Cosmology and astronomy
- Seasons aren't dictated by closeness to sun
- Season simulator
- How Earth's tilt causes seasons
- Are southern hemisphere seasons more severe?
- Milankovitch cycles precession and obliquity
- Precession causing perihelion to happen later
- What causes precession and other orbital changes
- Apsidal precession (perihelion precession) and Milankovitch cycles
Apsidal precession (perihelion precession) and Milankovitch cycles
Apsidal Precession (Perihelion Precession). Created by Sal Khan.
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- Talking about rotation, does anybody know if the earth's rotation is speeding up or slowing down? And it so what causes that?(12 votes)
- The Earth's orbit is slowing down due to the gravitational force of the moon acting on our planet(4 votes)
- to all who watched all the videos on precession, how many things are rotating.(6 votes)
- The planet around the poles, the planet around the sun, where the north pole points, where the perihelion occurs, and the eccentricity could be considered the absolute value of a sine function, so 5(12 votes)
- what is the difference btween an equinox and a solistice?(3 votes)
- an equinox is when a night and day is the same length of time. a solstice is the longest day and the longest night but may or may not be of different length.
: )(12 votes)
- Am I right to say that precession is a change in direction of the magnetic poles (north and south)?(3 votes)
- Technically, precession is the change in position of something.
What you are referring to is a geomagnetic reversal:
Precession is just the axis moving about the vertical line that is perpendicular to the Earth's orbital plane.(6 votes)
- Is this why we have leap years then?(4 votes)
- No, we have leap years because the orbit of the earth is not exactly 365 days it is about 365.242 days.(5 votes)
- why does the parahelium change?(3 votes)
- The precession of the perihelion* and aphelion in the ecliptic plane is caused for the most part by earth's gravitational interactions with Jupiter and Saturn. It is possible that the effects of general relativity have a small impact as well.(1 vote)
- Can I say the Sun's orbit around the black hole in the center of the Milky Way does affect the Earth's rotations and orbit in someway?(2 votes)
- Not really, the black hole is simply to far away to affect the Sun or earth at all.(3 votes)
- This is sort of unrelated, but whenever I see depictions of our Solar System, the planets are all lined up in the same direction relative to the Sun. ( you know, like this: (earth) (mars) (jupiter) ) Is that really the case? Is it even possible for planets to be on different lines relative to the center of their orbit?
Like this: - - - -(mars) - - - - - - - - -(jupiter)
See? Different lines relative to the Sun. Was that a helpful example?(2 votes)
- In answer to your first question, you might want to look up the word "syzygy". Really. It's a fun word which means an alignment of three or more celestial objects. And, yes, it happens fairly frequently for two planets and the Sun. For three or more planets in syzygy with the Sun, the occurrence happens less and less frequently as the number of planets gets larger, but it still does happen.
For your second question, as I understand it, let's start with the Earth's orbit around the Sun. The plane which is defined from that orbit is called the "plane of the ecliptic", because that is the plane in which eclipses happen. All the planets orbit the Sun in different planes, although they are all near to the plane of the ecliptic. Their planes are inclined to the ecliptic, so the measure of their inclination is called that planet's "inclination".
BTW, your illustration of Earth being below the plane of Mars and Jupiter was excellent, I understood exactly what you were getting at. Good job.(3 votes)
- Why do all these cycles happen?(2 votes)
- Because nothing we can see moves in perfect circles(2 votes)
- Hi, at4:30you said that because the perihelion itself is changing, so it takes only 21000 instead of 26000yrs for earth's rotational axis to trace out a circle, then i will assume that the Axial Precession is calculated using the Sun as a frame of reference like you said. But I also read that; a result of the Axial precession is that the North Star changes over time, 12,000 years ago the star Vega was the pole star, and because of the 26000yr cycle Vega will be the Pole Star again in 14000. In this case then it seems like the calculation of Axial precession is based on a specific point in space relative to the stars but not using the Sun as our reference. Can you please explain it to me?
