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 2: Long Hot Summer Days, we explain why summer days are long and winter days are short, and how the length of daytime contributes to seasonal variations in temperature.
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The Transcript (which can be used as a textbook)
Part A: Introduction: Daytime in summer lasts a lot longer than daytime in winter.
Part B: The Sun Above Us: The sun passes directly over the equator only twice a year. During the rest of the year, it is directly over some point on either side of the equator. We track the movement of the sun, explain what the equinoxes and solstices are, and explain the significance of the Tropics of Capricorn and Cancer.
Part C: Longer Days and Shorter Days: Why are summer days longer than winter days? Why do we get about 12 hours of daylight and 12 hours of night when the equinoxes occur? And does your location on Earth affect the way that the length of daytime and night time change?
Part D: Daylight Saving: Not really climate related, because daylight saving doesn’t change nature in any way! However, it does change what time we get up in the morning…
Part A: Introduction
The Sun. All life on Earth depends on it for the light and warmth that it brings to our planet. However, throughout the year, the weather changes. We saw in our last episode the reason that the Earth experiences seasons. It comes down to the fact that the Earth is tilted with respect to its orbit around the sun and this changes the angle from which sunlight hits the Earth. More direct sunlight produces warmer weather, while less direct sunlight produces cooler weather.
However, the tilt of the Earth and the direction from which sunlight hits the Earth also changes how long day and night last. For example, in Melbourne, Australia, on December the 21st, the day of the southern hemisphere summer solstice, sunrise occurs at 5:54 am and sunset occurs at 8:41 pm, so daytime lasts for 14 hours and 47 minutes. This is the longest day of the year for Melbourne. Night time lasts only 9 hours and 13 minutes. In contrast, on June 21st, the day on which the southern hemisphere winter solstice occurs, sunrise is at 7:35 am and sunset is at 5:08 pm. This is the shortest day of the year. The sun is up for only 9 hours and 32 minutes, while night time lasts 14 hours and 28 minutes.
These times are similar for all cities around the world that have similar latitudes: Sydney, Adelaide, Perth, Cape Town, and Santiago for example in the Southern Hemisphere, and Tokyo, Seoul, Beijing, Athens, Madrid, Washington DC, and San Francisco in the Northern Hemisphere, although for the Northern Hemisphere cities, the longest day is on the June solstice and the shortest day is on the December solstice (the reverse in other words of what Melbourne gets).
So in December, not only is sunlight shining onto the southern hemisphere more directly, it’s also hitting it for a longer amount of time which of course causes even more heating. The angle at which the sunlight strikes any given part of the Earth is by far the biggest factor that affects seasonal variations in temperature, but the length of day and night also plays a role.
So, how does the tilt of the Earth affect the length of daytime and night time? Well, let’s begin by following the sun on its movement, from the Earth’s point of view, over the Tropics.
Part B: The Sun Above Us
Now even though we now know that the Earth is turning and that’s what gives us day and night, from the Earth’s point of view it looks as if the sun rises every day and then sets every day and it’s often far more convenient to talk about the sun’s position in the sky and its movement across the sky than it is to talk about the Earth’s position in space.
The Tropic of Capricorn, 23.4° south of the equator, and the Tropic of Capricorn, 23.4° north of the equator are two important lines on Earth because the sun can only ever be directly overhead, straight up, if you’re standing between them, and the sun only passes overhead at any given point twice a year.
|Position of the Sun (Above the Earth) Throughout the Year|
|September 23 (September Equinox)||0° (Equator)|
|October 1||3.5° S|
|November 1||14.5° S|
|December 1||22° S|
|December 21 (December Equinox)||23.4° S (Tropic of Capricorn)|
|January 1||23° S|
|February 1||17° S|
|March 1||7.5° S|
|March 21 (March Equinox)||0° (Equator)|
|April 1||4.5° N|
|May 1||15° N|
|June 1||22° N|
|June 21 (June Solstice)||23.4° N (Tropic of Cancer)|
|July 1||23° N|
|August 1||18° N|
|September 1||8° N|
|September 23 (September Equinox)||0° (Equator)|
If I set up a camera behind my spotlight (which is our model sun of course), we can see what the earth looks like from the sun’s point of view on September the 23rd, the September equinox and where the sun is in relation to the Earth. That bright spot there is the actual spotlight reflecting back into the camera. The sun is in fact directly over the equator. Obviously we filmed the two scenes with the gymnasium lights on and with them off so that in the main image only the spotlight (our sun) was providing the illumination. Let’s see what happens as the Earth moves around the sun and fill in the table as we go.
