Jason Kendall's Introductory Astronomy Course

Adjunct faculty in Astronomy at CUNY Hunter (2015-2018) and William Paterson University (2011-2020)
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What Causes the Seasons?

In this video, I discuss the motions of the Earth, and how the Seasons come about. I also discuss Sidereal versus Solar days, the Ecliptic, and why there are thirteen constellations in the Zodiac.

Lecture Notes for This Video

Module 1: Foundations of Observational Astronomy: The Moon, the Seasons, and Mapping the Sky

  1. Navigating the Night Sky
  2. Angular Measurements and the Celestial Sphere
  3. The Celestial Sphere
  4. What Causes the Seasons?
  5. Lunar Phases and Months
  6. Lunar Eclipses
  7. What is a Solar Eclipse?
  8. Watching the Total Solar Eclipse of August 21, 2017
  9. Cosmic Distances using Parallax
  10. How do we know that the Earth is Round?
  11. How Big are the Sun and Moon?
  12. Geocentrism is False

Video Transcript

00:00:00:00 - 00:00:26:11
All right. Just just looking at the nature of what the sunrise and sunsets are like and the sun's motion across the sky, we know that the sun rises in the east and sets in the west just like the stars. But over time, the sun drifts slowly eastward relative to the stars. And what we're watching here is the diurnal motion of the star of the sun rising in the east and setting the west to various speed up over the course of the day.

00:00:26:13 - 00:00:49:29
However, the over the eastward drift relative to the stars takes about 365 to 5.25 days to return to the same location in the sky. That's one year. And therefore a leap day occurs every year, every four years, because that extra point to five days accumulates into one day. So we calendar ize that by adding a leap year once every four years.

00:00:50:01 - 00:01:12:20
But the sun itself just slowly drifts over time. And so we have to see why it does that with respect to the background stars. So let's take a look at the Earth has an orbital motion that makes it look a little different. The rate at which the earth goes around, the sun is different than the rate at which the Earth turns on its axis.

00:01:12:23 - 00:01:31:22
That creates a difference in the position of where the sun appears to be in the sky. The solar day is what we call noon to noon. That's what we call the day. Everybody says what time of day it is, and they say it's noon or 1 p.m.. That's time. According to the Sun. And that is the expected diurnal motion.

00:01:31:24 - 00:01:54:00
Well, the sidereal day, though, is the day according to the positions of the distant stars we call the normal day or the solar day, the wave this way. But if we look at the sidereal day, it's shorter. It is shorter by about 4 minutes. That means that the Earth has to turn just a little bit more to get the sun back in position for noon to noon.

00:01:54:00 - 00:02:32:03
Right. Otherwise, if we were telling this time by the stars we could do that. We could have said the day is defined by when, say, Orion is directly overhead or the stars directly overhead at midnight or something when it is due south or due east. If there was some particularly bright star that we could see all the time, maybe like Beetlejuice goes Supernova or something and we could see it in the daytime, it's possible that we could tell the time by Beetlejuice then if it lasted for centuries and centuries and most of human civilization or we had another distant star that we saw.

00:02:32:05 - 00:02:52:25
But anyway, we tell the sun time of day by the sun, but the sidereal day is different. We have three diagrams A, B and C, and we have a position on the Earth, Capital A and Capital A looks at the sun in in in the first image when the sun is at noon. So then what happens is as the earth goes around its orbit a little bit, right.

00:02:52:26 - 00:03:17:02
So we change from that that bottom dashed line to the dash ladies, that it's moved up a little bit. And if we go all the way around but have a pointed in the same direction, meaning looking at the same stars in the sky, stars are much more distant than the sun, so therefore they appear to be fixed to it on the sky as opposed to the sun, which appears to move with respect to these background stars.

00:03:17:05 - 00:03:38:22
So the stars themselves appear to come back to the same point in the sky in 23 hours and about 56 minutes. Now, then you got to move around a little bit further on the circle, just about a degree, which is 0.00.986 degrees, which is about 4 minutes of time to get to the same, to get to noon on the solar day in the bottom one.

