I have a
little shadow that goes in and out with me,
And what can be the use of
him is more than I can see....
- My Shadow, Robert
Me and My Shadow
- Mills Brothers
Use a sundial
to measure Solar Time. Get the current Standard
Time from The
Official U.S. Time Web page.
Most days, Solar Time is slightly different from Standard
Time (up to 16 min.
fast or 14 min. slow). This time difference is known as the Equation of Time.
Ever wonder, "Why is
Clockwise clockwise?" In the Northern Hemisphere, shadows
cast by the Sun move in a
clockwise direction; as a result, the hands of analog clocks are
made to move in the same direction. In fact, the word hour means
"the day" or "Sun's path."
Every day, shadows...
...are shortest at noon, and longest
at sunrise & sunset. On June 21, noon shadows are the shortest of any day during the year (for northern mid-latitude locations), and
vice-versa on December 21 (see solstice/equinox diagram).
Due to the geometry
sundials, the gnomon shadow is the same length for the entire day
(although its length varies from day-to-day according to the annual cycle
of change in the declination of the Sun).
...equivalent to time, and vice
versa. If you know the time difference between two locations, then you
can use the rate of the Earth's rotation (15°/hr or 1°/4 min) to
calculate the difference in longitude between the two places.
For example, Solar Noon occurs eight minutes later in
Washington, D.C. than it does on the Standard Time Meridian for the Eastern Time Zone
(75°W); how many degrees of longitude separate the two locations?
8 min x 1°/4 min = 2°
Therefore, the longitude of Washington, D.C. is 77°W. Simple, huh?
Well, it wasn't always so easy! Read The Illustrated Longitude, the story of
clockmaker John Harrison, who solved the problem that Newton
and Galileo failed to conquer--how to determine longitude at sea. See
also, Lost at Sea--the Search for Longitude from PBS/NOVA
Long story short, sundials must be corrected for
longitude (as well as the Equation
of Time) so that Solar Time reads the same as Standard
Did You Know...
...that Earth is eight light-minutes
from the Sun? Huh? That's right. At the speed of light (186,000 miles per
second, or 300,000 km/sec), it takes nearly eight minutes for sunlight to
reach the Earth. The Earth is connected to the Sun, but it is a
30 JUL 2002
|Sunspot No. Trend
(past 24 hours)
Credits: Real-time image courtesy SOHO;
sunspot number courtesy NOAA.
Updated: 29 JUL 2002
I'm Being Followed...
...by a moonshadow.
- Moonshadow, Cat
The Moon, especially the Full Moon, is bright enough to cast shadows at
night. So, how bright does an astronomical object have to be to cast
shadows? Good question! In fact, the question may be impossible to answer,
since there are so many factors involved. However it is possible to
quantify the brightness or magnitude of a light source. Oddly, negative
magnitudes are brighter than positive ones. The Sky & Telescope magnitude Web page lists the following magnitudes
for the Sun, Moon, brightest planet, and brightest star (in the night
- Sun = -26.7m
- Full Moon = -12.5m
- Venus = -4.4m
- Sirius = -1.5m
"Night is a shadow world. The only shadows we see at night are cast by the
moonlight, or by artificial light, but night itself is a shadow."
of the Sky, an essay by Diane Ackerman
During the pre-dawn twilight, have you
ever noticed that birds wake up and sing all at once? Ornithologists call
this phenomenon the "Morning Chorus." Plan to wake up early tomorrow to
see the sunrise and
listen to the symphony of nature--a delightful way to feel more connected
to the natural world!
Sandburg Center for Sky Awareness
A Fairfax County Public Schools Planetarium
Me and My Shadow
On a sunny day, drive a stake into the ground and observe how its shadow
changes throughout the day or year--a simple yet profound way to demonstrate the interconnection between the Sun and Earth! The
following Web resources may help you to more fully comprehend the deeper
meaning of these simple observations.
Making the Sun-Earth Connection
Ever notice the odd-looking figure eight that appears on many globes? It's called an
analemma. Visit the Analemma Web site to learn more about, well,
analemmas, including an explanation of how the analemma is derived from
the stake you drove into the ground.
See Dennis di Cicco's award-winning time lapse photograph of the Sun (showing the
analemma). See also Building an Analemma Curve, courtesy the Analemma Society.
Construct an indoor analemma; construct an outdoor analemma. Calculate and chart an analemma for any location at
any time of day.
A graph of the analemma for Washington, D.C. shows the Equation of Time
(Offset of the Sun) corrected for the eight-minute solar time difference between Washington, D.C. (77°W) and 75°W--the Standard Time Meridian for the Eastern Time Zone. Can you tell when the maximum & minimum
altitude of the midday
Sun occurs during the year?
