Unit 6 : Equatorial Sundials : Variations : Horizontal and Vertical Sundials : Tech Notes : Unit Questionniare

# Equatorial Sundials

The motion of the sun across the sky produces a complementary motion of shadows.  The tradition of using moving shadows to keep track of the passage of time goes back to prehistoric time.  Sundials are found around the world – in gardens, on walls and church towers, in public squares.  In fact, you can use just about anything as a kind of sundial.  Until accurate mechanical clocks were invented, sundials defined the time.

The kind of time that is kept by a regular sundial is local solar time.  Some sundials are adjusted to correct for the difference between their longitudes and the nearest time zone meridian, and if they are also corrected for the "equation of time" then they can show the same time your watch does – at least, when daylight savings time is not in effect.  In this section, though, we'll focus on the ordinary, traditional sundial that reflects the altitude and/or the azimuth of the sun.

Construction of the Basic Equatorial Sundial

The equatorial sundial is the simplest sundial to construct and also the easiest to visualize.  It consists of a flat surface aligned with the celestial equator, and a "gnomon" or post that is perpendicular to this celestial equator surface.  This means that the gnomon is directed towards the North or South Celestial Pole.  Also, note that the gnomon is NOT necessarily aligned with your local zenith.  In other words, the gnomon does not point straight up from the ground.

1) Where on the earth would you have to place an equatorial sun dial so that the gnomon points at the local zenith (i.e. straight up)?

At the North Pole
At the equator
At the South Pole

Around the gnomon, the equatorial sundial is marked in 15 degree segments.  Noon is marked at the lowest point on the dial, because at noon the sun is highest in the sky and on the meridian.  But remember, the sundial's flat surface may need to be tilted in order to be parallel to the celestial equator.  Only at the poles, as discussed in the answers to the question above, can the sundial's flat area (where the degree segments are marked off) lie flat on the ground.  In summary, anything placed parallel to the celestial equator will cast a shadow that changes 15 degrees each hour.

Why is the sundial marked in 15 degree segments?  Let Brisban show you the way!  Click on his icon to the left for No Frill's Help.

Arranging the Equatorial Sun Dial

To arrange an equatorial sundial properly, take your "latitude triangle"  (see description below) and arrange it vertically, in the N-S plane, with the angles 90 degrees latitude and longitude along the base.

A Latitude Triangle is a right-angled triangle where one of the angles is equal to your latitude; the other is equal to (90 degrees minus local latitude) for your latitude.   If you arrange it to stand vertically along a N-S line, then one edge points to your North Celestial Pole and the other points to the celestial equator.  This makes it particularly useful for arranging a sundial.

Latitude Triangle:  Angle A is the local latitude, and angle B is 90 degrees minus the local latitude.

What exactly is a Latitude Triangle? Click on Brisban's icon for No Frill's Help and find out!

Construct the dial face for an equatorial dial by marking 24 wedges of 15 degrees each around a point.  We do this because the sun's motion parallel to the celestial equator is 360 degrees in 24 hours which is 15 degrees per hour.

Attach the gnomon perpendicular to the dial face at this point.  Mark the hours 6A.M. to 6P.M. along the dial face clockwise.  Write the numbers with the bottom of the letters towards the edge of the dial face, that way you can read the sundial when you are "outside" of it (i.e. you aren't standing in the middle of the sundial reading the numbers).

Click on the following link to open a single page "sundial kit" using the design discussed above.  When the sundial kit is loaded on your browser, simply print it out by clicking on "File" on the menu at the top of your browser and selecting "Print."  Once you print the image, cut out the shapes and set it up.  Click here for the sundial kit.

2) An equatorial dial constructed according to the above instructions will actually only be useful during the summer half-year, between the vernal equinox and the autumnal equinox.  Why?

Because it's too cold to go out in the winter.
Because the Sun is South of the Celestial Equator in the Winter.
Because the Sun only goes through the Zenith in the Summer.

To get around this time of the year limitation, instead of using a flat dial face, we can make a sundial using a ring placed in the equatorial plane with marks equally far apart and a gnomon through the center.  This type is sometimes called an armillary dial and is very easy to make.  The picture on the left shows what an armillary dial looks like.