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 3: Following the Sun, we look at how the sun’s movement across the sky every day changes. In summer, the sun reaches a much higher angle in the sky than it does in winter. This affects the design of energy-efficient homes and the placement of solar panels.
The Episode 3 Question Sheet for Students:
QS3 Following The Sun.
Get the answers.
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The Transcript (which can be used as a textbook)
Contents:
Part A: Introduction: The sun rises in the morning, gets to its highest point in the middle of the day and then sets in the evening.
Part B: Solar Noon: The highest angle that the sun reaches on its daily journey across the sky is different every day. Why?
PART C: The Path of the Sun: It’s not just the highest angle that the sun reaches that changes; the path the sun takes every day changes. Knowing where the sun will be every day is really important in the renewable-energy industry. It’s also important because it’ll affect where you plant your flowers!
Part D: The Tropics: If you’re in the tropics, your shadow in the middle of the day will sometimes point north and sometimes point south. This isn’t the case in the rest of the world. Why are the tropics so special?
Part A: Introduction
Right now it’s early morning and the sun is rising behind me in the Eastern part of the sky.
The sun always rises in the eastern sky and gets higher and higher above the horizon until about midday, after which it gets lower and lower before setting in the western sky.
If you look at the shadows cast over a whole day, they start by pointing westwards, since the sun is over in the East. They’re also long because the sun is not at a very high angle above the horizon. In the middle of the day, the sun gets to its highest angle above the horizon and so the length of the shadows are at their shortest. As the sun starts heading back towards the horizon in the west, the shadows get longer and point more and more towards the east.
Because of the tilt of the Earth, the path that the sun appears to take every day actually changes throughout the year. Our knowledge of the paths the sun takes has helped us to build, for example, energy-efficient solar panels that are programmed to turn so that they can follow the sun as it moves across the sky which maximizes their output.
So how exactly does the sun’s position in the sky vary throughout the year? Well, let’s take a look.
Part B: Solar Noon
The time of day that the sun is at its highest point in the sky (let me freeze frame it) is called solar noon and also local noon occasionally.
Solar noon doesn’t necessarily occur at 12 o’clock midday because it depends on where you are in the time zone you’re in.
In my home state of Victoria, which is all in the one time zone, when the town of Marlo in the Eastern part of the state gets solar noon at exactly noon, 12 o’clock midday, solar noon doesn’t occur in Melbourne until 12:14 pm, because it takes another 14 minutes for the Earth to spin around to the point where Melbourne is in line with the sun.
Since accurate clocks were invented, we don’t usually notice when solar noon occurs of course, but it is still an important part of understanding how the sun moves across the sky.
If you’re in the Southern Hemisphere, in a place like Melbourne where I am now, at solar noon, which, as I said, is when the sun is at its highest point in the sky, which is right now in fact, the sun will be directly North of where you’re standing and your shadow will be pointing directly south. That way is south. It’s the opposite in the Northern Hemisphere, where your shadow will be pointing north.
Now when I say the sun is north of where you’re standing at solar noon (in a place like Melbourne), you have to look up at an angle of course to see it, but it’s still north and you have to face north to see it.
I can simulate the situation with my globe and my spotlight. In the morning the sun rises in the east. The shadows are long since the sun is still low and they point towards the west. As the Earth continues to turn, the shadows get shorter and shorter since the sun (from the Earth’s point of view) is getting higher and higher. At solar noon, when the sun is at its highest, the shadows are shortest. The shadow in the Northern Hemisphere points north, and the shadow in the Southern Hemisphere points south. Let me walk over to the other side. As the Earth continues to turn, you have to look towards the western sky to see the sun and the shadows get longer and longer in the easterly direction. Eventually, the sun sets of course and then everything is in shadow.
In the tropics, your shadow may point towards north or south at midday depending on the time of year. We’ll talk about this later.
Now at solar noon (which as I said is also called local noon), when the sun is at its highest point in the sky, it is directly above some point on the meridian that you’re standing on.
If you recall, meridian lines are the lines that run on the surface of the Earth from Pole to Pole, not to be confused with parallels. These two plastic figures have been placed on the same meridian: 150° East longitude. All places on any given meridian get solar noon at the same time.