Close examination of an almanac reveals that the latest sunrise and
earliest sunset do not occur--as one would expect--on the December Solstice (on average, 21 DEC), the day with
the fewest hours of daylight in the Northern Hemisphere. As it turns out,
the earliest sunsets occur in early December and the latest sunrises occur
in early January. [A similar situation occurs before/after the June Solstice (on average, 21 JUN).] A puzzling
mystery easily solved by the analemma! For details, see Why the earliest sunset, latest sunrise, and shortest
day of the year occur on different dates.
Explore the daily and annual cycles of change in the apparent path of the
Sun across the sky. Great Circle Studio's Solar
Calculator will calculate the Sun's altitude and azimuth for a user-specified location, date & time,
and data interval. A variety of output modes are available. Use this
information to predict how the length of your shadow would change daily
and annually (at the same time each day). Verify your predictions using
the SCSA Shadow Length
How can you determine the height of objects too tall to measure directly?
Using shadows, of course! Use the SCSA
Object Height Calculator to calculate the height of
tall shadow-casters, e.g., buildings, flagpoles, utility poles, trees,
Using user-specified times of sunrise and sunset, the SCSA Daylight Calculator calculates the number of hours
of daylight, also known as the Duration of Insolation (Incoming
The Solar Noon
Calendar calculates tables showing either the exact time of Solar
Noon for your location for each day of the year, or the Standard
Time Correction (the amount you have to add to, or to subtract from,
solar time on your sundial to get the time shown on your wristwatch).
The NASA J-Track Web page shows where on Earth the Sun is
currently directly overhead (see small Sun icon, correctly oriented with respect to latitude and longitude).
You Can Make a Sundial!
Tell time using shadows!
Use the SCSA Pole-to-Dial Converter-Calculator to convert
any vertical pole (e.g., a flagpole, utility pole, etc.) into a fully
functional reduced horizontal sundial featuring declination lines ("date
curves") for the equinoxes and solstices. Use the SCSA Object Height Calculator to calculate the height of
tall shadow-casting poles, e.g., flagpoles, utility poles, etc.
As its name suggests, the You Can Make a Sundial! Web site generates sundials
for a user-specified location. Several types of sundials are available in
a variety of output formats (GIF, PDF, and EPS). Start by making a
customized horizontal sundial similar to the "Sandburg
Sundial," a ready-to-use horizontal sundial available for downloading
in two file formats (some assembly required):
Print sundial templates using cover stock. For directions regarding set-up and
use, visit the How to Set Up & Use a Horizontal Sundial Web page.
For reference, visit the North American Sundial Society Horizontal Sundial Glossary.
Experiment with several other interesting types of sundials (designed for
You Can Construct a Sundial!
The preceding section features a
variety of ready-made, ready-to-assemble sundials--little if any
prerequisite knowledge is necessary to begin sundialing. Sooner or later,
you'll want to know what makes a sundial tick (pun intended)--at that
point in time, you are ready to construct a sundial from scratch. The
following information resources should help to get you started.
Two highly recommended books from Dover Publications, Inc.: Sundials: Their Construction and Use, by R.
Newton Mayall, Margaret W. Mayall, ©2000; and Sundials: Their Theory and Construction, by
Albert Edmund Waugh, ©1973. Similar content; complementary coverage.
Both books use the graphic (or geometric) method of sundial
construction--a simple, non-mathematical approach to constructing
sundials. In a word, these two books are a "must-have" for the novice
The first step in designing a sundial is to determine your exact location
(latitude and longitude):
- Use a relatively inexpensive Global Positioning System (GPS) receiver,
such as the Garmin
or eTrex Vista.
- U.S. Naval Observatory Complete Sun and Moon Data for One Day Web page: Use
Form A - U.S. Cities or Towns; select State or Territory; enter City or
Town Name; click "Get data"; among other useful information (especially
the time of "Sun transit"), the database returns the location name
and its longitude & latitude.
Start simple. Your first homemade sundial should be either a horizontal
sundial or an equatorial sundial. Before you
start [tips from the North American Sundial Society (NASS)]....
Equatorial Sundial -
The thickness of the gnomon determines the way in which the hour lines are
drawn on the two dial faces: hour lines are spaced exactly 15 degrees
apart and radiate from either the exact center of the dial face or
tangentially from a small inner circle representing the diameter of the
gnomon (for details, see Telling Time Using Shadows, Educator's Guide to
Equatorial Sundials). Use the SCSA
Equatorial Sundial Gnomon Length Calculator to
calculate the length of the upper & lower segment of the gnomon.
- Horizontal Sundial - Four methods for determining
the hour lines on the dial face, including the graphic method of
horizontal sundial construction, as well as three methods for calculating
the hour lines (in order of difficulty):
- Graphic (or Geometric) Method of Construction:
Your Own Horizontal Sundial, courtesy Paul R. Field, member, NASS.