00:03:38:25 - 00:04:05:24
So this is the differences between those two things. And as a result of this sight, this earth's motion around the sun and the difference between the sidereal and the solar day, the sun appears to move around the sky and it does so on a path. And that path is called the ecliptic. And that is the path the sun apparently takes in the sky because of the difference in time between the solar day and the sidereal day.

00:04:05:27 - 00:04:36:24
And because that happens, because the earth is going around the sun. The sun traces a path that looks like that curves the red. The the red dash thing is the celestial equator, which is the projection of the Earth's rotation, the earth's equator out into space. And the reason that the ecliptic, which is the path the sun takes that around the sky is different than the celestial equator is because the Earth's rotation axis is tilted with respect to its orbital axis.

00:04:36:29 - 00:04:55:19
But more on that later. The zodiac are the constellations through which the sun passes on its path around the ecliptic. So we have the celestial equator, which is the projection out into space of the Earth's equator. We have the ecliptic, which is the path that the sun takes in the sky and the zodiac of the constellation is through which it passes.

00:04:55:21 - 00:05:20:05
So let's look at those particular The zodiacal constellations in October sits in Virgo, in November, it's in Libra, December, it's in Scorpius. Most of December, though, it's in over you guess different one Sagittarius in January, Capricornus in February, Aquarius in March, Pisces in April, Aries and May. Taurus in June. Gemini in July. Cancer roughly in August and Leo roughly in September.

00:05:20:09 - 00:05:39:28
These are rough things, but you'll notice they all sound very familiar. Those are the astrological zodiacal constellations, and you're very familiar with them from, from people spouting about them. But this is what this means. That's all that it means. So let's take a closer look. We can look at these zodiacal constellations, and now we're taking a much tighter look.

00:05:40:01 - 00:06:06:03
Now we're isolating the sun. In this video at noon, all through out a number of years. And we see the progression of the sun as we're going really fast. We're doing many days per month. And there we go. We see I put the artwork up, I put the grid, the boundaries of the constellations up, and the lines that commonly shown asterisms as well as the names.

00:06:06:06 - 00:06:28:11
Now, of course, we don't see these artwork in the sky, the red boundaries in the sky, nor do we see the names or any of those. The lines connecting them. But the ecliptic is the path that the sun is going on. Again, given the given time of year, given date shows where it is in the sky, it's kind of weird that that line seems to be moving, but it's not.

00:06:28:11 - 00:06:52:07
It's the sun that's moving along the line and we're looking at noon or 1 p.m. or noon, depending on what the daylight savings hour is, where the sun is looking. We're looking faint due south. Every solar day. So this is where isolating it's solar day by solar day. If we were isolated by sidereal day, then the stars would stay fixed and then the sun would appear to move.

00:06:52:14 - 00:07:21:20
As you can think about it, just imagine that we're fixing the stars in the background and the sun would then definitely seem to move with respect to those background stars and background constellations. Notice over that we've looked at a number of years. It's now on the simulation up to 2030 and we're maintaining the same constellations. The sun has maintained the ecliptic to a greater or lesser extent for thousands of years now.

00:07:21:22 - 00:07:46:03
The Earth's precession on its axis will change. Things will change the position of the ecliptic, or more specifically, it'll change where the ecliptic meets the celestial equator. When we get to that point, that is the autumnal equinox. When we get to that point, that's the winter solstice. This is the vernal equinox or spring equinox. And up here is the summer solstice.