When someone says, for example, 7 am, the am stands for ante meridiem which is Latin for before midday. When you say 5 pm, the pm stands for post meridiem, which is Latin for after midday. The Latin word meridiem is where we get the English word meridian from. At solar noon, what people called midday before accurate clocks were invented, the sun is directly above some point on Earth that is on the same meridian that you are on.
The angle of the sun above the horizon is called the sun’s elevation or sometimes the sun’s altitude. At sunrise, the sun’s elevation is zero degrees.
In this animation, the highest elevation the sun gets to is 52 degrees above the horizon. The sun always gets to its highest elevation at solar noon.
However, as we’ve seen, because the earth is tilted, the sun’s rays hit any given part of the Earth at a different angle at different times of the year. This means that the path the sun takes every day (rising, getting to its highest point, and then dropping back down) changes.
This is the situation in the middle of the day in Melbourne on June 21st every year. The sun strikes it at a shallow angle, even at solar noon.
It’s now about 12:20 and the sun is at its highest point in the sky. As you can see, it’s not very high above the horizon. It is in fact only about 29° above the horizon.
The shadow cast by a 1 metre ruler at solar noon is 1.82 metres long on June 21st, the June solstice. It’s quite a long shadow because as I said, the sun is not very high above the horizon, only 29° above the horizon and that’s the highest it reaches on this day.
Basically, this is what’s happening. There’s a 1 metre ruler being held vertically in Melbourne on the June solstice. I’ll extend the flat surface that the ruler is standing on, and then show sunlight coming in from the sun. The angle of the sunlight hitting Melbourne creates a shadow that I’ve represented with a dotted line. This angle here between the horizontal shadow and the sun is about 29°. Now it takes a bit of effort to visualize and to understand that someone standing there holding a 1m ruler is exactly the same as the situation shown in the diagram, but the diagram and the photo are the same. I can reposition the photo and show you what I mean. The two triangles are identical.
When Georgina and I filmed this scene, we were kind of upside down, because we come from a land down under. I hope this helps. Remember, this is the situation on the June solstice at solar noon when the sun is at its highest point in the sky.
Now six months later in December, on the day of the southern hemisphere summer solstice, it’s a very different situation.
| Elevation of the Sun at Solar Noon and Shadow Length  in Melbourne Throughout the Year | ||
| Date | Angle of Sun
(Elevation) |
Length of 1 m Ruler’s Shadow (m) |
| December 21
(December Solstice) |
76° | 0.26 |
| January 21 | 72° | 0.32 |
| February 23 | 62° | 0.53 |
| March 22 | 52° | 0.79 |
| April 21 | 40° | 1.19 |
| May 19 | 33° | 1.57 |
| June 21 (June Solstice) | 29° | 1.82 |
| July 28 | 33° | 1.53 |
| August 25 | 41° | 1.13 |
| September 22 | 51° | 0.77 |
| October 21 | 63° | 0.51 |
| November 18 | 71° | 0.34 |
When the sun reaches its highest point in the sky, on the day of the summer solstice here in Melbourne, it’s at an angle of about 76° above the horizon. The 1 metre ruler casts a shadow that’s only about 26 cm long.
We can compare real life with the diagram again. In real life the sun is at an angle, as shown by the shadow, of 76° above the horizon and it’s exactly the same angle in the diagram! Once again, rather than letting you do all the mental gymnastics of comparing the photo with the diagram, I can turn the photo and flip it around so that you can see that the photo and the diagram are showing exactly the same situation. The two triangles are the same again.
So, depending on the time of year, the angle of the sun above the horizon at solar noon is different. Let’s actually look at what happens throughout the year.
In Melbourne, in the middle of the day of the summer solstice, the shadow of a 1 metre ruler is 0.26 metres long because the sun is high in the sky at an angle of 76° above the horizon.
At solar noon on January 21, the sun is at an angle of about 72° above the horizon and the shadow of a vertical 1m ruler is 0.32 m long.
The highest angle that the sun reaches in the middle of the day changes as we move through February, March, April, May, and June and as we’ve seen, the sun doesn’t get very high on the day of the June solstice. After June, the sun rises to a higher and higher angle as we move through July, August, September, October, November, and back to December.
We aimed to capture these images monthly on around the 21st of every month, to coincide with the December and June solstices, but sometimes it was cloudy. We also tried to use the same camera position every month but though we didn’t exactly nail it, the changing angle of the sun is still pretty obvious.