[Note: The horizontal sundial is derived from the equatorial sundial. See
- Horizontal Sundial Hour Line Calculator:
Calculates hour, half-hour, and quarter-hour lines for a user-specified
latitude. Special thanks to Peter Daykin, Derbyshire Sundials, for
this time-saving calculator! Use an FCC utility to convert from either deg/min/sec to
decimal degrees, or decimal degrees to deg/min/sec.
- Microsoft® Excel Spreadsheet: Horizontal_Dial.xls,
courtesy Dr. Robert L. Kellogg, Treasurer, NASS. Enter your latitude &
longitude. In order to calculate the longitudinal offset of your location
from the Standard Time Meridian (STM) in your time zone, enter the longitude of the STM and your
longitude (again). Enter "yes/no" in response to query, "Correct for
longitude?" Enter size of dial (radius, in centimeters). Note results in
columns labeled "Time (hh.mm)" and "Dial Angle (degrees)."
- Mathematical Calculation (of Hour Lines): Five
simple sundial projects for you to make, courtesy Sundials on the
Internet. See Project 2 - A horizontal sundial. Check your answers
using either the hour line calculator
or horizontal dial spreadsheet. See also
Trigonometry, courtesy NASS.
[Note: Project 3 provides further proof that the horizontal
sundial is derived from the equatorial sundial. See Steps 3 & 6.]
Horizontal Sundials and the Earth's
The rotation of the Earth around its axis causes a daily cycle in the Sun's apparent path
across the sky that can be observed indirectly using a horizontal sundial.
In the northern
mid-latitudes, the Sun rises in an easterly direction, arches across the southern sky, and sets in a westerly direction.
Facing south, the Sun rises on your left and sets on your right. Sun
shadows fall in the opposite direction as the Sun. Therefore, morning
times are located on the right (or western) side of the dial plate of a
horizontal sundial (shown upper left); afternoon times are located on the
left (or eastern) side. Because the Earth rotates counterclockwise (as
viewed from above the Northern Hemisphere), shadows cast by the Sun move in a clockwise direction. See a six-hour time
lapse movie that shows the clockwise motion of the gnomon shadow around
the dial face of a horizontal sundial from roughly 6 a.m. to 12 noon: sundial.avi (1.02 MB); sundial.mov (1.01 MB). [Time-lapse movie courtesy Film & Video
Equatorial Sundials and the Earth's Revolution
Although horizontal sundials are more familiar to most people (due to the
fact that horizontal sundials are by far the most
common type of sundial), experience has shown that an equatorial sundial
is better suited for making the connection between the Earth's rotation and solar
time-keeping (see Are You Clock-wise?
sidebar, left), as well as the connection between the Earth's revolution around the Sun
and the annual cycle
of change in the Sun's apparent path across the sky.
The subsolar point is the point on the
Earth's surface at which the
Sun is at the zenith at
local solar noon. On any given day, the subsolar point moves east-west
along a single line of latitude as the Earth rotates counterclockwise.
The latitude of the subsolar point varies between zero
degrees (0°) at the equinoxes and ±23.5° at the solstices. The latitude of the subsolar point
varies directly with the declination of the Sun. The analemma--the odd-looking figure eight that appears on many globes--neatly traces the annual north-south
migration of the subsolar point (caused by the tilt of the Earth's axis of rotation and the revolution of the Earth around the Sun).
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 subsolar point moves east-west along the Equator. From the March
Equinox to the September Equinox, when the subsolar point is located 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 subsolar point
is located in the Southern Hemisphere (between 0° and 23.5°S
latitude), the gnomon shadow falls on the lower dial face (see example).
Put a little theory into practice--assemble a simple equatorial sundial [courtesy StarDate Online and the University of Texas McDonald
Observatory/SCOPE (Southwestern Consortium of Observatories for
Public Education)] and empirically observe where the gnomon shadow falls
as the seasons change. Gain valuable insight by checking the NASA J-Track Web page to see where on Earth the Sun is
currently directly overhead (see small Sun icon, which is correctly
oriented with respect to latitude and longitude). Related activity: calculate & chart a
projection of the analemma.
Light & Shadow - Suggested Activities for Grade K-12
From NASA Liftoff to Space Exploration, a set of sundial Web
pages (upper elementary, middle school):
- The Sandburg Planetarium Equatorial Sundial template
(print using cover stock) - Designed for use with a pencil-sized
gnomon, approximately 1/4" (7 mm) in diameter.
instructions (courtesy John Hoy)
- Equatorial Sundial Activity
Questions - Provide differentiated instruction by assigning multiple
choice questions only, as appropriate. [Teacher's Answer Key available
From StarDate Online and the University of Texas McDonald
Observatory/SCOPE (Southwestern Consortium of Observatories for
Public Education), the SCOPE solar poster educational activities and
- How Sundials Work
- Building a Simple [Equatorial] Sundial
- Pondering Sundials
Bill Nye the Science Guy Earth's Seasons video, Disney
Educational Productions, No. 68A93VL00 - Entertaining demonstrations showing the reasons for the seasons.