- The Speaker did not say 21,000 "instead of" 26,000. Two separate figures were given: a) 26,000yr "axial" precession cycle, and b) 21,000yr "perihelion" precession cycle.(2 votes)
We've learned that axial precession, it's not a change in the tilt or the obliquity of our rotational axis, it's a change in the direction. And over a long period of time, 26,000 years, it kind of traces out a circle. And the main affect of that is that if we wait long enough that our rotational axis, or you could say almost the North Pole, will be pointed in a different direction. And so if our rotational axis is pointed in a different direction after a long enough time, then the absolute point in our orbit, if we use the sun as our frame of reference, the point in our orbit when we are most pointed away from the sun, or when the Northern Hemisphere is most pointed away from the sun, will be earlier in the orbit. Now I emphasize that that won't necessarily mean earlier in our calendar, because our calendar, by definition, takes into consideration, I guess, or it's more based on when we are furthest tilted away from the sun or furthest tilted towards the sun. So even though if we wait 1800 years, like the example I gave, we will be most tilted away from the sun. The Northern Hemisphere will have its winter equinox at an earlier point in the orbit. According to our calendar, it will still be December 22nd. If our calendar instead was based-- and it's not based on this-- but if our calendar was based on the exact point in orbit, then our year would be about 20, 25 minutes longer every year and then the date for the start of winter actually would go back. 1800 years later, the date of the start of winter would be November 22nd. But that's not how we measure our calendar. Our calendar is actually measured from equinox to equinox. From December 22nd or 21st, there's slight fluctuations depending on the calendar, but that will always be the date that we are most pointed away from the sun. That will not be necessarily the date at this exact position relative to the sun itself. And that's why the actual perihelion does change. Because if this is always December 22nd, and if we, at first, assume that the perihelion is always at the same fixed point in space relative to the sun, although that's not exactly the case, but if we make that assumption, then it will be further and further after that December 22nd, further and further after that time, that we are most pointed away from the sun. And that's why you have this kind of pushing back of the perihelion. Now, what I want to add to this video is that the perihelion itself is also changing. So if I draw the sun again, and right now our orbit looks something like this, and I'm going to exaggerate the eccentricity of it so that the perihelion and the aphelion are a little bit clearer. So right now this is the perihelion. This is the aphelion based on the way I drew it right over there in different colors. I don't want to show that's necessarily where Earth is. Perihelion and aphelion, there is also a rotation of this, of the perihelion. And sometimes this is called the precession of the perihelion, or perihelion precession, or apsidal precession. These are all very hard to say. And so if we wait several thousands of years our orbit might look a little bit like this. The actual perihelion will have rotated. So our orbit will look like this. The actual ellipse would have rotated a little bit. You wait a little bit longer, it might look like this. And obviously, I'm once again talking about over thousands and thousands of years. From a year-to-year basis, you really wouldn't notice the difference. But what that does is, is we talked about the axial precession, that this change in direction of our rotational axis, it takes 26,000 years to complete one period. So 26,000 years from today, our polar axis, if we don't think about our rotational axis, if we aren't too concerned about the actual change in tilt, which there will be some small change in tilt, but 26,000 years from now, our pole will roughly point in the same direction again. We would have completed one whole period of axial precession. However, it does not take 26,000 years for whatever our date of perihelion is today. So it's in January. I actually don't know the exact date. You can look that up. But whatever that data is in January, it will not take 26,000 years for it to be that date again. And it would have taken 26,000 years if the perihelion itself were not changing, if it always stayed fixed over here, if we did not have this apsidal precession. But since it is also changing, you could kind of say it is over 1,000 years moving in that direction while our January date is moving in that direction, they will actually meet sooner, so that the precession will be back on whatever date it is on January, less than 26,000 years from now. And actually the exact time, and I haven't done the calculation, but this is what I've read, is that it will be 21,000 years from now. And then on top of that, if that's enough for you that not only is the direction of Earth's rotational axis changing and the tilt is changing and that the perihelion and the aphelion are also rotating around, it's also the case that the eccentricity of the orbit itself is changing. So over long periods of time, Earth's orbit becomes more or less eccentric. And we've learned that almost circular has-- well if you're circular you have no eccentricity and then you can become more and more eccentric, which means you're more and more of kind of this flattened out ellipse. And these cycles occur, these eccentricities cycles occur over approximately 100,000 years. And so to revisit the Milankovitch cycles-- and once again this is a theory. We're not sure whether this is necessarily causing our ice ages or whether this is necessarily a major influence over long-term climate change, but the theory of Milankovitch cycles is that over long periods of time, if the eccentricity changes enough and if it coincides with when the perihelion and the seasons also coincide, maybe that's enough to start an ice age. Or maybe that's enough to take us out of an ice age. And actually, if you want to throw something even more on that, the actual plane of our orbit also changes over time mainly because of interactions with the outer planets. Anyway, I'll leave you there. As you could imagine, this is a very complex topic, but hopefully you now have an appreciation of all the different ways our orbit can change and maybe start to think about how that might affect our weather. Although, we don't necessarily know how it does or whether it even really does affect going into or out of ice ages.