Now though these angles are for Melbourne, they would be similar for all places at similar latitudes. In the Northern hemisphere they would be reversed though. The longest midday shadows would be seen in December and the shortest in June.
PART C: The Path of the Sun
The tilt of the Earth doesn’t just change the highest angle the sun reaches, it also changes the direction that you have to look to see the sunrise and in fact the path that the sun takes every day as it moves across the sky.
Even though the sun always rises in the Eastern sky, which is behind me at the moment, it turns out that it only rises exactly east on two days of the year: on the day of the spring equinox and on the day of the autumn equinox. For the rest of the year, the sun rises either to the north of east, or to the south of east.
This is the sunrise on September 23rd, that is, on the day of the September equinox in Melbourne. The sun rises directly east on the two equinoxes everywhere on Earth, so this point here was directly east of the camera. Three months later, on the day of the December solstice, the sun doesn’t rise directly east but to the south of east. I’ve lined up the footage that we shot, using this tree as a landmark, so that you can see the relative position of the sun at sunrise on these days. We also filmed the sunrise on the day of the June solstice. In June the sun rises north of east.
So, the sun rises here in September, about here in October, about here in November and about here in December. It then starts rising further and further north again every day: January, February, March and in particular, exactly east on the day of the March equinox, and then April, May and June. After the June solstice it starts rising further south again every day: July, August, and back to September.
The same holds true for sunsets. On the two equinoxes, this is the September equinox sunset, which we filmed from the other side of the lake, the sun sets directly west of wherever you’re standing anywhere on Earth. For the rest of the year, it sets either a little north of west or a little south of west. Once again all of this is because of the tilt of the Earth.
I can show you why using my globe onto which I’ve placed three figures: one on the equator, one at 30°N latitude and one at 30°S latitude. On either of the two equinoxes when neither hemisphere is facing the sun more than the other, you can see that at sunrise the three figures have to face directly east along the parallels to see the sun rising. As I said, the sun rises directly east on the two equinoxes everywhere on Earth.
In December, the southern hemisphere is tilted towards the sun so now the figures don’t face directly east anymore as the sun rises, but to the south of east. The exact angle varies depending on your latitude, but in Melbourne it’s about 30° south of east while on the equator it’s about 20° south of east.
Here we see the situation in June, when the Northern Hemisphere is facing more towards the sun. In June, you have to face slightly north of east to see the sun rising.
So the sun rises in the general direction of east, but not necessarily exactly east, it gets to its highest angle above the horizon in the middle of the day, and then it sets in the general direction of west. Let’s put all this together and follow the sun for the whole year.
This is what happens in Melbourne where I live and in places that have a similar latitude to Melbourne. On the day of the March equinox, typically March the 21st, the sun rises directly east, rises to an angle of about 52° and then sets directly west. Three months later, on the day of the June solstice, which falls on around June the 21st, the sun rises north of east, and reaches an angle of 29° above the horizon before setting again north of west. Three months later, on the day of the September equinox which falls on around September the 23rd every year, the sun rises directly east again and does what it did on the day of the March equinox. On around December 21st every year, the day of the December solstice, the sun rises south of east, rises to an angle of 76° and then sets south of west.
These angles are the same for all places on Earth that have the same latitude as Melbourne. In Sydney, which is about 4° further north of Melbourne (that is closer to the equator), the sun’s path is slightly different and the angles shown would be 4° higher.
So, in the southern hemisphere summer which starts in December, the orientation of the Earth results in (a) the sun hitting the southern hemisphere more directly and (b) the sun being in the sky for longer. Both of these produce warmer weather (although the direction from which the sun hits is a bigger factor). In the southern hemisphere winter, which starts in June, the orientation of the Earth results in (a) the sun hitting at a lower angle and (b) the sun being in the sky for less time, and both of these result in cooler weather. It’s the exact opposite in the northern hemisphere.
In the northern hemisphere in places with similar latitudes to Melbourne the sun rises at the opposite angle to what I see from where I live in Melbourne but the behaviour otherwise is the same.
People in the Southern Hemisphere generally have to face towards the north in the middle of the day to see the sun, but people in the Northern Hemisphere have to face towards the South. The exact angle depends on where you are and on the time of year.
Knowing where the sun is going to be is really important in the renewable-energy industry. Solar panels work best if they are directly facing the sun. These solar panels have been mounted onto movable gimbals (which are kind of like motorized hinges) and they’ve been programmed to actually point directly towards the sun as it moves across the sky. This maximises their efficiency. The engineers can pre-program the direction that the solar panels will need to face because they know exactly where the sun will be every minute of every day.
