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
Louis Stevenson
 Me and My Shadow
- Mills Brothers
Use a sundial
to measure Solar Time. Use RealPlayer to hear the exact Standard Time from the US Naval
Observatory Master Clock Voice Announcer.
Most days, Solar Time is slightly different from Standard
Time (up to 16 min.
fast or slow). This time difference is known as the Equation of Time.
Are You
Clock-wise?
Ever wonder why is
clockwise Clockwise? As it turns out, for objects in the Northern
Hemisphere, shadows cast by the Sun move in a clockwise direction. In
fact, the word hour means "the day" or "Sun's path." For details,
see the How Sundials Work Web page.
Longitude is...
...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° per hour or 1° per 4
minutes) to calculate the difference in longitude between the two places.
For example, Solar Noon occurs eight minutes later in
Wash., 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 Wash., 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.
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
Time.
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
long-distance connection!
I'm Being Followed...
...by a moonshadow.
- Moonshadow, Cat
Stevens
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
sky):
- 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."
- Soul
of the Sky, an essay by Diane Ackerman
The "Morning
Chorus"
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!
28 MAR 2002
Today's sunspot
number is
179 /
Sunspot No. Trend
(past 24 hours) |
| / |
Increasing |
| -- |
Steady |
| \ |
Decreasing |
Credits: Real-time image courtesy SOHO;
sunspot number courtesy NOAA.
Updated: 27 MAR 2002
ARCHIVES
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Sandburg Center for Sky Awareness
A Fairfax County Public Schools Planetarium
Me and My Shadow
Making the Sun-Earth Connection
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.
The Analemma
Ever notice the odd-looking figure eight that appears on many globes? It's
called an analemma. See the analemma for Washington, D.C. (can you tell when the maximum altitude of the midday Sun
occurs during the year?), then 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).
Sun Calculators
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) by visiting the SCSA Shadow Length
Calculator.
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,
etc.
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 watch).
The NASA J-Track Web page shows where the Sun is currently
directly overhead (see small Sun icon, correctly oriented
with respect to latitude and longitude).
You Can Make a Sundial!
Equatorial Sundials and the Equinoxes
Although horizontal sundials are more familiar to most people, 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.
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 lat.),
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 lat.), the gnomon shadow falls on the lower dial face.
Put a little theory into practice--assemble a simple equatorial sundial (courtesy NASA Liftoff to
Space Exploration) 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 the Sun is currently
directly overhead (see small Sun icon, which is correctly oriented with
respect to latitude and longitude).
Editor's Note: From the March Equinox to the September Equinox,
use the Northern Hemisphere equatorial sundial template.
From the September Equinox to the March Equinox, use the Southern Hemisphere template. Be sure to fold the
Southern Hemisphere template so that the dial face and gnomon point
downward rather than upward (as directed).
Me and My Shadow - A Rule-of-Thumb for Safe Sun
Exposure
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.
The
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
levels).
Light & Shadow - Suggested Activities for Grade K-6
- Just Me and My Shadow - Kids create crazy creatures
with shadows and a little sunshine. From Sesame Street Parents.
- Smithsonian Astrophysical Observatory Everyday Classroom Tools,
featuring K-2, 2-4, and 4-6 threads for each activity:
- From NASA Liftoff to Space Exploration, a set of sundial Web
pages (upper elementary):
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
number.
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
Day
 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.)
- "Sun-Earth-Moon
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 National Capital
Astronomers (NCA), Northern Virginia Astronomy Club (NOVAC), and Analemma
Society have volunteered to be our guides for a day of sundialing, safe sunspot observing, and Moon-watching.
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