Time

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.

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

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.

Homework Complete your own shadow stick observations this weekend.

Mini Review What can be found out through observations made with a shadow stick?

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

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

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.

Mini Review To perform simple calculations using the equation of time.