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 look at how the changing seasons affect life on our precious Earth and then examine what causes seasons.

A 4-minute or so excerpt followed by a 1-minute description.

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, they only get one day a year; a six-month long day followed by a six-month long night!!)


 The Transcript (which also features low-resolution screen shots)
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.

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.

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.

Of course nature doesn’t know or care about leap years. Plants and animals simply respond to the changing seasons every year. However, having leap years in our calendar helps us, because an accurate calendar means that the seasons begin on the same calendar date every year. If we didn’t have leap years, then after a few hundred years, the southern hemisphere summer wouldn’t begin in December, but would begin in June.

Now since different parts of the Earth experience different weather patterns throughout the year, it helps if we use the concepts of latitude and longitude to identify exactly where a place is on Earth.

A place’s latitude is the angular distance it is from the equator measured from the centre of the Earth.

Lines of latitude, they’re actually called parallels, are marked on most globes. The equator has a latitude of 0°. This line is at 15°S latitude, this line is at 30°S latitude and so on.

The South Pole is at a latitude of 90°S.

It’s probably easier to picture if we look side on and cut into the Earth. This point has a latitude of 15°S, since a line drawn from the centre of the Earth to this point is at an angle of 15° to the line that goes to the equator. 30°S is here, and so on until we get to the South Pole which, as I said, has a latitude of 90°S. This point has a latitude of 15°N and we can continue to the North Pole which has a latitude of 90°N. So, for example, everywhere along this line has a latitude of 30°N. Melbourne, the city I live in, is at a latitude of about 38°S.

A place’s longitude is the angular distance either west or east of what’s called the “prime meridian”, a line that connects the North Pole and the South Pole that passes through the town of Greenwich near London.

This map on the left shows what the Earth would look like from above the North Pole. Everywhere on the yellow dotted line, the prime meridian, has a longitude of 0°. We then have 10° West and 10° East, 20° West and 20° East and so on until we get to 180°, which can be called 180° west or east. So, Athens in Greece is at a longitude of about 24° east, New Delphi in India is at a longitude of about 77° east, and New York in the USA is at a longitude of about 74° west. Any line that runs from the North Pole to the South Pole is called a meridian (and I’ll just mention again that lines of equal latitude are called parallels). The meridians drawn in on the globe on the right are 6° apart, while the parallels are 4° apart.

For what it’s worth, the meridian that Greenwich is on was chosen to be the prime meridian because of the important astronomical work that was done at the Royal Observatory Greenwich in the 1700s and the 1800s that allowed ships to calculate their position at sea. It’s now a museum and people go there to see the displays. I actually visited the Royal Observatory recently and did what a lot of tourists like to do. I’m standing on the prime meridian of the world. Everything that way is to the east and everything that way is to the west in longitude.

The longitude of a place doesn’t really affect the climate that it experiences but its latitude certainly does. At a simple level, it’s always hot near the equator and it gets on average cooler the further you are from the equator, that is, as your latitude changes.

So, the Earth spins once every 24 hours, which is what we call a day. The side of the planet that faces the sun is having daytime and the other side is in night time. A year is the time it takes for the Earth to orbit the sun, which is about 365 days.

During the course of the year, we experience seasons. What causes the seasons? Let’s take a look.

Part D: Summer, Autumn, Winter, Spring

So, we know that the Earth orbits the sun. The imaginary flat surface that the earth is on is called the orbital plane (a plane is any imaginary flat surface). The reason that the Earth experiences seasons comes down to the fact that the Earth’s axis, the imaginary line running from the North Pole to the South Pole, isn’t perpendicular (that is, at right angles) to the orbital plane but is tilted at an angle of 23.4°.

This 23.4° tilt of the Earth is the reason you’ll usually see globes tilted at this angle.

So the Earth moves around the sun in a more-or-less circular path, it’s not exactly a circle but it’s fairly close, but, while it moves, it remains tilted at this angle of 23.4°.

Now the key thing here is that the tilt doesn’t change and the direction that the axis points also doesn’t change.

When the Earth is here, the axis is aligned like this and that’s the way it will stay. Three months later it’s still tilted in exactly the same way. Another three months later it’s the same, three months later it’s the same again and then three months after that (that is, a year later) it’s back to where it started. The reason that the axis direction stays the same is that the Earth is like a giant spinning top.

Now the Physics here is a little complicated, but, basically, when something is spinning it tends to keep the same orientation. Spinning tops seem to defy gravity, but they don’t really. So as the Earth moves around the sun, it keeps the same orientation.

So let’s get to the seasons. Seasons occur because of the tilt of the Earth. The tilt of the Earth as it orbits the sun changes two things: firstly, throughout the year, the direction from which sunlight hits any particular part of the Earth changes and secondly the length of daytime that any particular part of the Earth experiences changes. Let’s look at the direction first.