The program includes a brief segment (entitled "Try This") featuring a
home-made horizontal sundial--the "Cardboard Sundial of Science"--that
provides a fast-paced introduction to several fundamental concepts in
sundialing, including the clockwise rotation of the gnomon shadow, style angle versus latitude (the style should be parallel
to the Earth's axis), the noon line, and hour lines. The segment ends with a broadbrush
introduction to the Equation of Time (EoT).
- Equatorial Sundial Activity, two versions: HTML; PDF
- Dial Face Template (print using cover stock)
[Editorial Commentary: The program script
does not refer to the phenomenon as the EoT, rather it simply points out
the observed difference between Solar Time and Standard Time ("noon won't
quite stay noon; 9 in the morning won't quite stay 9 in the morning"). The
program fails to mention the fact that the time difference could be
caused, in part, by a difference in longitude between the location of the
observer and the Standard Time Meridian. The complex topic of correcting
Solar Time for Standard Time is perhaps better covered by the classroom
teacher rather than Bill Nye's "MTV" presentation style.]
Tracing the Analemma
- SCSA Calculate and Chart the Analemma - A year-long
projection of the Sun's image on a horizontal surface. (See also Related
- From the Smithsonian Institution, Eyes on the Sky, Feet on the Ground - Hands-On
Astronomy Activities for Kids. See Chapter Two - The Earth's Orbit, Activity 2-4: The
Analemma (constructing a ceiling analemma).
- Two interrelated activities from Paper Plate Education: Sub-Solar Cup; and Analemma Project.
Me and My Shadow - A Rule-of-Thumb for Safe Sun
The Sun is a star that radiates energy at all
wavelengths of the electromagnetic spectrum; some wavelengths of solar
radiation are hazardous to plants and animals. Visit the EPA Stay Healthy in the Sun Web site for information
about the health risks posed by ultraviolet (UV) radiation, as well as the
steps people can take to protect themselves from overexposure to the Sun.
An easy way to tell how much ultraviolet (UV) radiation exposure you are
getting is to look for your shadow:
- If your shadow is taller than you are (in the early morning and late
afternoon), then your UV exposure is likely to be low.
- If your shadow is shorter than you are (around midday), then you are
being exposed to high levels of UV radiation. Seek shade and protect your
skin and eyes.
Ultraviolet Index (UV Index) overview includes a link to EPA's new SunWise
School Program regarding Sun safety. The National Oceanic and
Atmospheric Administration (NOAA) produces a daily UV Index U.S. map (showing predicted exposure
Sunspots and the Solar Cycle
What is "The Solar Cycle?" In a regular cycle, the Sun undergoes a period
of great activity called the "solar maximum" (predicted to occur during
2000-2001), followed by a period of quiet called the "solar minimum." One
way scientists track solar activity is by observing sunspots. Sunspots are
relatively cool areas that appear as dark blemishes on the face of the
Sun. During solar maximum there are many sunspots; during solar minimum
there are few. See sidebar (left) for today's sunspot
For more information about sunspots, Solar Maximum, and the Sun-Earth
Connection, visit the SCSA Themes Web page. For classroom teachers, a couple of
suggested sunspot-related activities...
Sun-Earth Day, Astronomy Week/Day, Sky Awareness Week, and Space
Plan to celebrate Sun-Earth Day--a national celebration of the
Sun, the space around the Earth (geospace), and how all of it affects life
on our planet--on 20 March 2002. Its theme is: "Celebrate the
Equinox and the Seasons." Celebrate Astronomy Week/Day: Astronomy Week is
April 15-21, 2002; Astronomy Day is Saturday, April
20th. Celebrate National Sky Awareness Week (NSAW), April
21-27, 2002. Its theme is: "THE SKY - Where Meteorology Meets the
Heavens and the Earth."
Locally, the Sandburg Planetarium will
host two special events:
- "Astronomy Day at
Huntley Meadows Park," Saturday, 20 April 2002. (Rain Date:
Sunday, 28 April 2002.)
Day" on Mon., 06 May 2002, from 8:30 a.m. 'til 2:40
p.m. (Rain Date: Tue., 07 May.) The event is timed to coincide as
closely as possible with Space
Day (Thu., 02 May) and the Last
Quarter Moon (Sat., 04 May). Amateur astronomers from the Maryland
Sidewalk Astronomers (MSA), National Capital Astronomers (NCA), Northern Virginia Astronomy
Club (NOVAC), and Shenandoah Astronomical
Society (SAS) have volunteered to be our guides for a day of
sundialing, safe sunspot observing, and Moon-watching.