 We take the seasons for granted. It's always cold in the winter and warm in the summer and somewhere in between during spring and autumn. But when you ask most people what causes the seasons, the answer is usually a guess that the earth must be closer to the Sun during summer time and farther away from the Sun during winter time. But in fact, it is not the case at all as we're about to find out. So to help us understand the seasons, let's begin with our familiar model of the celestial sphere. So we have the celestial equator. We have the Sun's path. That is the ecliptic. Now, let's just change our perspective slightly. Instead of letting the celestial equator be parallel to our screen, we'll just tilt this so that the ecliptic is parallel to our screen. So we can envision the earth rotating on its axis and it's tilted slightly with respect to the ecliptic. In fact, the celestial equator and the ecliptic are tilted by about 23 and a half degrees. It's closer to 23.4 degrees in real life, but this is close enough. That also means that the north celestial sphere must be tilted by 23 and a half degrees from a line that is perpendicular to the ecliptic. So this axial tilt of earth has a name. We call it the obliquity of earth and it's just the rotation axis being tilted from the ecliptic and it's more commonly known as axial tilt. So when we look at our celestial sphere, we see that the celestial equator is tilted, the entire system is tilted. But remember, it is really not earth at the very center of the universe. It is, in fact, the Sun that occupies the center of, well, our solar system. So the earth is rotating around the Sun at the same time it's rotating on its axis. So when we see the Sun projected on the sky during the vernal equinox, the earth is, well, on the opposite side of the vernal equinox from the Sun, you might say. We look up, we otherwise would imagine seeing the Sun against the vernal equinox. That means that when we are looking at the Sun, there it is. We have the celestial equator going straight across our field of view. We have the ecliptic tilted by 23 and a half degrees. And notice that we're now marking off the hours of right ascension. So the Sun is at zero hours of right ascension. It is on the vernal equinox. It's also got a declination of zero degrees. It's landed right on the equator. But when we come to the Sun on the 21st of June, it's now three months later and the Sun has now reached six hours of right ascension. Notice that its declination is about 23 and a half degrees north. So we give that a positive value. And in the northern hemisphere, we call this the summer solstice. Three months later, the Sun has now moved to 12 degrees. So it's opposite where it was at the vernal equinox. So it's now 12 degrees around our imaginary clock face. It's back to a declination of zero for the autumn equinox. Then in honor about the 21st of December, the Sun is now south of the celestial equator. So now it has a negative value of the declination of 23 and a half degrees. And since we're three quarters around our 24 hour clock face, we're now at 18 hours for the winter solstice. And then, of course, back again the following year for the vernal equinox. So now that we understand obliquity in terms of the axial tilt and how this affects our perception of the Sun, let's understand how this translates in terms of increased temperatures in the summertime and lower temperatures in the wintertime. You notice that the axial tilt of the Earth always points in the same direction, regardless of where Earth happens to be in its orbit. This has the effect of allowing one hemisphere to be tilted toward the Sun during, say, summertime in the Northern Hemisphere. And in wintertime in the Northern Hemisphere, our hemisphere is tilted away from the Sun. This is not to say that the Northern Hemisphere is closer to the Sun in the summertime or farther from the Sun in the wintertime. Rather, we're just simply saying that the hemisphere is tilted toward or away from the Sun as Earth goes around in its orbit. So let's see what this means in terms of temperatures. Well, first of all, we know that the days are longer in the summertime. We can imagine ourselves here in Baltimore, we're watching the Sun rise from some position in the northeastern part of the sky, and it's going to rise at about 4.40 in the morning, on average, on the day of the June solstice, at least for our latitude here in Baltimore. It's going to reach a maximum altitude as it transits the meridian, and then it's going to set some time in the early evening, around 7.36. And it's going to be somewhere in the northwest. So the Sun has to spend a fair amount of time to cross from its sunrise position all the way over to its sunset position. This gives us about 15 hours of daylight around the June solstice in Baltimore. So we have a longer path for the Sun to travel, and therefore it takes more time. That means the Sun is up in the sky longer, and therefore it's going to be able to cook us a little bit longer. So the ground absorbs more heat because the Sun is up for more time. But that sunlight is also more concentrated. So if we just imagine, for example, a single column of the sunlight reaching down to the surface of the Earth, you notice that it's fairly well concentrated. It's almost a perfect cylinder. It's tilted just a little bit off. The Sun never reaches the zenith during the summertime at our latitude here. But we're still getting a fairly direct concentration of sunlight. So this means that there's more energy per square meter or kilometer or whatever square area you choose. But there's more energy to warm the ground much more efficiently. By contrast, in the wintertime when the northern hemisphere is tilted away from the Sun, well, now we have a Sun that's going to rise later in the morning and it's going to set earlier earlier in the afternoon. And the reason for this is because the Sun is now rising from the southeast and setting in the southwest. So it has a much shorter path to travel through the sky. Therefore, we're only getting about nine and a half hours of daylight during the winter solstice. And therefore, we're getting less time for the Sun to