Learning Outcomes To distinguish between a solar day and a
sidereal day. To explain and interpret data provided by shadow
sticks. To describe how a sundial can be used to determine apparent
solar time. To explain the seasonal variations in the times of
sunrise and sunset. To perform simple calculations using the
equation of time.
Slide 3
Solar Day v Sidereal Day A solar day is the time it takes the
Sun to arrive in the same position as the day before. This happens
in exactly 24 hours. This is not the Earth's true rotation though.
As the Earth rotates it is also orbiting the Sun. It moves in
space. If we measure the rotation not by the Sun but by a Star then
we find the Earth rotates in 23 hours, 56 minutes and 4 seconds.
This is a sidereal day and explains why the Earth rotates 4 minutes
earlier than 24 hours. Likewise the time it takes for the Moon to
show Earth two full moons (29.5 days) is different to its sidereal
period of rotation (27.2 days) To distinguish between a solar day
and a sidereal day
Slide 4
Slide 5
Shadow Sticks An interesting project is to plot the shadow of
the Sun over the course of a day at various times of year. Every
hour place a mark with the time at the end of the shadow of the
stick. Plot regularly over the midday period. You may find the
shortest shadow does not occur at midday on your watch. The stick
is showing apparent solar time. At the end of the day you will end
with an interesting pattern. Repeat the exercise on another date
but this time put another date or use another pencil so you can see
the difference between the dates. The point of this that the Sun
casts different shadows at the same time of day throughout the
year. This happens because of Earth's elliptical orbit and axis. To
explain and interpret data provided by shadow sticks.
Slide 6
Homework Complete your own shadow stick observations this
weekend.
Slide 7
Mini Review What can be found out through observations made
with a shadow stick?
Slide 8
Sundial The purpose of a sundial is to tell the time of day
based on a shadow the Sun causes on its surface. The sundial is
typically a horizontal or vertically aligned disc that has points
around its surface marking time. There is sometimes an
interpretation chart on it as well. In the middle is usually a
large protrusion called a gnomon. It is this that causes the shadow
to fall on the disc. The angle of this gnomon is important as it
can make the difference between a correct and incorrect reading.
Sundials have been used for centuries to tell the time but there
are two disadvantages to them: 1. They are useless when no Sun is
visible 2. The time they tell is usually inaccurate to the casual
observer due to the equation of time and require special
understanding. This problem is addressed in the equation of time
page. Telling time by the position of the Sun is a difficult
process. We do this by a formula called the equation of time The
Sun does not move relative to Earth, the changing of the Sun's
position is due to the movement of the Earth. To describe how a
sundial can be used to determine apparent solar time
Slide 9
Slide 10
Slide 11
The Equation of Time EOT = apparent solar time mean solar time
This is simply EOT = sundial time - clock time You can also find
out mean solar time by subtracting EOT from apparent solar time
Mean solar time = apparent solar time EOT And it just keeps getting
better! You can work out apparent solar time by adding EOT to mean
solar time Apparent solar time = Mean solar time + EOT
Slide 12
You can see from the diagram on the right that the times when a
sundial is accurate are few in the year. 4 times in fact. This is
caused by the axis of the Earth (which gives us our seasons) and
also Earth's orbit. Sometimes Earth is near the Sun and orbiting
quickly, sometimes it is far away and orbiting slower.
Slide 13
Mini Review To perform simple calculations using the equation
of time.