Simple Shadow Casting Sundials

By Dr. Harold Williams

of Montgomery College Planetariumhttp://montgomerycollege.edu/Departments/planet/

All Sundials• All sundials tell time by the position of the sun in

the sky. Mostly they tell local apparent sun time.• At sunrise the sun is just coming up on the

horizon.• At midday, noon, the sun is crossing the meridian,

the line that runs from north to south through the zeinth, (the point over head).

• At sunset the sun is just going down on the horizon.

Most Sundials

• Most sundials work by casting shadows with a “gnome,” shadow caster.

• The shadow is projected on a surface that is a dial face telling you the time.

Equatorial Sundial with Gnome pointing toward the pole

Equatorial Sundials across the planet

Equatorial Sundial Summer

Equatorial Sundial Winter

The Sun reaches itsZenith (Peak) atHigh Noon…

NOT!The Sun Peaks in Takoma Park at 12:16 PM EST on March 18, 2006

Corrections: 1) Daylight Savings 2) Time Zones 3) Earth’s 23.5o Axis Tilt 4) Earth’s Elliptical Orbit

The Sundial Compared to Mean Time

Equation of Time

-20

-15

-10

-5

0

5

10

15

20

0 100 200 300 400

Day of the Year

Ad

d o

r S

ub

tra

ct

Min

ute

sT

o G

et

Sta

nd

ard

TIm

e

Some Sundial Patterns for Here

• Analamic & Horizon Sundial for 39 North http://www.montgomerycollege.edu/Departments/planet/sundials/analamic_&_horizontal39.pdf

• Analamic Sundial for 39 North http://www.montgomerycollege.edu/Departments/planet/sundials/ana39.pdf

Some Additional Sundial Resources

• North American Sundial Society http://sundials.org/

• British Sundial society http://www.sundialsoc.org.uk/

• Sundials on Mars http://www.astrobio.net/news/article625.html