However, gimbals are expensive. Most solar panels and solar hot water systems like these ones are fixed in position. In Australia (in the southern hemisphere), they are installed so that they are facing northwards. This maximises the amount of sunlight they receive. In the northern hemisphere though, they are installed facing southwards. In the little farming town that my parents grew up in in Greece, in the northern hemisphere, just about every house has a solar hot water system, and every single one of them points towards the south. So, though we often don’t really notice it, the direction from which the sun hits us can often be a really important thing to know. Properly installed solar panels and solar hot water systems can reduce your energy bills by hundreds of dollars every year.
Many houses designs make use of the changing angle of the sun to provide extra warmth in winter when the sun doesn’t get very high in the sky.
In the Southern Hemisphere, houses are often built with large north-facing windows which allow plenty of winter sunlight into the house in the middle of the day. As we’ve seen, the sun is at a low angle in the sky at solar noon in winter. The sunlight heats the home and reduces the need for additional heating from heaters. This house is quite warm in winter, even when it’s cold outside. However, does the house overheat under the summer sun? Well, no. The windows have eaves, which are the parts of the roof that overhang the walls, so that the summer sun, which reaches a much higher angle in the sky, is blocked. It’s not always possible to incorporate this design into a house of course, but it can be very effective. In the northern hemisphere, south-facing windows are required.
Now if you ever want to put a plant in a garden bed, it’s really important to know where north and south are. If the tag says that the plant needs full sun, then you don’t want to plant the plant too close to the southern side of a tree or a house or a fence or whatever because the sun shines from the north (in the southern hemisphere) and your plant won’t get full sun. It’ll spend too much time in the shade.
Instead, you have to choose a spot that has a clear view to the North. Here’s a garden bed I prepared earlier, well, it’s actually my dad’s handiwork, and nature’s. These plants get plenty of sun.
Part D: The Tropics
In the tropics, your shadow in the middle of the day can sometimes point south and sometimes point north.
If you’re standing on the equator, for example, in December, in the middle of the day you have to face southwards if you want to see the sun and your shadow will be pointing to the north. At midday in June though, you have to face northwards and your shadow will be pointing southwards. On the equinoxes, the sun is directly overhead at solar noon on the equator, so your shadow is directly underneath you. Regardless of the time of year, the sun always reaches a fairly high angle in the sky in the tropics and so the tropics don’t ever get really cold.
So the Earth is heated by the sun but not all parts of the Earth are heated equally. Regions near the equator are always warm to hot and regions near the poles are always cold. When the southern hemisphere is tilted towards the sun, sunlight strikes it with more intensity and for longer, since the days are longer, so it gets summer. The northern hemisphere experiences winter. Six months later, it’s the opposite.
Now because the tropics get more direct sunlight than the rest of the planet, you’re looking at various beaches in tropical north Queensland, the ocean water in the tropics is much warmer than it is in the rest of the planet. This warmer water often causes severe tropical storms called tropical cyclones. The wind speed in a tropical cyclone can peak at nearly 300 km/hr, which is enough to knock down buildings and cause all sorts of other damage.
The strong winds in tropical cyclones spin clockwise in the southern hemisphere, but spin anticlockwise in the northern hemisphere. So how exactly do tropical cyclones form and why do they spin in different directions in the two hemispheres?
Closely related to this is the question that has plagued people for generations. Does the water draining from a container, like a bath tub, drain differently in the southern hemisphere compared to the northern hemisphere. Well, I’ve done some experiments and it’s what we’ll be looking at (along with tropical cyclones) in our next episode. See you then.
Credits:
12 HOURS in 30 SECONDS – Moving Shadows from morn to eve (Royalty free footage ) Time Lapse by der Naut. Creative Commons License.
Hurricane Irma DEVASTATES Naples, FL 150 mph gusts! by Bob McCallan. Creative Commons License.
Storm Surge PSA – Debbie (30 seconds) by FEMA. Creative Commons License.
The Life Of Cyclone Debbie (HD View From Satellite) uses satellite footage captured by the Japan Meteorological Agency’s Himawari-8 satellite.
Satellite Animation Sees Category 4 Hurricane Irma Approach South Florida by NASA Video