Student activity
Making an equatorial sundial
Background information
Sundials have been used to tell the time by different societies for more than 5 000 years.
The Greek historian Herodotus (484-425 BC) stated that sundials originated with the
ancient Chaldeans and Sumerians who lived in Iraq. They used vertical rods on their
buildings to cast shadows in order to tell the time and the date, and were the first people
to divide the day into 24 hours, the week into seven days and the year into 12 months.
There are many different types of sundials. The equatorial sundial is just one example.
The equatorial sundial
The circular dial plate of the sundial lies parallel to the plane of the Equator. The
gnomon, which casts the shadow, passes through the centre of the dial and points
towards the South Celestial Pole. The gnomon is tilted according to latitude (38 degrees
for Melbourne). Since the Earth rotates once on its axis every 24 hours, covering 360
degrees, the Earth rotates 15 degrees every hour. The hour lines are therefore equally
spaced at 15 degree intervals.
As the Earth spins, the Sun appears to move across the sky. During summer, in the
Southern Hemisphere, the Sun rises in the south-east and sets in the south-west but
during winter, the Sun rises in the north-east and sets in the north-west.
In summer, the Sun has to move further to travel across the sky and reaches a higher
altitude in the sky than in winter. As a result, the summer shadow of the gnomon lies on
the uppermost face of the sundial, while the winter shadow lies on the face underneath
the dial plate.
http://museumvictoria.com.au/scienceworks/education/
38
Student activity
The following diagram illustrates the path of the Sun on:
(a) The longest day of the year: Summer Solstice, which usually occurs on
December 22.
(b) The two days when the day and night are equal in length: Spring Equinox, usually on
September 23 and Autumn Equinox usually on March 22.
(c) The shortest day of the year: Winter Solstice, which usually occurs on June 22.
On the Autumn Equinox and the Spring Equinox, the Sun rises due east and sets due
west. On these days there is no shadow cast on either side of the dial plate. The shadow
falls on the edge of the dial plate. Thus the sundial cannot be used to tell the time on
these two days of the year.
Making clock time equal solar time
Clock time is based on the average Sun time. In order for our sundial to read clock time,
we need to take some important factors into account.
Time correction
The sundial needs to be corrected for two main reasons. We need to compensate for:
1. The path of the Earth around the Sun being elliptical and the fact that the Sun is
slightly off centre from this elliptical path.
2. The tilt of the Earth’s axis (23.4 degrees to the plane of the ecliptic).
We can use the Universal Equation of Time graph to make the necessary correction.
http://museumvictoria.com.au/scienceworks/education/
39
Student activity
The Universal Equation of Time
For simplicity, the following correction averages for each month can be used for younger
students.
Summer
Dec (- 5 min)
Jan (+ 7 min)
Feb (+ 14 min)
Autumn
Mar (+ 8 min)
Apr (correct)
May (- 3 min)
Winter
Jun (+ 1 min)
Jul (+ 6 min)
Aug (+ 4 min)
Spring
Sep (- 5 min)
Oct (- 14 min)
Nov (-14 min)
Longitudinal correction
Longitudinal correction should also be taken into account when adjusting the sundial to
read clock time. The Sun rises at different times at different places (or longitudes)
around the world. A standard system was developed so time could be referred to
universally. The 360 degrees of longitude of the Earth’s circumference is divided into 24
zones, each covering 15 degrees of longitude. (In practice the zones are altered in a few
places to better fit the boundaries of the countries and islands.) These zones are called
Standard Time Zones.
All places within these zones have the same defined clock time based on a set longitude
within these zones. The zero reference is taken to be at the Royal Greenwich
Observatory in the outskirts of London.
http://museumvictoria.com.au/scienceworks/education/
40
Student activity
Australia is divided into three Standard Time Zones: Western Standard Time (Western
Australia), Central Standard Time (South Australia and Northern Territory) and Eastern
Standard Time (Queensland, New South Wales, Victoria and Tasmania). Sydney and
Melbourne have the same clock time but Perth and Melbourne have a two-hour
difference. Adelaide and Melbourne have only half an hour difference in clock time.
Australian Eastern Standard Time (AEST) is referenced at 150 degrees longitude.
Melbourne is located at 145 degrees longitude. This five degree shift translates to an
error of 20 minutes between clock time and solar time for Melbourne. In order for the
sundial to read clock time, 20 minutes needs to be added to our sundial time in
Melbourne. (If the sundial is to be used outside of subtract an additional four minutes for
every one degree longitude east of Melbourne or add an additional four minutes for
every one degree west of Melbourne.)
Lastly, during summer we move our clocks forward one hour to maximise the length of
evening. This means that the sundial will be an hour slow so an hour is added onto the
sundial time during daylight saving. In Victoria daylight saving usually begins the last
weekend in October and ends the last weekend in March.
Finding north using sunrise and sunset times
The sundial is designed so that the gnomon points to the South Celestial Pole along the
north/south line. The Sun will be on the north/south line when the Sun is midway
between sunrise and sunset. If you measured the time the Sun was at its highest point
(or midway between sunrise and sunset), then you will be able to use the Sun to find due
north. Sunrise and sunset times for the year can be printed from the Melbourne
Planetarium web site. The following example shows how to calculate the time that the
Sun is midway between sunrise and sunset.
Example
In the month of May, 2008:
Thursday 1st
SUNRISE
7:01am
SUNSET
5:33pm
1. Convert the Sunset time to a 24 hour clock by adding 12 hours to the sunset time.
12 hours + 5 hours and 33 minutes = 17 hours and 33 minutes
2. Subtract the Sunrise time.
17 hours and 33 minutes - 7 hours and 1 minute
= 10 hours and 32 minutes (sunrise to sunset)
3. Divide this value by 2.
10 hours and 32 minutes / 2
=5 hours and 16 minutes
4. Add Sunrise time
5 hours and 16 minutes + 7 hours and 1 minute = 12 hours and 17 minutes.
This means that on Thursday May 1, the Sun will be on the north/south line at 12:17pm.
This is the time you should position your sundial so that the north face of the dial points
north.
http://museumvictoria.com.au/scienceworks/education/
41
Student activity
Please remember: NEVER look directly at the Sun.
Follow the instructions on the template on the next page to make your sundial.
What you need





A4 template of sundial
A4 piece of cardboard
glue
scissors
piece of stiff cardboard (half an A4 sheet)
What to do
1. Paste the sundial template onto the cardboard.
2. Cut around the dial following the heavy black line (illustration 1).
3. Fold out the tabs along the dotted lines (illustration 2).
4. Fold the dial plates (north and south circles) back-to-back and stick them together
with glue (illustration 3).
5. Cut the V slot in the centre of the dial with the scissors (illustration 4).
6. Cut out and fold the gnomon (illustration 5).
7. Push the narrow end of the gnomon through the north face of the dial (illustration 6).
8. Stick the tabs at the bottom of the dial to a piece of cardboard to act as a stand.
9. Cut out the table of corrections for each month and stick this somewhere on the
stand to be used for reference.
10. Follow the instructions on how to position the sundial so that the north face points
towards due north. The gnomon should stand at 90 degrees to the dial face pointing
towards the South Celestial Pole (illustration 7).
For more activities and information visit: http://www.sundials.co.uk/intro.htm
http://museumvictoria.com.au/scienceworks/education/
42
Student activity
http://museumvictoria.com.au/scienceworks/education/
43