00:07:46:05 - 00:08:23:05
And that's when the sun is highest in the sky. At noon, that's when the sun is up. Only 12 hours. And this is when the sun is up for the shortest amount of time during the year. And there it is. And that was used. Video was used using stellarium. So let's go back again and look at it. Instead of moving through the sky, we can see as the earth moves around the sun, we look in the direction of the sun and it appears to be in these constellations and the tilt of the Earth's axis of rotation with respect to that orbital plane means that the band, the blue band that goes all the way around

00:08:23:05 - 00:08:58:21
is not the same as the as the orbital axis, which is the dashed line. That thing is tilted. So this kind of diagram is a little bit goofy and still we can then look at it from the other perspective to say, Well, what's in the night sky at midnight during these times? It's like, okay. So in September, Pisces is up at midnight, in December, Geminis up at midnight or at least 100%, 180 degrees away from the sun in the sky in March, Virgo is 100%, 180 degrees.

00:08:58:22 - 00:09:22:29
Explain saying how 180 degrees away is from the sky, from the sun. And in June Sagittarius is. And this gives rise to what we see as the summer constellations in the winter constellation. Gemini is a winter constellation. Sagittarius is a summer constellation, Pisces is a full constellation, Virgo is a spring constellation. And this shows the zodiacal positions in the constellations at midnight.

00:09:23:02 - 00:09:44:07
Again, we're not taking into account the difference between the Earth's orbit and the and Earth's orbit. And it's the axial tilt we don't really take into account with this particular thing. But we do in this diagram. See, now, now we're seeing in this diagram where the positions of the winter and summer solstices are where the autumnal and vernal equinox is.

00:09:44:07 - 00:10:20:06
Ah, the vernal equinox occurs when the sun is traveling on the ecliptic and it crosses from the south of the solstice equator to north of the celestial equator. It's also where we define the right ascension and declination of the right of the equatorial coordinate system. If we look at it from a purely a geocentric standpoint, which we shouldn't try to guess, it's going to helpful in this regard because now we can look at it will be apparently see and we see that in the sky straight up from the Earth's axis rotation is the North Celestial Pole.

00:10:20:08 - 00:10:43:28
The celestial equator is the extension of the Earth's equator out into space, and the sun appears to make a path in the sky that is tilted 23 and a half degrees. But really, that tilt is actually the or Earth's orbit around the sun. And the axis is tilted with respect to 23 and a half degrees. So this is the geocentric view of things.

00:10:44:00 - 00:11:09:13
But really we should have a heliocentric view, which we've shown. But this is this helps us understand what the ecliptic is and why it seems to go up and down based on how we appear to see it. Once again, Geo centrism is dead wrong. Let's look again for the reason for the seasons. Frequently when we look at the seasons, we say, it's warmer in the summer and cooler in the winter for the Northern Hemisphere.

00:11:09:13 - 00:11:35:22
It's warmer in the summer for everybody. It's whatever summer it is, it's warmer in your summer and cooler in winter. Why is that? So what is the reason for the seasons? Reason for the season is the tilt of the Earth's axis with respect to its orbital plane, The altitude of an object off the horizon is the height of that celestial object measured at an angle above the horizon.

00:11:35:25 - 00:11:56:29
And the sun has a higher altitude in the summer than in than it does in the winter in the northern hemisphere. And the solstice are the points in the celestial sphere with the sun reaches its northernmost and southernmost points. And in the northernmost point is around June 21, and the southern hemisphere becomes the southernmost point in the winter solstice.

00:11:56:29 - 00:12:23:04
As about December 21. The equinoxes are the play plate. Other places in the celestial sphere, the sun crosses the celestial equator. That's about March 21st for the spring and the autumnal equinox is about September 22nd. So let's see what the effect is on that. When we look at the sun on the solstice is well, actually on the summer solstice, specifically, the sun appears to be very high in the sky, casts very short shadows.

00:12:23:04 - 00:12:48:12
The earth is warm, the sun is warm, and the it rises very far north of east and it sets north of west. And when it's at noon, it is high in the sky that's belied by those kind of simulations they show which will look at the sun. But if you think about it, the sun in the previous simulations looked high in the sky at noon when it was in the summertime.

00:12:48:14 - 00:13:12:01
Well, we didn't take into effect into account the fact that the sun rose and said we were just jumping forward 24 hours at a time in that simulation. But if we watch the sun, we would see it would rise very far north of east, get to that high point and set very far well north of west. And so notice that the length of the shadows is short and the days then, therefore, because the sun is in the sky longer would be longer.