warm us. But likewise, the same column of light that we saw in summer is now spread out over a much wider area because the Earth is or rather the northern hemisphere is tilted away from the sunlight. So there's less energy to heat the ground with and it's not very efficient. But notice that even while we're experiencing winter in the northern hemisphere, the southern hemisphere is now receiving more concentrated sunlight. The southern hemisphere is tilted toward the Sun. So our winters are the southern hemisphere's summers. So this tilt of the Earth allows us to come up with some very interesting alignments during the solstices and the equinoxes. So, for example, during the June solstice, we can imagine a single column of sunlight striking at a latitude of 23 and a half degrees north. This makes the southern or rather this makes the midday summer sun arrive at the zenith. So if you happen to be at this particular latitude, you're going to get a midday sun directly overhead. And it's for this reason that we call this the Tropic of Cancer. So the midday sun is always overhead on the June solstice. However, at 66 and a half degrees northern latitude, somewhere above what's called the Arctic Circle, the sun never really sets below the horizon. It gets, you know, reasonably high for this latitude. But even at its lowest altitude above the horizon, the sun never quite fully sets. We just get like a momentary dusk before the day begins again. So we're getting 24 hours of daylight at the Arctic Circle and all points north. Meanwhile, at the North Pole itself, we're getting something like this, the midnight sun. And this is true of any latitude north of the Arctic Circle in summertime. The sun will get low on the horizon, but will never fully set. OK, so now at the North Pole, the sun has reached its highest altitude above the horizon. It will be at all year long. In fact, the sun has been above the horizon every day since the date of the vernal equinox. It's just that this is as high as the sun gets. Notice that at its highest, the sun is still relatively low in the sky. And this is why, even though it is up for 24 hours a day at this time, the North Pole is still cold because that angle that the sun is reaching the surface at is still very shallow. Likewise, at all latitudes south of the Antarctic Circle, 66 and a half degrees, there in 24 hours of darkness, the sun never really rises above the horizon. There's a momentary dawn like event, but it's soon replaced with darkness once again. So 24 hours of darkness all the way down to the South Pole. And here, the sun has reached its lowest altitude below the horizon. So the good news is that from this day on, the sun will start to get a little bit closer to the horizon. But the South Pole is still going to remain in darkness until the autumn equinox. So that is the June solstice. It's the longest day in the Northern Hemisphere, and therefore the shortest day in the Southern Hemisphere. But six months later, by December, the situation is now completely reversed. Notice that the Arctic Circle and North are now experiencing 24 hours of darkness. The Arctic Circle might experience just a brief dawn event, but it doesn't matter. It'll soon be dark once again. Likewise, at the North Pole, the sun has set as far below the horizon as it's going to be. In fact, it's been below the horizon every day since the autumnal equinox. However, at the Antarctic Circle, the sun is still going to remain just above the horizon all through a 24 hour period. And indeed, at the South Pole, the sun is experiencing what the North Pole experienced during its summer solstice, only now it's reversed. So now the South Pole is getting the sun at its highest altitude above the horizon. And it has been experiencing 24 hours of daylight every day since the autumn equinox. Now, that means that if you are at 23 and a half degree south latitude, where we call the Tropic of Capricorn, then the midday sun is directly overhead. So you're experiencing your peak summer day here in the December solstice. So we have the southernmost path of the sun at all latitudes and the longest day in the southern hemisphere. That means that the equinox, which literally translates into equal night, means that we're getting approximately 12 hours of day and night in both hemispheres. The midday sun will be directly overhead if you are at the equator. But you notice I said it's approximately 12 hours of day and night. It's not exactly. And there's a reason for that, which we'll get to in a moment. And at the poles, well, it depends on whether or not it's the vernal or the autumnal equinox, but you're going to get either 24 hours of dawn at the vernal equinox at the north pole, or you'll get 24 hours of dusk at the at the autumnal equinox at the north pole and vice versa for the south pole. So once again, just remember that whatever season we are experiencing in the northern hemisphere, the southern hemisphere is necessarily experiencing the reverse season and the amount of seasonal variation will vary as a result of latitude. So, for example, let's take a look at the tropics. We'll just use the equator since it's literally right in the middle of the tropics. At all days of the year, the sun is up above the horizon for exactly 12 hours. However, the difference in the sun's location is negligible. It's always going to be high in the sky at some point during the day. It'll be highest on the equinox, but it never really sets very low even on the solstices. So the tropics really don't experience much in the way of seasonal temperature variations. It's at the pole, however, it's going to be the greatest seasonal variation. The temperatures are always going to be low for sure, but you notice the dramatic change from the June solstice, where you get peak daylight for 24 hours, all the way to the December solstice, where you get, well, peak darkness I guess for 24 hours. That means that at mid-latitudes, such as here in Baltimore, we are experiencing middle-grade seasonal variations. The sun does get high in the sky during the June solstice, never quite to the zenith, however, but still it gets pretty high in the sky and we get correspondingly warm