Seasons

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Slide 1

In this presentation, we’re going to talk about the seasons of the year and what causes the different seasons to occur. A lot of people think that Earth is closer to the Sun during the summer, making it hotter, and farther away from the Sun during the winter, making it cooler. This would make sense, but, unfortunately, it is incorrect. The reason that the Earth has different seasons is because it tilts on its axis! In this presentation, we’re going to explore this idea in more detail.

Slide 2

Let’s look at the topics we’re going to discuss. We’ll start by discussing the seasons that people on the Earth experience. Next, we’ll look at what we mean by the rotation and revolution of the Earth. Once we understand what we mean by these terms, we’ll be ready to answer our main question: why do we have seasons? We’ll then move on to talk about why different regions of the Earth experience different seasons at the same time, and finally have a look at some models of the relative motion of the Earth, Sun and moon.

Slide 3

So: just what are seasons? These are the main periods into which each year is divided. Each season is characterised by its own particular weather conditions. In many parts of the Earth, the year is divided into four seasons: Summer, Autumn, Winter and Spring. In the tropics, however, there are just two seasons: a rainy season and a dry season. The seasons occur at different dates on different parts of the Earth. For example, when it is Summer in the non-tropical parts of Australia, Canada is experiencing Winter. Different regions of the Earth experience different seasons because the Earth does not stand straight up and down. It tilts. The Earth’s axis is an imaginary pole that passes straight through the Earth from its north pole to its south pole. This pole is not vertical. Scientists believe that a long time ago, a large celestial object called Theia collided with the Earth, causing it to tilt on its axis.

Slide 4

In non-tropical regions of the Earth, we experience four seasons, in the order Spring, Summer, Autumn and winter. Spring follows the Winter and precedes the Summer. It is often rainy and warm. When we move to Summer, the weather becomes sunny and very hot. Things start to cool down in the Autumn, which is often cloudy and windy, and is followed by the Winter, which is cold. In many parts of the Earth, for example in the Snowy Mountains and some parts of Tasmania, snow can fall in winter.

Slide 6

As the Earth orbits the Sun, it spins around on its axis. This is called the rotation of the Earth. The Earth makes one complete turn on its axis every day (or, more precisely, every 23 hours and 56 minutes). As the Earth spins, different parts of it face the Sun. When our part of the Earth faces the Sun, we say that it is day time. When our part of the Earth faces away from the Sun, we say that it is night time.

Slide 7

As the Earth spins on the axis, it is also spinning in another direction. This time, it is spinning around the Sun. We call the motion of the Earth around the Sun the “revolution of the Earth”. It takes about a year (365.25 days) for the Earth to travel around the Sun once. The path that the Earth follows around the Sun is called the Earth’s orbit.

Slide 8

Now let’s think about just why we have seasons.

Slide 9

First, we need to know that the Earth’s orbit around the Sun doesn’t trace out a perfect circle. In fact, it traces out an ellipse (or oval). So, there are times when the Earth is closer to the Sun: it’s closest on around the 3rd of January and farthest away on around the 4th of July. But, this is not what causes the seasons. The different seasons occur because the Earth is tilted at an angle of about 23.5 degrees on its axis. The axis always points in the same direction during the revolution of the Earth, but this means that the Sun’s rays hit different parts of the Earth directly at different times of the year. For example, during December, the Sun shines directly onto the Southern hemisphere, so we experience Summer here in Australia in December, while the Northern hemisphere experiences Winter. The tilting of the Earth’s axis causes different season to be experienced on different parts of the Earth at the same time.

Slide 10

Summer occurs when the Sun’s rays hit a region of the Earth directly. For example, the Sun’s rays hit the Southern hemisphere directly during December, January and February, so we experience Summer in the Southern hemisphere at this time, while the northern hemisphere experiences winter as it is not hit so directly by the Sun’s rays. Consequently, it is hotter in the Southern hemisphere during December, January and February. The opposite occurs during June, July and August: it is hotter (and Summer) in the Northern hemisphere and cooler (and Winter) in the Southern hemisphere. Some effects of the tilt are that the days are longer during the summer, and shorter during the winter. The Sun strikes the Earth at a much steeper angle in regions that are experiencing winter.

