# Solar Motion from Australia

## Article excerpt

At noon throughout the year the Sun has a north-south and east-west motion around the meridian. Earliest/latest sunrises and sunsets do not occur at the solstices and the effect is more pronounced with decreasing latitude. This phenomenon is calculated for 25 Australian cities and the following observations are recorded:

1. The latest sunrise occurs before the June solstice and the earliest sunset after.

2. The earliest sunrise occurs before the December solstice and the latest sunset after.

3. The effect at the December solstice is more pronounced than for the June one.

4. The effect increases with decreasing latitude.

INTRODUCTION

An observer plotting the location of the Sun at noon over a year would notice it altering its position in two directions. It changes its elevation (angle above the horizon) and shifts east and west of the meridian (line passing from north to south over the viewer's head). These two displacements of the Sun in the sky create a figure eight shape.

This paper investigates the causes of these observations. Specific values are calculated with the use of simple algorithms. The perspective is an Earth-centred view.

The time from sunrise to the Sun's crossing of the meridian, and from the meridian to sunset depends on both the declination of the Sun (angular distance from the celestial equator) and the latitude of the observer. Even though the longest and shortest days are the same date for all places in Australia, the earliest and latest sunrise and sunset are latitudinally dependent. Darwin, as the most northerly Australian city, is selected as an example. Finally, the procedure is repeated mathematically to obtain a table of results for 25 Australian cities.

PLACE IN THE CURRICULUM

The astronomy material presented here encompasses the first two of Kepler's Laws of Planetary Motion, namely, that the planets move in elliptical orbits with the Sun at one focus, and an imaginary line joining the Sun to a planet sweeps out equal areas in equal intervals of time. In addition, an effect of the tilt of the Earth's spin axis to its orbital plane, other than the traditional one of its being the cause of seasons, is examined. The topic presents a varied treatment of reference frames, as the shift here is from laws and concepts couched in a view from outside the Earth to those of an observer on Earth.

One of the aims of the National Science Curriculum is, 'an interest in and understanding of the natural world' (National Science Curriculum, 1988:18) and the Sun has a great influence on this planet. Just some of the environmental applications of this work involve the placement of rooms in a home for natural lighting and warmth, the pitch of the roof over an outdoor living area (as well as its positioning) and the location of gardens and selection of vegetation for them.

The variation of the Sun over a year at noon could be observed in a lesson with free downloadable astronomical software. However, an extended activity of a plot with a shadow stick to a horizontal or vertical surface could be added in the junior or primary science curriculum. The details given should allow teachers to translate the concepts into simpler terms for their younger students but the analysis also encourages physics students to derive results for their own locality as a research or investigative topic. The theme provides a ideal context for the use of Information and Communication Technology within Science or ICT itself. This real situation is an opportunity for advanced mathematics students to incorporate trigonometric functions, parameterised curves, graphs of single effects and the compounding of sinusoidal ones.

NORTH-SOUTH MOTION

The elevation or north-south variation of the position of the Sun at noon over a year is a result of the tilt of the spin axis of the Earth to the vertical of its orbit. …

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