In December, the Earth is always in this position. Because of the tilt of the Earth, the Southern Hemisphere is kind of pointing towards the sun and receives more direct light from the sun, and so it gets heated by the sun more than what the Northern Hemisphere does.

I can set up a camera here just behind the spotlight so that we can see what the Earth looks like in December from the sun’s point of view. The sun is shining more directly down onto the southern hemisphere and so the southern hemisphere is going to get a lot hotter than the northern hemisphere.

If we imagine a beam of sunlight with all its energy striking the southern hemisphere, the energy is concentrated in a small area which heats up a lot. Here the southern hemisphere is experiencing summer. An identical single beam of light striking the northern hemisphere at the same distance from the equator hits at an angle such that the energy spreads out over a wider area. That part of the Earth therefore doesn’t heat up as much. So in the northern hemisphere it’s winter.

Another way of looking at is to consider lots of sunlight hitting the Earth in this position. Most of the sunlight is hitting the southern hemisphere, so the southern hemisphere is obviously going to be warmer at this time of year.

Six months later, in June, the Earth has moved to the other side of the sun but it keeps the same orientation. From the sun’s point of view (as you can see, I’ve placed a camera back behind the spotlight which is our sun), the Earth now looks like this, with the northern hemisphere facing more towards the sun, although obviously the real Earth is continuously rotating.

Now, the sun is shining more directly onto the northern hemisphere so the northern hemisphere experiences summer while the southern hemisphere experiences winter.

Now regions near the equator, which are called tropical regions, are hot all year round because of the more direct angle that the sun strikes those regions, and the poles are always cold once again because of the angle that sunlight strikes them.

Summer and winter have nothing to do with the Earth’s distance from the sun because the Earth moves around the sun in more or less a circle. Rather, it’s the angle of the Earth with respect to the sun that gives us summer and winter. More direct sunlight creates warmer weather while less direct sunlight creates cooler weather.

(If a scale model of the sun was built to match this globe (that has a diameter of 32cm), it would have to be placed about 3.8 km away to accurately reflect the sun-Earth distance.)

Between summer and winter we get autumn and spring. Let’s take a look.

In March, the Earth moves a quarter of the way around its orbit to this position where neither of the two hemispheres is facing more to the sun, so the weather is generally neither all that hot nor all that cold. It stays hot near the equator but just gets on average cooler and cooler the further from the equator you are.

In this position, the southern hemisphere is coming out of summer and into autumn, while the northern hemisphere is coming out of winter and into spring.

If we do a half lap, passing June and getting to September, a similar situation arises. Neither hemisphere is facing the sun more than the other. The southern hemisphere is coming out of winter and into spring while the northern hemisphere is coming out of summer and into autumn.

So from the sun’s point of view, the Earth’s looks kind of like these images at different times of the year. As I’ve said more direct sunlight creates warmer weather while less direct sunlight creates cooler weather.

These four positions actually have names and they occur on more or less the same dates every year. The Earth is in this position on around December 21st every year. It’s called the summer solstice in the southern hemisphere and the winter solstice in the northern hemisphere, although it’s often simply called the December solstice to avoid confusion.

On the day of the December solstice, the sun is directly above what’s called the Tropic of Capricorn. The Tropic of Capricorn is marked on most globes. If you’re standing on the Tropic of Capricorn in the middle of the day on December 21st, the sun will be directly overhead and you will be the closest person on Earth to the sun! Having said that though, I’ll just repeat, summer and winter have nothing to do with how close you are to the sun but rather it’s got to do with the tilt of the Earth and the direction from which sunlight hits it.

The day on which the December solstice occurs is the longest day of the year in the southern hemisphere, that is, it’s the day with the longest amount of daylight, and it’s the shortest day of the year in the northern hemisphere. I’ll explain why in our next episode.

Six months later, on around June 21st, the Earth is in this position, and it’s called the summer solstice in the northern hemisphere and the winter solstice in the southern hemisphere, or simply the June solstice to avoid confusion. This is typically the shortest day in the southern hemisphere and the longest day in the northern hemisphere. On this day, the sun is directly above the Tropic of Cancer.

The Tropic of Cancer is as far north of the equator as the Tropic of Capricorn is south of the equator.

On around March 21st every year, the Earth is in this position. It’s called the equinox, the autumn equinox in the southern hemisphere and the spring equinox in the northern hemisphere and, quite often, the March equinox. The sun is directly over the equator and no matter where you live, you’re going to have 12 hours of daytime and 12 hours of night time.