On October the 1st, the sun is no longer over the equator, but about 3.5° south of the equator. In the middle of the day on October the 1st, it will pass directly overhead if you’re standing 3.5° south of the equator.
A month later, on November the 1st, the sun will pass overhead 14.5° south of the equator. On December the 1st the sun is 22° south of the equator and on December the 21st, the December Solstice, it gets to 23.4° south of the equator. This line is called the Tropic of Capricorn. As we’ve seen, the Southern Hemisphere gets warmer weather because the sun shines more directly onto it and the daytime is longer as well, for reasons I’ll explain soon.
The word solstice comes from the Latin Sol (which means sun) and the Greek stasi (which means to stand still or to stop). The word station as in train station also derives from stasi: the train stops at the train station. On the day of the June Sol-stasi… the solstice as we now say in English, the sun stops moving towards the south (from the Earth’s point of view) and starts moving northwards again.
On January the 1st, the sun has moved back to 23° south of the equator, on February the 1st to 17° south, on March the 1st 7.5° south and then on March 21st, the equinox, the sun is directly over the equator again.
The word equinox comes from the Latin “equi” meaning equal and the Greek “nix”, which was changed in Latin to “nox” meaning night. In modern Greek, night is called ?????. At the equi-nixta, the equinox, the length of daytime and night time are the same: 12 hours each.
Technically, though, the equinox is defined as the moment that the sun is directly over the equator. It turns out that the day on which the equinox occurs, daytime is actually a few minutes longer than night time. So it’s close to 12 hours of day and 12 hours of night, but not exactly.
By the way, the word nocturnal also derives from nox and nyxta and it describes an animal that is most active at night. Possums are supposed to be nocturnal but I filmed this one during the day while on holidays once. Maybe it had insomnia! Who knows?!
After the March equinox, the sun moves north of the equator (from the Earth’s point of view) and it keeps moving north until about June 21st, the June solstice, when it reaches the Tropic of Cancer (although the plastic arm there is hiding it a little). As I said, the word “solstice” means “sun stop”, so on the day of the June solstice, the sun stops moving north and starts moving southwards again. As the months pass, it eventually gets back to being directly over the equator again at the September equinox.
So, from the Earth’s point of view, the sun’s path across the sky changes throughout the year. This changes the angle that you have to look to see the sun throughout the year and it changes the length of day and night throughout the year. Let’s see why.
Part C: Longer Days and Shorter Days
At any given moment, half of the Earth is facing the Sun, and is therefore in daytime, while the other half is facing away from the sun and is therefore experiencing night time. However, that doesn’t mean that any given place on Earth gets 12 hours of day and 12 hours of night. That only happens twice a year on the equinoxes. The number of daylight hours for the rest of the year actually varies. Let me explain why.
Let’s look at an Equinox first. On about September 23rd every year, the sun is directly above the equator and the Earth is not tilted towards the sun or away for the sun at all. So, everywhere on Earth gets about 12 hours of daylight and 12 hours of night. Here for example is Melbourne, in the Southern Hemisphere and here’s Tokyo in the Northern Hemisphere. I’ve put some plasticine onto the cities so that we can see them more clearly.
Tokyo and Melbourne aren’t exactly on the same line of longitude, that is on the same meridian, but they’re pretty close at 140°E and 145°E and they’re both about the same distance from the equator which means they have similar latitudes although one is north and one is south of course. So, given their similarity on either side of the equator, they make a good pair of cities to compare and contrast.
