Shedding Light on the Sun and Earth Episode 1: Seasons Using exceptional demonstrations and animations, the Shedding Light on the Sun and Earth series introduces students to the essentials of climate science. We examine what causes seasons, why the days are longer in summer than they are in winter, how the movement of the sun across the sky affects the renewable-energy industry, and a whole lot more. In Episode 1: Seasons, we briefly look at how the changing seasons affect life on our precious Earth and then examine what causes seasons. Contents: Part A: Introduction. We explain the scope of the series. Part B: Seasonal Variations: A brief analysis of how the seasons affect life on Earth. Part C: A Day and a Year: What causes day and night? Why does the sun rise in the eastern sky and why does it set in the western sky? What is a year? What is a leap year? What are latitude and longitude and why are they so important? Part D: Summer, Autumn, Winter, Spring: What causes seasons? What is the Tropic of Cancer and what is the Tropic of Capricorn? Why is it always hot at the equator? Part E: Tropical, Temperate, and Polar: What do these three terms mean? Why don’t tropical regions experiences four seasons like temperate regions do? Why do the North and South Poles get only one day a year? (Yes that’s right, the Poles get only one day a year; a six-month-long day followed by a six- month-long night!!) SLOSE Episode 1: Seasons www.liacoseducationalmedia.com Page 1 of 11 Shedding Light on the Sun and Earth Episode 1: Seasons Part A: Introduction Why do we have seasons? Why do we have day and night and why are the days longer in summer than they are in winter? Why are places near the equator always hot and why are the north and south poles always cold? Why do tropical cyclones (also called hurricanes and typhoons) rotate anticlockwise in the northern hemisphere but clockwise in the southern hemisphere? And does the same thing apply to water draining out of a bath tub? Every morning the sun rises and provides the heat and the light that all living things on Earth need to survive. In this series we’re going to look at the basics of how the sun illuminates and warms our planet by different amounts depending on where you are and on the time of year and we’re going to look at some simple climate science. We’re going to look at how (a) the rotation of the Earth, (b) the movement of the Earth around the sun and (c) the tilt of the Earth causes the seasons to change, the length of daytime and night time to change and a whole lot more. In this episode, we’re going to focus on the seasons. Part B: Seasonal Variations Seasons! Throughout the year, the weather changes. In summer it’s usually warmer, in winter it’s usually cooler, and in between we get spring and autumn (which is often called fall in American English). Winter is characterized by cool to cold weather with long nights and short days. Plants don’t usually grow all that well in winter, partly because of the cold weather, but also because of the fact that in winter the sun rises later and sets earlier than it does in summer. Therefore literally less light shines onto plants in winter and since plants need light to grow, they don’t grow as well when there’s less light around. The lack of plant growth results in less food availability for many animals. Many bird species will have already flown large distances to warmer climates where there is more food. Spring brings warmer weather and longer days, which means more light. Plants start to grow more and most flowering plants flower in spring. Many animals bring their off-spring into the world during spring because they know, in their instinctive animal kind of way, that there will be plenty of food around. Summer comes next and the weather is the hottest it will be all year. The days are at their longest and the nights at their shortest. The flowers that bloomed in spring transform into a bonanza of ripe fruit for animals and humans to enjoy. SLOSE Episode 1: Seasons www.liacoseducationalmedia.com Page 2 of 11 After summer, we come into autumn. Temperatures begin to drop and night time lasts longer than daytime again. Many trees start to lose their leaves and many mammals breed during autumn so that their babies can be born during spring. When the Northern Hemisphere is having summer (we filmed the beach scenes you’re watching in Greece in late June), the Southern Hemisphere is having winter. The snow scenes were filmed in the mountains just outside of Melbourne in July. All the seasons in fact are reversed in the two hemispheres! So, why do we get different seasons throughout the year and why when it’s winter, for example, in the Southern Hemisphere is it summer in the Northern Hemisphere? Let’s begin by looking at the basics. Part C: A Day and a Year. The word “day” has two meanings. It can mean the time that the sun is above the horizon. When the sun rises it’s a new day and we have daytime. When the sun sets, it’s the end of the day, after which we have night (or night time), when the sun is below the horizon. But the word “day” can also mean a whole 24-hour period. Sunday is a day, followed by Monday and so on. It’s usually pretty obvious which of the meanings of “day” someone is talking about. Daytime and night time are caused by the spinning of the Earth. Here we’ve set up a spotlight to represent the sun and I have a globe that of course represents the Earth. The side of the Earth facing the sun is having daytime, while the side of the Earth facing away from the Earth is having night time. The reason a whole day is 24 hours is that it takes that long for the Earth to spin around once. So, for example, the time it takes Australia to go all the way around once and to end up where it started from (in real life of course, not in this animation) is 24 hours. The Earth rotates from west to east, anticlockwise if you look at it from the North. So at sunrise, when the part of the Earth you’re standing on comes into daylight you have to look in an easterly direction to see the sun, although not necessarily exactly East. We say the sun is at a low angle in the sky, or that it’s on or just above the horizon. SLOSE Episode 1: Seasons www.liacoseducationalmedia.com Page 3 of 11 In the middle of the day, the sun reaches its highest point, after which it starts going back down. The highest angle that the sun reaches above the horizon varies depending on where you are and on the time of the year. At sunset, the sun drops to the horizon and you have to look towards the west. So the sun rises in the eastern sky and sets in the western sky. Well that’s what it looks like to us because of the fact that the Earth is spinning. I can show the sun’s path diagrammatically if I display just a small section of the earth. The sun rises in the east, gets to its highest point in the middle of the day, and then sets in the west. It does this every day, of course. Daytime is usually hotter than night time because the radiant heat from the sun heats up the side of the Earth that it’s facing. At night, the side of the Earth facing away from the sun radiates heat back out into space and so it cools down. One year, is the time it takes for the Earth to go all the way around the sun to back where it started from. This movement is called one orbit and we say that the Earth is orbiting the sun. For what it’s worth it’s orbiting in a more-or-less circular path at a distance of about 150 million km from the sun. In one year, the Earth rotates 365¼ times, which means that there are 365¼ days in one year. But how do we account for the ¼ day. We simply set what is called a normal year or common year to 365 days, and have a leap year of 366 days every four years. This averages out to 365¼ days. Let’s take a quick look at what happens. In the first normal year, the Earth goes almost but not quite all the way around the sun and ends up about here 365 days later, slightly short of where it started 365 days earlier. In the following normal year, it does the same thing and again falls slightly short after another 365 days. In the third normal year, it falls short again another 365 days later, but in the fourth year, the leap year that has 366 days, the extra day allows the Earth to move back to where it was 4 years earlier, and the cycle starts again. Nothing here is to scale by the way, but that’s basically how leap years work.