拱线进动 （近日点进动） 和米兰柯维假说

we've learned that axial precession it doesn't change 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 effect 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 then if our rotational axis is pointed in a different direction after a long enough time then the 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 it was very 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 farthest away from farthest tilted away from the Sun or furthest tilted towards the Sun so on when even though if we wait 1800 years like the example I gave we will be most tilted away from the Sun we will have our or the northern hemisphere will have its winter 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 where's base and it's not based on this but if our calendar was based on the exact point in orbit 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 dates actually would then the the date for the start of winter actually would go back it would you know 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 the 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 that we are at that at this exact position at this exact position relative to the Sun itself and that's why the actual Peary perihelion does change because if this is always December 22nd and if we at first assumed 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 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 if I draw the Sun again and right now our orbit may be right now our orbit looks something like this and I'm gonna exaggerate the eccentricity of it I am going to exaggerate the eccentricity of it just 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've drew it right over there we do it in different colors I don't want to show that that's necessarily where Earth is for helium and aphelion there is also a rotation of this of the of the perihelion and sometimes this is called the precession of the precession of the perihelion or perihelion precession or apps' it'll press peps it'll 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 our orbit will look like this the actual perihelion will have rotated so our orbit will look like this the actual the actual ellipse would have rotate a little bit you wait a little bit longer it will go it will look like 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 26 26,000 years so 26,000 years from today are our polar axis if we don't if we don't if we don't think about our rotational axis if we don't 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 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 an it's in January I actually don't know the exact date you could look that up but whatever that date 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 Hylian itself were not changing if it always stayed fixed over here if it was if it if we did not have this app Siddal precession but since it is also changing you can kind of say it is over thousands of years moving in that direction while our while our jet our date 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 21,000 years from now and then on top of that if that's not enough for you that the actual 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 so and these cycles occur so these eccentricity cycles occur on over approximately 100 100 thousand 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 Milankovitch cycle or 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 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 can mention 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 we don't necessarily know how it does it or whether it even really does affect going into or out of ice ages