On around September 23rd every year, we get another equinox. Once again, there are 12 hours of daytime and 12 hours of night time and the sun is directly over the equator (although I’m not really showing that in this animation, because of the angle that we’re looking at it from).

So, basically, the tilt of the Earth is responsible for the seasons because the way the Earth is orientated changes how directly the sunlight hits any given part of the Earth.

However, not every place on Earth gets four clear cut seasons, and in fact, not every place on Earth has 365 days in a year. Let’s take a look.

Part E: Tropical, Temperate, and Polar

The Tropic of Capricorn is at a latitude of 23.4° South and the Tropic of Cancer is at a latitude of 23.4° North. Now where have you heard that number before? Well, it’s exactly the same as the angle of the tilt of the Earth with respect to the orbital plane.

The Tropic of Capricorn and the Tropic of Cancer are important because the sun can only ever be directly above your head if you’re standing somewhere between them. Regions between the Tropic of Capricorn and the Tropic of Cancer are called the tropics. The tropics don’t typically get four seasons, but only two: the wet season and the dry season.

The beautiful and tropical city of Townsville, for example, is at a latitude of 19° South. The climate graph shows the average daily maximum temperature of each month and the average monthly rainfall. As you can see, it’s quite warm for the whole year although it is a little cooler in the Southern Hemisphere winter months of June, July, and August. However, the difference between the wet season and the dry season is quite pronounced. There is no exact start and end of the wet season and the dry season but this pattern in rainfall is fairly consistent. Temperature and rainfall statistics like these ones are very common in tropical areas.

By the way, the average daily top temperature shown here for, say, January is the average of the highest temperature that was reached each day (during January). The records go back for decades, so the average is for all January days since data recording began. You can also find an average for the highest temperature it actually got to in January for all the Januaries that they have statistics for and that figure would be higher, but that’s not what is being shown here.

Another important set of lines on the Earth are the Arctic Circle and the Antarctic Circle, which together are called the Polar Circles.

The Arctic Circle is at a latitude of 66.6° North and the Antarctic Circle is at a latitude of 66.6° South. Now 66.6° = 90° – 23.4°, so the Polar circles run 23.4° from the two Poles because that’s the tilt of the Earth with respect to the orbital plane. Regions between the Poles and the Polar Circles are called Polar regions.

Now there are 365 days in a normal year and 366 days in a leap year, but that’s not 100% true. The South Pole and the North Pole get only 1 day a year. Daytime at the poles lasts for about 6 months, which is then followed by about 6 months of night time. As you can see, as the Earth turns in December, the south pole is in full sunlight as is the rest of the South Polar Region, and the sun doesn’t set at all. If you are at the South Pole in December, you would see the sun just circling around the sky, very low on the horizon, once every 24 hours. However, the North Pole in December experiences continuous night time because it’s facing away from the Sun. The sun sets on the South Pole in about March and it doesn’t see the sun again for six months. However, the sun rises at the North Pole in March and stays up for six months. In June, everywhere north of the Arctic Circle receives continuous sunlight. You can see here that the North Pole is facing the sun in June, but the South Pole is in darkness.

No-one actually lives at the poles because they’re way too cold, but the USA runs a scientific research station at the South Pole that has been continuously staffed since the 1950s. This post marks the exact location of the South Pole. From here, everywhere you face is north.

So we have the tropics and the polar regions. The regions between the Tropics and the Polar regions are called the Temperate Regions. The word temperate means not too hot and not too cold. Most people on Earth live in the temperate regions, and it’s the temperate regions that experience the most noticeable change of four seasons: summer, autumn, winter, and spring.

Now as I said, the tilt of the Earth is responsible for seasons because places warm up more if sunlight shines more directly onto them.

However, as I mentioned earlier, the tilt of the Earth also changes the length of daytime throughout the year, and quite obviously if the daytime lasts longer, then the region that’s getting a longer day will warm up more because sunlight hits it for longer. So in our next episode, we’re going to look at how the tilt of the Earth affects the length of night and day. See you then.

Credits:

Written and directed by Spiro Liacos.

https://en.wikipedia.org/wiki/File:Division_of_the_Earth_into_Gauss-Krueger_zones_-_Globe.svg by Hellerick. This file is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.

https://commons.wikimedia.org/wiki/File:Orthographic_Projection_Polar_North.jpg by Lars H Rohwedder is licensed under the Creative Commons Attribution-Share Alike 2.5 Generic2.0 Generic and 1.0 Generic license.

Climate data for Townsville sourced from the Australian Bureau of Meteorology via Wikipedia.

The Royal Observatory, Greenwich is a must-see if you plan to visit London. Visit https://www.rmg.co.uk/royal-observatory for details.