temperatures. During the equinox, we get kind of comparable medium temperatures, ideally, obviously we're not taking things like weather into account, but we're going to get about 12 hours of daylight. And then by December, the sun has reached a very southern path. And so here's the same view just seen from a southwestern direction. So the idea that the sun reaches its lowest point, or rather its southernmost declination on the day of the December solstice, means that the sun will soon be getting higher and higher in the sky each and every day until the June solstice. And this idea was not lost on ancient cultures. In fact, their lives depended on understanding when the sun would return, so to speak, because this meant that with it came the warmer weather and then they could plant and they could harvest and they could potentially survive another brutal cold winter. So the ancient people of what is now modern-day Ireland built this. This is what is now called New Grange. It is at 54 degrees northern latitude. Even though temperatures in Ireland perhaps aren't quite as brutal as they can be in other parts of the world, they were still hard enough. People would huddle together during the winter time, sharing food, just literally trying to survive. And so they built this as a way of commemorating when the sun would return. And there are many such examples like this all over Ireland, but this one is certainly the largest. So the entrance is normally closed by the closing stone. You can see it's actually bolted to the right of the doorway and then we have a box up top, that's the light box. And New Grange is oriented such that it's facing the southeast. In fact, it's facing a very particular azimuth of the southeast because it is on this date at this particular azimuth that the rising sun will appear on the day of the December solstice. And so it passes through the light box and it illuminates the interior chamber, creating this eerie shaft of light that only really shows up essentially once a year. So this was clearly a way for ancient cultures to celebrate when the light would return. So the main thing is that the axial tilt is the reason for the season and it results into phenomena. So in summertime, we have longer days and therefore more time for the sun to warm the surface. By contrast in winter, we have shorter days. Therefore, the sun is up for less time to warm the surface. Also in the summertime, the sunlight is much more concentrated which means that there's more energy to heat the ground efficiently whereas in winter it's less concentrated and so there's less energy to heat this ground efficiently. And that's for Earth, but do other planets have seasons? Well, it turns out some do and some really don't and it's all because we have a wide variety of obliquities or axial tilts in the solar system. So for example, Mars, Saturn and Neptune have obliquities that are relatively similar to Earth's. There are going to be some slight variations in seasonal temperatures not the least of which is because they are all orbiting the sun farther away than we are here on Earth. But the obliquities mean that the variations from season to season are on the same level or the same degree you might say that we experience here on Earth. By comparison, Mercury, Venus and Jupiter are essentially not tilted with respect to the ecliptic, with respect to their orbital paths around the sun. So they won't experience very much in the way of seasonal variations. So when we look at Jupiter, for example, we see essentially a three-degree obliquity there's essentially little axial tilt to speak of so there's not much seasonal variation from a Jovian summer to a Jovian winter. By contrast, Uranus has the most extreme seasonal variations. It has an 84-year orbit around the sun. That means that if you're at the poles a season or actually a day can last 42 years. So if you imagine you're standing at the North Pole of Uranus at the front of the picture, so to speak. Well, you're seeing the sun just rising and for the next 21 years, the sun will get higher and higher overhead until you reach the left-hand side of the picture and now the sun is at its highest point in the sky and then for the next 21 years, the sun will gradually set as you descend into Uranian autumn in the North Pole and vice versa for the South Poles. So axial tilt really is the reason for the season but there is one important complication. Earlier I said that we get approximately 12 hours of sunshine during the equinoxes but that assumes that we're not worrying about the effects of our atmosphere. So if you imagine we're standing here on the surface of the earth, it is pre-dawn on the day of, let's say, the equinox and that means that we should not be able to see the sun from here. The sun's light is passing overhead, you might say. So the sun is over the horizon. In reality, though, we have an atmosphere and this atmosphere means that the sunlight gets bent. So the atmosphere almost acts kind of like a lens and changes the direction of the sunlight. That means that when we look at this image of the sun, we are seeing what appears to be a sun that is already risen above the horizon even though it has not. So this means that we get a few minutes of sunrise a little bit earlier than the actual sunrise. So we get a little bit of extra sunlight, a few minutes extra sunlight. Likewise, after the sun has set, the same atmospheric refraction keeps the sun to appearing above the horizon so we are getting a few extra minutes in the evening as well. So the atmosphere does play a role in exactly when the light shows up and for how long it's going to be. So even though we experienced the winter solstice on December 21st, it takes about a month and a half for most of that heat to escape the northern hemisphere so we get into the coldest days in February. Then as we get to the summer solstice, it still takes about another month and a half or so until about late July, early August to really experience the warmest temperatures. It just takes the ocean and the atmosphere that much time to heat up. Still, none of this would be true if we didn't have an axial tilt so the axial tilt is always the reason for this season.