Slide 11

This picture shows us how the Sun shines on the Earth during the Earth’s revolution around the Sun. In December, January and February, the Southern Hemisphere receives more direct sunlight, and experiences Summer, while the Sun shines on the Northern Hemisphere at a more extreme angle, and they experience winter. In June, July and August, the Northern Hemisphere experiences Summer, and the southern hemisphere experiences Winter. In between these two extremes are the northern Autumn and southern spring in September, October and November. In the northern hemisphere, the temperatures get cooler and the days get shorter, but in the southern hemisphere, the temperatures get warmer and the days get longer. The opposite occurs in the southern spring and northern autumn, which occurs in March, April and May.

Slide 12

Let’s talk a little more now about why we experience different seasons at different times of the year.

Slide 13

The Earth’s axis always points in the same direction. This means that different parts of the Earth get different amounts of sunlight at different times of the year. The southern part of the axis points most towards the Sun during December, January and February, so the Southern hemisphere receives more direct sunlight at these times and is warmer. We call this Summer. However, in June, July, and August, the southern part of the axis points furthest away from the Sun. So, during this time, the Southern hemisphere receives the least direct sunlight and is cooler. We call this time of the year Winter. We can predict when these different seasons are going to occur because the Earth’s orbit does not change from year to year.

Slide 14

The solstices are the days when parts of the Earth experience their largest number of hours of daylight or their smallest number of hours of daylight. There are two solstices each year, and they occur when the Earth’s axis is tilted the most towards or away from the Sun. The winter solstice is the shortest day of the year. In the Southern hemisphere, it occurs during the winter on June 20th or 21st. The summer solstice occurs in the Northern hemisphere at this time. The Summer solstice is the longest day of the year. It occurs on December 21st or 22nd in the Southern hemisphere. At this time, the Northern hemisphere experiences its winter solstice.

Slide 15

Another way to think of the solstices is to think of the moments in the year when the apparent part of the Sun is farthest south or north from the Earth’s equator. The celestial equator marked on the animation is the apparent position of the Earth’s equator when projected into the sky. On the day of the summer solstice, the sun appears to be at its highest elevation. On the day of the winter solstice, the Sun appears to be at its lowest elevation. There are two other days of the year that are of interest. They are the spring and autumn equinoxes. On these days, the Sun is over the celestial equator, and the lengths of the days and nights are roughly equal. The spring equinox occurs on the 22nd or 23rd of September, and the autumn equinox occurs on the 20th or 21st of March.

Slide 16

Now let’s look at some animations that allow us to visualise how the Sun, Earth and Moon move with respect to each other.

Slide 17

First we watch the moon orbit the Earth. This occurs once every 27.322 days (or one lunar month) and the moon rotates once on its axis during this period. This makes the moon look like it is keeping perfectly still, but it is, in fact, spinning.

Slide 18

Now we look at the way the Earth completes one revolution of the Sun. One complete revolution takes about 365.25 days. As the Earth orbits the Sun, it rotates on its axis. It takes 23 hours and 56 minutes for the Earth to complete one rotation on its axis.

Slide 19

Finally, we put it all together. Not how the Earth is tilted on its axis. As the Earth revolves around the Sun, it rotates on its axis, and the moon revolves around it, also slowly rotating on its axis.

Slide 20

Let’s just summarise the main points we’ve covered in this presentation. Each year is divided up into seasons. In most parts of the world there are four seasons in the year: Spring, Summer, Autumn and Winter. In the tropics, there are only 2 seasons.

The Earth rotates on its axis. It takes 23 hours and 56 minutes for it to complete one rotation. This rotation gives rise to day and night as different parts of the Earth face towards and away form the Sun. It revolves around the Sun completely in 365.25 days, which is about one year.

The tilt of the Earth’s axis gives rise to the seasons as the Earth moves around the Sun. The Earth’s axis always points in the same direction, so different parts of the earth receive more direct sunlight at different times as the Earth revolves around the Sun.

Because of this, different regions of our plant experience different seasons at the same time. When it is Summer in the northern hemisphere, it is winter in the southern hemisphere and vice versa.
The last thing we looked at were some animations showing models of the Sun, Earth and Moon as the Earth revolved around the Sun, and the moon revolved around the Earth. This allowed us to get an idea of how these three celestial bodies move in relation to one another.