Multiple Choice (circle correct answer within brackets)

1. The gnomon (or style) of an equatorial sundial represents the Earth's [axis, Equator] of rotation.

   A. axis

2. The dial plate of an equatorial sundial represents the plane of the Earth's [axis, Equator].

   A. Equator

3. The upper dial face of an equatorial sundial represents the [Northern Hemisphere, Southern Hemisphere].

   A. Northern Hemisphere

4. The lower dial face of an equatorial sundial represents the [Northern Hemisphere, Southern Hemisphere].

   A. Southern Hemisphere

5. 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].

   A. longitude (meridian)

6. Relative to a horizontal surface, the gnomon of an equatorial sundial should be inclined at an angle equal to your [latitude, longitude].

   A. latitude

7. Relative to a horizontal surface, the dial plate of an equatorial sundial should be inclined at an angle equal to the [complement, supplement] of your latitude.

   A. complement

8. [Complementary, Supplementary] angles are two angles for which the sum of their degree measurements equals 90 degrees.

   A. Complementary

9. 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.

   A. counterclockwise; clockwise

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

    A. opposite

11. For equatorial sundials located in the Northern Hemisphere, morning times are located on the [left, right] side of the upper dial face (facing south).

    A. right (see example)

12. During the day, the gnomon shadow appears to move [clockwise, counterclockwise] around the [upper, lower] dial face.

    A. Answers will vary depending upon the location of the sundial and time of year. For Northern Hemisphere locations, the gnomon shadow appears to move clockwise around the upper dial face from the MAR Equinox to the SEP Equinox; from the SEP Equinox to the MAR Equinox, the gnomon shadow appears to move counterclockwise around the lower dial face. The opposite is true for Southern Hemisphere locations.

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

    A. rotation

14. The Earth's [rotation, revolution] causes the gnomon shadow to move from the upper dial face to the lower dial face, and vice versa.

    A. revolution

15. 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."

    A. earlier.

Short Answer (write clear and concise answers)

1. When doesn't an equatorial sundial work?

   A. When the sky is overcast. At night (after sunset/before sunrise). On the day of the equinoxes (see Question No. 2, below).

2. Explain why the gnomon (or style) of a properly oriented equatorial sundial will not cast a shadow on the dial plate on the day of the equinoxes (MAR 20, SEP 22).

   A. Theoretically, the gnomon (or style) of a properly oriented equatorial sundial will not cast a shadow on the dial plate during the equinoxes. Because the dial plate of an equatorial sundial is parallel to the Earth's Equator, the Sun is directly over the edge of the dial plate on the equinoxes, when the Sun is at the zenith along the Equator. From the March Equinox to the September Equinox, when the Sun is at the zenith in the Northern Hemisphere (between 0° and 23.5°N latitude), the gnomon shadow falls on the upper dial face; from the September Equinox to the March Equinox, when the Sun is at the zenith in the Southern Hemisphere (between 0° and 23.5°S latitude), the gnomon shadow falls on the lower dial face. See Equatorial Sundials and the Sun's Apparent Path Across the Sky.

3. The Earth rotates once every 24 hours (approximately). How many degrees does the Earth turn in one hour? Hint: One complete rotation of the Earth is 360 degrees. Verify your answer by using a protractor to measure the angle formed by the sundial center and two adjacent hour lines on the dial face.

   A. 15 degrees per hour. Before assembling the equatorial sundial template, students should use a protractor to measure the angle formed by the center of the sundial (vertex) and two adjacent hour lines on the dial face (rays); every hour equals 15 degrees.

4. Why do time zones generally run north-south instead of east-west? Why are time zones nominally 15 degrees of longitude in width? Hint: There are 24 Time Zones around the world.

   A. Lines of longitude run north-south. Longitude is equivalent to time (see No. 3, For Further Thought), therefore time zones run north-south. All time zones are one hour wide; the Earth rotates at the rate of 15°/hr.

5. 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 equatorial sundial.

   A. As the Earth rotates counterclockwise in the Northern Hemisphere (clockwise in the Southern Hemisphere), the gnomon shadow moves clockwise around the upper dial face (and vice versa on the lower dial face). As the Earth revolves around the Sun, the gnomon shadow moves from the upper dial face to the lower dial face (and vice versa) because the line of latitude where the Sun is at the zenith (a.k.a., the subsolar point) moves north-south between the Tropics of Cancer and Capricorn in an annual cycle. The analemma--the odd-looking figure eight that appears on many globes--neatly traces this annual north-south solar cycle. (For a detailed explanation, see Question No. 2, above.)

For Further Thought

1. Are you clock-wise? Why is "clockwise" clockwise? Remember, mechanical clocks were invented in the Northern Hemisphere.

   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.

2. Why is it incorrect to say that 12 noon is 12 p.m.? Hint: The "m" in p.m. means "meridian" or "midday."

   A. "Ante" means before; "post" means after; "meridian" means midday. "p.m." literally means, "after midday." Noon is exactly midday--it is neither before nor after 12 o'clock.

3. Longitude is equivalent to time (and vice versa). Explain. What would be the difference in longitude between two sundials separated by a 12-minute time difference? Hint: Reduce the rate of the Earth's rotation from degrees/hour to degrees/minute.

   A. Longitude and time are equivalent: there are 360 degrees of longitude (0-180 degrees east & west of the Prime Meridian); the Earth rotates 360 degrees per day (24 hours). Therefore, each degree of longitude is equal to four minutes of time (based upon the Earth's constant rate of rotation of 360°/24 hr, or 15°/hr, or 1°/4 min).

   15°/hr or 15°/60 min reduces to 1°/4 min, therefore a 12-minute time difference equates to a difference of three (3) degrees in longitude: 12 min/1 x 1°/4 min = 3° longitude

4. Would your sundial read the same time as another sundial 100 miles directly north of you? Would the gnomon shadows be the same length?

   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.

5. Does an equatorial sundial work the same north and south of the Equator? Would an equatorial sundial work at the North and South Poles?

   A. In principle, an equatorial 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 faces.

   At the poles, an equatorial sundial is effectively a horizontal 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.

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.

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.