# Why does perihelion and aphelion move around so irregularly? **URL:** \ **Category:** Factual Questions\ **Created:** [July 6, 2018, 6:34am UTC](https://boards.straightdope.com/t/why-does-perihelion-and-aphelion-move-around-so-irregularly/817367 "2018-07-06T06:34:44Z")\ **Posts on this page:** 6\ **Page:** 2
**Author:** ![watchwolf49](https://avatars.discourse-cdn.com/v4/letter/w/e9c0ed/32.png) [@watchwolf49](https://boards.straightdope.com/u/watchwolf49)\ **Post date:** [July 7, 2018, 3:13pm UTC](https://boards.straightdope.com/t/why-does-perihelion-and-aphelion-move-around-so-irregularly/817367/21 "2018-07-07T15:13:03Z")
> [@DPRK](#): > > Not everything is periodic; there are secular effects as well. > > I think a reasonable answer to the OP is: 1. The date of apsis increases by about 1.7 days per century. 2. In the short term, the apsides of the Earth-Moon barycentre are pretty regular. 3. The remaining variation principally depends on the phase of the Moon at the time the barycentre reaches perihelion/aphelion; e.g. a first quarter will make the perihelion happen at a later date, and a last quarter will make perihelion happen at an earlier date. I’m curious as to what these secular effects are … the three effects you’ve listed are eminently periodic … The center-of-gravity of the Earth is irregular: volcanoes, earthquakes and the general fluid motion within the Earth … these would have some effect on the time of the apsides but only when we start looking at millions of years … ---
**Author:** ![watchwolf49](https://avatars.discourse-cdn.com/v4/letter/w/e9c0ed/32.png) [@watchwolf49](https://boards.straightdope.com/u/watchwolf49)\ **Post date:** [July 7, 2018, 5:04pm UTC](https://boards.straightdope.com/t/why-does-perihelion-and-aphelion-move-around-so-irregularly/817367/22 "2018-07-07T17:04:16Z")
> [@rat\_avatar](#): > > If you aren’t scared away by tensors … [snip] The key to dealing with tensors is to spray them with Round-up every two weeks during the growing season, this will kill about 90% of them so repeating the following year just about gets rid of them all … Also important is to express your systems of partial differential equations in Cartesian coordinates using unit vectors … this prevents tensors from taking root in the first place … ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [July 7, 2018, 5:05pm UTC](https://boards.straightdope.com/t/why-does-perihelion-and-aphelion-move-around-so-irregularly/817367/23 "2018-07-07T17:05:19Z")
> [@rat\_avatar](#): > > I think it is still an open question, but aren’t most of the changes generally attributed the secular acceleration of the Moon and secular slowing down of the Earth’s rotation to tidal friction? I believe that’s right. We’re talking about the phases of the Moon, and the main contribution to the secular acceleration of the Moon’s longitude are the tides which are accelerating the Moon and slowing down the rotation of the Earth. Lesser terms result from the shapes of the Earth and Moon and from perturbations due to other planets. [/QUOTE] > [@](#): > > The Milankovitch cycles mentioned earlier and the precession of the equinoxes seems to be mostly related to the precession of the Earth’s axis over time. Yes, the Earth experiences precession of the equinoxes, and in addition the perihelion of the Earth’s orbit advances (apsidal precession). These effects add together and account for the date of perihelion advancing by 1.7 days per century. > [@](#): > > As VSOP87C has about a thousand terms in each series, so I am sure I am missing something. If you can help feed my most likely unhealthy interest in ephemerides I would appropriate any tips on where to look. Obviously this isn’t an analytical solution and to avoid wasting others time I am not claiming this is a solved problem. I am just curious about secular effects I have missed. The relevant lunar theory is called [ELP 2000](https://en.wikipedia.org/wiki/Ephemeride_Lunaire_Parisienne). Of course there are tens of thousands of terms, but at least you can go in there and explicitly see the polynomials expressing secular effects and see how much is due to the tides, how much to other planets, etc. > [@watchwolf49](#): > > I’m curious as to what these secular effects are … the three effects you’ve listed are eminently periodic … I meant secular effects on the phase of the Moon. It’s well-known it is not completely periodic, the biggest effect on its ecliptic longitude being the tides. They are actually accelerating the moon. In principle the Earth could become tidally locked to the Moon, but supposedly the Sun will have exploded before then so I wouldn’t prematurely take it into account. ---
**Author:** ![Anne\_Neville](https://avatars.discourse-cdn.com/v4/letter/a/b9e5f3/32.png) [@Anne\_Neville](https://boards.straightdope.com/u/Anne_Neville)\ **Post date:** [July 9, 2018, 12:42pm UTC](https://boards.straightdope.com/t/why-does-perihelion-and-aphelion-move-around-so-irregularly/817367/24 "2018-07-09T12:42:46Z")
I suspect this also has something to do with our calendar not being a perfect match for the length of the year. A year has 365.2422 days. That’s inconvenient. We deal with it by adding an extra day every four years, but that’s not perfect. ---
**Author:** ![watchwolf49](https://avatars.discourse-cdn.com/v4/letter/w/e9c0ed/32.png) [@watchwolf49](https://boards.straightdope.com/u/watchwolf49)\ **Post date:** [July 9, 2018, 2:35pm UTC](https://boards.straightdope.com/t/why-does-perihelion-and-aphelion-move-around-so-irregularly/817367/25 "2018-07-09T14:35:16Z")
Ha ha ha ha … From the OP’s citation: > [@](#): > > The table presented below gives the date and time **(Greenwich Mean Time)** of all perihelions and aphelions of Earth for the 21st Century (2001-2100). **If Daylight Saving Time is in effect, add one hour to the times listed.** For conversion to other time zones, see Time Zones and Universal Time. Emphasis mine What’s wrong with this sentence? ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [July 9, 2018, 7:48pm UTC](https://boards.straightdope.com/t/why-does-perihelion-and-aphelion-move-around-so-irregularly/817367/26 "2018-07-09T19:48:05Z")
> [@Anne\_Neville](#): > > I suspect this also has something to do with our calendar not being a perfect match for the length of the year. A year has 365.2422 days. That’s inconvenient. We deal with it by adding an extra day every four years, but that’s not perfect. It’s more than just that. The soltices and equinoxen change because of that, but they move in a very regular pattern. In non-leap years, they move almost 6 hours forward and then jump back almost 18 hours in leap years. There’s no pattern like that for the perihelion and aphelion. [Previous page](https://boards.straightdope.com/t/why-does-perihelion-and-aphelion-move-around-so-irregularly/817367.md?page=1)