Humans have been observing the Sun for thousands of years
and sometimes included it in religious ceremonies or artworks. Early
civilisations observed the Sun moving across the sky daily, at a regular speed and
forming shadows on the ground. This added to their thoughts on the concept of
time and led to the development of devices to measure the passage of time, such
as sundials.
Although there are many modern examples, remnants of sundials
have been found in Babylonian, Greek, Egyptian and Roman settlements. There are
numerous designs including three that students can easily make at home: Nodal,
T Bar and Gnomon.
NOTE: Some resources on sundials may mention sundials
showing shadows moving in a clockwise direction. This is only true for the
Northern Hemisphere as shadow movement across sundials in the Southern
Hemisphere is in an anti-clockwise direction.
Measuring time with the Sun
To make measuring time using the Sun easier, we say that it
takes one day (broken into 24 hours of even length) for the Earth to rotate on its
axis 360o. This is known as a solar day and the hours as solar
hours.
This way of measuring time assumes that the Earth’s orbit
around the Sun is circular and that it travels at a constant speed. In fact,
Earth’s orbit is more of an elliptical shape (a squished circle) and it travels
at different speeds at some points in this orbit as it is affected by the Sun’s
gravity. The average day length is actually 23.93 hours as some ‘days’ are
longer than others. If we want to say that one year is a complete orbit of the
Earth around the Sun, so it will appear in exactly the same spot in the sky, we
have to add about 4 minutes to every year.
Image from Wikipedia Commons
This makes it very confusing for students trying to measure
time with sundials, but a simple explanation is to let them know that solar
hours are different from the hours we measure with clocks. There is a
difference in the number of hours of sunlight at different times of the year so
a sundial that divides the time into twelve even sections would have different
length ‘hours’ at different times of the year. In modern times, we have decided
that all measurements of time all over the world will be the same length to
save confusion.
This PALMS activity to demonstrate how the Earth’s rotation
around the Sun affects the sunlight shining on Earth could be tried at home
with some adult assistance.
This Crash Course Kids You Tube video discusses
how the Sun’s apparent movement in the sky changes shadows throughout the day.
Students will need a protractor to make these sundials. They
can download and print one from this website which also explains how to use one
if they need some help.
Nodal Sundial
· A nodal sundial uses a pole-like object placed
at its centre point to cast a shadow onto a base divided into hours.
Photo by Ben Carlisle, accessed 03/04/2020
· Examples of nodal sundials can be seen in
ancient Roman artefacts and Egyptian ruins. Ancient Egyptians were thought to
have positioned tall obelisks to cast shadows on marks on the ground
Image from Wikipedia Commons
Image from Science and Inventions, accessed 06/04/2020
· Students can make their own simple nodal sundial
after watching the video here. The supporting PALMS teacher notes can be found here and the student worksheet, here. and student worksheet can be found here.
T Bar Sundial
· Evidence of T Bar sundials has been found in
paintings on the tomb walls of Egyptian pyramids. One of the earliest found is
made from green schist (a metamorphic rock). They would more likely have been
made of wood, so they were easy to transport, and been used by Egyptian work
overseers to measure out the working hours of slaves and peasants.
· Sailors also used this kind of system to tell
time but using the shadow of the ship’s yardarm. Once the Sun had passed over
the yardarm, the captain would distribute the sailors rum ration. This has been
accepted as a colloquial saying, indicating it’s an acceptable time to drink
alcohol.
· Students can make their own simple T Bar sundial
after watching the video here. The supporting PALMS teacher notes can be found here and student worksheet here.
Gnomon Sundial
· Gnomon
sundials are more complex than the other two mentioned in this blog post and
require more mathematical knowledge. They are also more commonly found today as
decorative pieces in parks and gardens.
Image from Wikipedia Commons
· Arab philosophers of the 9th and 10th centuries
were amongst the first advanced scientists and mathematicians in Europe. They
discovered that a gnomon, a triangular shadow maker, would indicate hours of
exactly the same length of standard time, no matter what time of year it was.
The angle between its base and top must be the same as the latitude of the
location in which it is used. This is because when it points south, it lies
parallel to the Earth’s axis of rotation.
· Students can make their own simple gnomon sundial
after watching the video here. The supporting PALMS teacher notes can be found here and student worksheet can be found here.
· There is a lovely example of a large gnomon
sundial at Cottesloe Beach in Western Australia
Other Examples and Resources
- NASA’s Mars Rovers carry sundials as calibration instruments.
- Interesting article about the Sundial of Human Involvement at The Australian Botanic Garden at Mt Annan, NSW including how you can make your own. This interesting sundial just happens to use some beautiful pieces of columnar basalt as the hour markers too!
- The Royal Museums Greenwich has some great Learning At Home activities on their website related to this topic. The website also gives information on the origin of Greenwich Mean Time that all world time zones are measured against (https://www.rmg.co.uk/schools-communities/learning-resources/ks2-learning-home)
- This website gives a brief history of sundials and some alternative designs to make at home.
- A more complex explanation of how sundials work can be foundhere.This website is more appropriate for adults wishing to learn more.