Here we can see that the Americas are in daytime. As the Earth turns, Melbourne and Tokyo both come into sunlight that is, they see the sun rise, at the about the same time, and then after the Earth rotates one half turn—12 hours later—they both go into night time, that is, they see the sun set, at about the same time. (Although just to be accurate, the sun sets slightly earlier in Melbourne because it is slightly more to the East.)
Now what happens as the Earth moves around? Three months later, on around the 21st of December every year, the December solstice, the Earth moves to this position. Now we have a completely different situation.
The southern hemisphere is facing more towards the sun and the sun is directly over the Tropic of Capricorn. As the Earth turns, Melbourne is hit by the sun way before Tokyo is, even though the two cities are roughly in line with each other. Tokyo doesn’t see the sun until about 2 hours later. In fact Melbourne gets nearly 15 hours of daylight (14h:47min) on December 21st.
After midday, which occurs for both cities at about the same time, the sun starts going down and sets in Tokyo much earlier than it does in Melbourne, about 3 hours earlier in fact. Tokyo gets only about a 9½ hour day in late December.
So, in December, Melbourne (and the Southern Hemisphere in general) gets long days and short nights, while the reverse is true for places in the Northern Hemisphere which get short days and long nights.
So, not only is the Southern Hemisphere receiving more direct sunlight, the actual length of the day is longer so it actually receives more sunlight. This means that the southern hemisphere gets hotter.
If we look from the side of the Earth on the December solstice so that we can see the sunlit side of the Earth and the side having night time and show the paths that Melbourne and Tokyo take as they go around and around, it’s pretty clear that Melbourne and other cities in the Southern Hemisphere spend most of the time in sunlight and not much time in darkness. The opposite is true of Tokyo and other cities in the Northern Hemisphere, which spend most of their time facing away from the sun. Six months later, it’s going to be the other way around.
So as I said, the days are longer and the nights are shorter in summer and the opposite is true in winter.
Three months after the December Solstice, on around March 21st every year, the Earth moves another ¼ of the way around its orbit and we’re back to an equinox, the March equinox.
The sun rises in Melbourne and Tokyo at about the same time, they both get 12 hours of daylight and then the sun also sets in both cities at more-or less the same time.
So wherever you are on the days on which the equinoxes occur, you’re going to get 12 hours of day time and 12 hours of night.
Three months later on about June 21st, the Earth moves to this position and we have the June Solstice. Now, the Northern Hemisphere is facing more towards the sun, and the sun is over the Tropic of Cancer. As the Earth turns, Tokyo sees the sunrise, but Melbourne doesn’t see the sunrise until a few hours later. On June 21st, Tokyo gets a 14½ hour day, while night time last only about 9½ hours. It’s approximately the reverse in Melbourne, which gets only about a 9½ hour day and about 14½ hours of night time. Melbourne sees the sunset first, but the sun stays up in Tokyo for another few hours.
So the length of day and night changes throughout the year which means the sun rises and sets at different times throughout the year.
|December 21||June 21|
|Melbourne||38° South||5:54 am||8:41 pm||14:47||7:35 am||5:08 pm||9:32|
|Tokyo||36° North||6:46 am||4:31 pm||9:44||4:25 am||7:00 pm||14:34|
|Singapore||1° North||7:00 am||7:03 pm||12:03||7:00 am||7:12 pm||12:11|
|London||51.5° North||8:03 am||3:53 pm||7:50||4:43 am||9:21 pm||16:38|
|Anchorage||61° North||10:13 am||3:41 pm||5:27||4:20 am||11:42 pm||19:21|
So these are the sunrise and sunset times and the day lengths for Melbourne and Tokyo on the solstices. In December, Melbourne gets long days and Tokyo gets short days, and the reverse is true in June. As I said earlier, these times are similar to all locations that have similar latitudes. At different latitudes things are different. The length of day and night in Singapore, 1° north of the equator, barely changes at all throughout the year. Places on and very near the equator get pretty much 12 hours of day time and 12 hours of night time every day.
Now London, 51.5° north of the equator, has only about 8 hours of daylight on the day of the December Solstice, but nearly 17 hours of daylight on the day of the June Solstice. However, though the days are long at that time of year, London doesn’t usually get very hot because the sun’s rays are on too low of an angle to give London a hot summer, although it’s still much hotter than it is in winter of course. The days and nights have similar lengths in cities like Berlin and Vancouver.
