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EPISODE 7:
Why are summer days longer than winter days?
What is the difference in daylength between summer days and winter days?
Is this difference the same everywhere on Earth?
When do we get our longest day?
When do we get our shortest day?
How long is an average day?
The STAR LAWS Episode III Question Sheet for Students:
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The Microsoft Word version.
The Transcript:
The word astronomy comes from the Greek words astron which means star and nomos which means law. Astron nomy therefore translates literally as STAR LAWS. Astronomy is the study of the laws of nature that govern how stars and other bodies in space appear to move across the sky, how they form, and how they change over time.
In this episode, we’re going to explain why summer days are longer than winter days.
Hi Everyone. It’s Spiro here. Why are summer days longer than winter days? Well, that’s what we’ll be looking at in this episode of the STAR LAWS series. We’re also going to look at how daylength changes throughout the year and at how the change in daylength varies depending on your latitude.
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’s look at an example first. In Melbourne, Australia (the city I live in), on the day of the southern hemisphere summer solstice, which is around December the 21st, sunrise occurs at about 5:54 am and sunset occurs at about 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 the day on which the southern hemisphere winter solstice occurs, typically on around June 21st, 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).
The reason for this difference in daylength throughout the year comes down to the fact that the Earth is tilted with respect to the orbital plane. (The orbital plane is the imaginary flat surface that the Earth moves on as it orbits the Sun.) The tilt of the Earth doesn’t change as the Earth orbits the sun.
Here I’ve set up a spotlight in the gym that models the sun, and I’ve placed a globe representing the Earth on a trolley. I can film the Earth from behind the spotlight and observe what the Earth looks like from the sun’s point of view at different times of the year.
In December, the southern hemisphere is tilted towards the sun and the northern hemisphere is tilted away from the sun. The reverse is true in June. You can see here that the northern hemisphere is tilted more towards the Sun. At the equinoxes in March and September, the sun is directly above the equator and neither hemisphere is tilted towards or away from the sun.
So, how does the tilt of the Earth affect the length of daytime and night time?
Well, 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 2½ hours later.
Here it is here just coming into the sunlight. 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.
We can also look from the side of the Earth on the December solstice, so that we can see the sunlit side of the Earth in daytime and the side facing away from the Sun that is experiencing nighttime, and show the circular paths that Melbourne and Tokyo take as they go around and around.
You can see that the circular path that each city takes every day is pretty much the same size, but because the southern hemisphere is pointed more towards the Sun, Melbourne spends much more of its time in daylight (that is, moving along the part of the circular path coloured orange) and much less time in night (along the part of the circular path coloured red). Tokyo though, spends most of the time in nighttime (the red-coloured part of its circular path) and much less time in daylight (the orange-coloured part of its circular path). 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 about 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 about 12 hours of day time and about 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. There it is there. 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.
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. 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.
So, if I list just the Northern Hemisphere cities in order from closest to the equator to furthest from the equator, we can clearly see that as we get further from the equator, there’s a greater variation in daylength between summer and winter.
This graph shows the variation between the longest day of the year and the shortest day of the year at different latitudes. The longest day of a location like Oslo in Norway which is at a latitude of 60 degrees north is much longer than the longest day of a location like Cape Verde which is at a latitude of 15 degrees north. However, Oslo’s shortest day is also much shorter than Cape Verde’s shortest day. On average, over the whole year, every place on Earth is in daylight for about half the time and in night for about half the time.
The graph here doesn’t specify the latitude north or south of the equator because it doesn’t need to. The daylengths of the longest day and the shortest day depend on the latitude of the location, but for Northern Hemisphere locations, the longest day will be in June and the shortest day will be in December and for Southern Hemisphere locations, the longest day will be in December and the shortest day will be in June.
The timeanddate.com website provides data about daylength for any location on Earth. If you hover over the Sun, Moon, and Space tab, and then click Sun Calculator, it’ll take you to the Sunrise and Sunset Calculator – City Lookup page. I live in Melbourne and I’ve been to this page before so I could just click Melbourne, but just to show you, if I type in Melbourne and select it, it takes me to this page which provides lots of Sun-related information about my location, including the current date and time and today’s daylength. If I scroll down, the page provides information about sunrise and sunset times, daylengths and more for every day of the month. On November the first for example, the sun rose at 6:13 am and set at 7:54 pm. Daylength was 13 hours and 41 minutes. It even tells us the difference in daylength from the previous day. November the first was 2 minutes and 11 seconds longer than the previous day!
And so, summer days are longer than winter days because of the tilt of the Earth. Near the equator, there’s not much variation in daylength throughout the year, but as you get further and further from the equator, the variation in daylength between summer and winter gets bigger and bigger. Thanks for watching, see you next time.
CREDITS:
Produced by Liacos Educational Media
Some of the content of this video first appeared in Shedding Light on the Sun and Earth Episode 2: Long Hot Summer Days.
Opening and end titles music by Humanoide_Media via Pixabay.
https://pixabay.com/users/12661853/?tab=music&order=latest&pagi=1
https://pixabay.com/music/main-title-star-wars-style-march-165111/
https://pixabay.com/music/main-title-invasion-march-star-wars-style-cinematic-music-219585/
Star-Wars-style opening crawl generated at the STAR WARS Intro Generator website. https://starwarsintrogenerator.com/
Screen recordings were taken from the timeanddate website: https://www.timeanddate.com/