 360°/24 hr which reduces to 15°/hr, therefore all time zones are 15 degrees of longitude (or one hour) wide.
A. For the same time of day, shadows should be shorter for EarthDials located near the latitude of the subsolar point and grow progressively longer for more northerly and southerly locations.
A. "...when it is noon at any given place, it is noon at all other places on the same meridian (having the same longitude); and in places having different meridians, it is forenoon if they are west and afternoon if they are east of the given place." Quote courtesy Sundials: Their Construction and Use, R. Newton Mayall and Margaret W. Mayall, Dover Publications, Inc., ©2000, p. 34. As a general rule of thumb, shadows should be longer to the north, although there are exceptions to the rule.
A. "...when it is noon at any given place, it is noon at all other places on the same meridian (having the same longitude); and in places having different meridians, it is forenoon if they are west and afternoon if they are east of the given place." Sundials: Their Construction and Use, R. Newton Mayall and Margaret W. Mayall, Dover Publications, Inc., ©2000, p. 34
A. In principle, an EarthDial sundial works the same north and south of the Equator. In the Southern Hemisphere, the gnomon should point toward the Celestial South Pole, and morning & afternoon times would be reversed on the dial face.
At the North and South Poles, a horizontal sundial is effectively an equatorial sundial that works for six months of the year. (See horizontal sundial latitude applet, NASA Liftoff to Space Exploration How Sundials Work Web page; gradually increase latitude to 90 degrees, at which point all hour lines are spaced exactly 15 degrees apart like the dial face of an equatorial sundial.) For example, at the North Pole an equatorial sundial will work from the MAR Equinox to the SEP Equinox; at the South Pole an EarthDial (a horizontal/equatorial sundial) will work from the SEP Equinox to the MAR Equinox.
A. Daylight Saving Time (most obvious), difference in longitude (between the observer and the Standard Time Meridian), and Equation of Time. For an overview, see Solar Time Versus Standard Time.
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.
A. Answers will vary.
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.
A. Students should look for Moon shadows near the date of the Full Moon (when the Moon's apparent magnitude is greatest).
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.