Anchorage in Alaska, 61°N north, gets only about 5½ hours of daylight in December, but more than 19 hours of daylight in June. Oslo, and Helsinki are similar. Once again, cities this far North don’t really get very hot, because the sun strikes them at too low an angle.
Part D: Daylight Saving
Many countries and states use Daylight Saving Time in the warmer months when the days are longer. We put the clocks forward one hour in spring and put them back again in autumn. Daylight Saving doesn’t of course change the way the Earth spins or the length of daytime or anything like that. It just allows us to make better use of the daytime. So how does it work?
In Melbourne, on the day of the winter solstice, the sun rises at about 7:35 am and sets at about 5:08 pm. Now let’s just say school starts at 9 am and finishes at 3 pm (for students, not for teachers; we always work till much later, but anyway…). There are only about 1½ of daylight between sunrise and the start of school on this day and only about two hours of daylight after school before the sun sets. Now these times are in what is called Standard Time.
However, the sun rises earlier and earlier every day until, on the day of the summer solstice, it rises about 2½ hours earlier than it did on the day of the winter solstice. It also sets about 2½ hours later. So, in the summer months the sun rises really early, but if you wake up and want to do something, you’re limited, because you have to be at school by 9. This is where daylight saving time comes in.
Daylight saving isn’t really just about changing the clocks, it’s really about doing everything earlier.
Instead of leaving this huge amount of time between sunrise and school, or work, or whatever, during daylight saving time school actually starts an hour earlier. And of course, instead of finishing at 3 pm standard time, it finishes one hour earlier, which gives everyone more time in the afternoon to do whatever they want to do. However, instead of changing the times shown on timetables at schools, the times on bus timetables, train timetables, flight schedules, TV schedules and whatever else, we simply put the clocks forward. 8 am standard time becomes 9 am daylight saving time, 2 pm standard time becomes 3 pm daylight saving time and so on.
So daytime is longer anyway during the warmer months, but by starting school, or work or whatever an hour earlier but still calling it 9 am daylight saving time, it gives us an extra hour after school to do things… just don’t forget to fit in your homework as well.
A good way of remembering which way to change the clocks is this simple saying: spring forward, fall back. In Spring we put the clocks forward, in Fall (or Autumn) we put the clocks back.
Remember, these sunrise and sunset times are for Melbourne, but they’re similar for all places around the world that have similar latitudes to Melbourne.
For countries and states that use it, daylight saving time typically goes for about 6 months of the year.
It’s used in a lot of Australia, New Zealand, most of Europe, most of North America and a handful of other places. The countries and states shaded in orange have used it in the past but don’t any more, and the countries shaded in red have never used it. Countries or states near the equator don’t generally use daylight savings because near the equator, in Singapore for example, the length of day and night doesn’t really vary all that much throughout the year, so there’s not much point to using daylight saving time.
For me personally though, living in Melbourne, I love having the extra time after work to do things.
So, the length of day and night changes throughout the year because the Earth is tilted and the sun hits the Earth from different angles throughout the year. The change in day length is more pronounced the further a place is from the equator.
So, if the sun hits us at a different angle at different times of the year, is it obvious? Can we tell? Well, it is obvious and we can tell although we don’t always look up.
The angle of the sun changes throughout the day and it changes throughout the year and it affects the way we install solar panels, place plants in a garden and a whole lot of other things and so it’s the changing angle of the sun that we’ll be looking at in our next episode. See you then.
https://en.wikipedia.org/wiki/File:Mercator_projection_SW.jpg by Strebe. This file is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.
https://commons.wikimedia.org/wiki/File:DaylightSaving-World-Subdivisions.png by Paul Eggert. This file is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.
Day lengths, sunrise times, sunset times, and a whole lot more data can be found at the Time and Date website (www.timeanddate.com). For example, hover over the Sun and Moon drop-down menu item, and then click Sun Calculator. Search for your city or town and a graphic and a table will be displayed. An example is shown below.