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EPISODE 9:
In which direction does the Sun rise every morning?
In which direction does the Sun set every evening?
What path does the Sun follow during the day?
How does the Sun’s path change throughout the year?
Does your location change the way that the Sun appears to move across the sky?
How does the Sun’s path affect the renewable energy industry and the growth of plants?
The STAR LAWS Episode IX Question Sheet for Students:
The PDF version.
Google The Google Doc version. Google
The Microsoft Word version.
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 look at how the Sun’s path across the sky changes throughout the year….
Hi Everyone, it’s Spiro here. Welcome to Episode 9 of the STAR LAWS series. The path that the Sun takes every day across the sky changes throughout the year.
We saw in our last episode that the sun reaches a higher elevation in summer than it does it winter thanks to the Earth’s tilt.
Now the tilt of the Earth doesn’t just change the highest angle that the Sun reaches, it also changes the direction that you have to look to see the sunrise and in fact the entire path that the Sun takes every day as it moves across the sky. In this lesson we’re going to look at how the daily path of the sun across the sky changes throughout the year AND look at examples of how this affects us.
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.
Basically, the Sun’s path moves northwards every day between March and June. I can’t show you the Sun’s path for every single day, but, for example, its path on May the 6th, halfway between the dates I’ve already shown so, in other words, 6 weeks after the March equinox and 6 weeks before the June solstice, looks something like this red curve. Now, from the June solstice onwards, the Sun’s path stops moving northwards and starts moving southwards again. On August the 7th, about 6 weeks after the June solstice, the Sun’s path is very similar to the path that it followed on May the 6th. Now to avoid clutter, I’m just going to show the four key astronomical dates from now on. 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.
It then starts moving northwards again and the cycle, the yearly cycle, continues.
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.
In the tropics though, things are a little different. At the equator, the Sun’s path on the four key dates looks something like this. The Sun basically rises straight up every day. On the days of the two equinoxes, the Sun passes directly overhead at solar noon. For half the year, from the March equinox through to the June solstice and back to the September equinox, it appears to be north of you and for the other half of the year it appears to be south of you.
Your shadow at solar noon therefore points southwards for half the year and northwards for the other half of the year.
Now in Iceland, at 65°N, the path of the Sun looks something like this. At high latitudes, the Sun’s path is angled quite a lot with respect to the horizon and the sun never gets to a very high angle at solar noon, which generally results in cooler weather.
So, the Sun’s daily path across the sky changes throughout the year thanks to the Earth’s tilt and the exact angles are affected by your latitude. Near the equator the Sun always reaches a fairly high angle at solar noon but as you get further from the equator, the sun’s elevation at solar noon gets lower and lower.
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 house 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.
And so, we see that the STAR LAWS governing the Sun’s apparent movement across the sky affects us in lots of different ways. Thanks for watching. See you in the next episode.
CREDITS
Produced by Liacos Educational Media
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/