00:13:12:04 - 00:13:34:03
That's important to note. But the soot but the short shadows are a key. Next, we can look at the winter solstice. Obviously, since the sun is rises south of the east and set south of west and it gets lower in the sky. The shadows are in general longer and the sun is up for shorter periods of time. It's a very familiar thing.

00:13:34:03 - 00:13:57:13
You know, the winter days as they approach, the days get shorter and the sun rises later and it sets earlier. And that's what we see. That's our that's what we see here. So in the in the winter solstice, the sun does not is not up above the horizon as much. And at the equinoxes, the sun rises due east and sets due west.

00:13:57:18 - 00:14:23:23
And that's the effect of this tilt of the Earth's axis and the earth going around the sun. All right, so why do we care about shadows? This is why. Because the angle of incidence of the source of light on the earth distributes the light differently. If something is beaming straight down on a surface, the light doesn't spread out that much.

00:14:23:25 - 00:14:50:26
And in fact, the beam at the spot is smaller. But if there's a larger angle than the beam of light, the all the like, it's spread out onto a larger surface area. So when you have the sun straight up, the beams of light do not spread out over larger surface areas and therefore you have more intense heating per square inch on the surface of the earth.

00:14:50:28 - 00:15:19:17
That's what the left hand side flashlight ideas representing. But in the winter solstice, when the sun is low to the horizon at noon, the sun and the sun's light is spread out over a larger surface area per square inch. And therefore it is less intense per square inch and therefore there's less in the intensity of light is less, which means the energy that is received by the ground is less per square inch.

00:15:19:19 - 00:15:43:05
That means there is not more energy and it doesn't get as hot. That's the reason for the seasons. It is the angle of incidence of the light on the earth, distributing the light less and less intensely at winter than it does in the summer. Let's look at that effect on the next slide here. We see it quite explicitly when we look at the left hand side.

00:15:43:08 - 00:16:14:16
The effect of that flashlight thing on the left is that the smaller ground area is covered by the light from the sun for the same amount of rays of beams. Let's see if three beams they would had a smaller surface area, which means they would be more intense and deliver all their light into a smaller surface area, which means that surface area would get more energy per square inch on the surface in the wintertime or late in the afternoon or even late in the day when the sun is setting.

00:16:14:16 - 00:16:37:23
It's actually less intense y because the beat, the light from the sun is being spread out per square inch over a larger surface area. And that's what we see on the right. So in the wintertime, it's all day like this and it's always like this at sunrise and sunset for every day of the year. But at noon, let's just look strictly speaking, at noon at noon in the winter, it's like the right hand side.

00:16:37:28 - 00:17:05:07
And at noon in the summer, it's like on the left hand side, the intensity of the light is what creates the heat, the heat capacity per square inch on the surface of the earth. And that's where the seasons come from. It is not due to the distance between the earth and the sun. In fact, we are closer in the northern hemisphere to the sun during the winter than we are at summer.

00:17:05:12 - 00:17:25:25
And that's just the position of the it's the nature of the ellipticity of the Earth's orbit, which is very small, very small. So if it were about the distance, then we would be hotter in the winter or it's not the angle of incidence of the sun's light on the surface of the earth is drives everything. That's what we have is the reason for the seasons.

00:17:25:29 - 00:17:44:11
They're caused by the relative tilt of the Earth's axis with respect to its orbital plane. And it's not due to the distance from the sun. All right. So we talked a little bit about the Zodiac. We talked a little bit about rising and setting of the sun. We talk about the sun's motion throughout the day. The sidereal day, the solar day, all of these things.

00:17:44:14 - 00:18:02:20
We'll talk to you next time. Hey, if you like this video and other videos like it in my series, please leave a thumbs up and subscribe to the channel. I'm going to be going through and redoing a lot of my old stuff and doing a lot of new things too, in this coming time. So please stay tuned. See you next time.