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Earth's orbit & precession affected by Venus & Jupiter?

  1. Nov 3, 2015 #1
    I watched a program that said that some of Earth's past climate change was due to variations in Earth's orbit and precession due to the the gravitational perturbations caused by Venus and Jupiter. Why would that be so?

    I know that the orbits of the planets are eliiptical generally and they these elliptical orbits (while remaining elliptical) can shift in their orientation (perhaps their plane?)

    So are these the causes of the changes to the orbits of Venus and Jupiter and is that how they affect Earth?

    If so, is the effect on the change of Earth's precession due to to the tugging by Jupiter and Venus on different portions of the Earth to cause precession and/or obliquity shifts of the Earth?

  2. jcsd
  3. Nov 4, 2015 #2

    D H

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    Jupiter and Venus make the Earth's orbit about the Sun not quite elliptical. One key effect is apsidal precession. It takes slightly longer (a bit less than five minutes longer) for the Earth in its orbit about the Sun to advance from perihelion to perihelion (the anomalistic year) than it takes the Earth to come to the same position with respect to the "fixed stars" (the sidereal year).

    Digging deeper, there are a number of other effects that need to be taken into account. I'll discuss three of them. One is axial precession. The Earth's rotation axis is inclined by about 23 degrees from its orbital axis. The Moon and Sun make this axis of rotation precess. This makes the tropical year (the basis of our seasons and our calendar) shorter than the sidereal year by a bit over 20 minutes. Spring and summer are currently longer in the northern hemisphere than are autumn and winter. The combination of axial precession (caused by the Moon and the Sun) and apsidal precession (caused by Jupiter and Venus) means that this will not always be the case. The situation will be reversed in 13000 years or so: Northern hemisphere spring and summer will be short compared to august and winter.

    The next effect is the uneven distribution of land vs ocean in the northern and southern hemispheres. Land is currently concentrated in the northern hemisphere. Even though perihelion occurs in early January, northern hemisphere winters are in general colder than are southern hemisphere winters, and northern hemisphere summers are in general warmer than are southern hemisphere summers. Ice ages are a consequence of mid-July temperatures in the far north (60 degrees north latitude). Cool summers means multiyear ice and snow can accumulate. Warm summers prevent that multtiyear accumulation of snow and ice.

    The last effect is the eccentricity of the Earth's orbit. Right now, it's a fairly small value, about 0.0167. This varies from nearly 0 to about 0.06. The Earth would be primed for entering an ice age right now if the eccentricity was significantly larger. But it isn't. We have a nice long stretch before the next ice age hits.
  4. Nov 4, 2015 #3
    If the earth's precession is due primarily to the sun and moon; and the moon's orbital eccentricity is relatively stable though the moon's distance from the earth is increasing; what is it about the influence of the earth and sun that changes the earth's precession ?
  5. Nov 4, 2015 #4
    I mean the moon and sun rather, and what changes about them to change the Earth's precession. Sorry
  6. Nov 4, 2015 #5

    D H

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    The Earth is not quite spherical. The Earth's rotation instead makes the Earth an oblate spheroid. That the Earth's rotation axis is inclined with respect to the Moon's orbit about the Earth and with respect to the Earth's orbit about the Sun means that the Moon and Sun can (and do) exert torques on the Earth. It is these torques that cause the Earth's rotation axis to slowly precess. This is called axial precession, and is almost entirely due to the Moon and Sun.

    There's another very different meaning to the term "precession." Perturbations from other bodies make the orbits of the planets not quite Keplerian. In a Keplerian orbit, the eccentricity, inclination, longitude of ascending node, and argument of periapsis are all constant. The eccentricity of the Earth's orbit isn't constant, nor is the inclination, nor is argument of periapsis (or in this case, the argument of perihelion). The location in space where the Earth makes its closest approach to the Sun is not fixed. It instead very slowly changes over the millennia. This is called apsidal precession, and the Moon and Sun have very little to do with this. This is almost entirely the consequence of perturbations from Jupiter and Venus.
  7. Nov 5, 2015 #6

    D H

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    I didn't quite answer this question in my previous posts.

    Astronomically, there are three distinct concepts that qualify as a year:
    • The sidereal year, which is the time it takes the Earth takes to complete one orbit the Sun,
    • The anomalistic year, which is the time from one perihelion passage to the next, and
    • The tropical year, which is the time from one March equinox and the next.
    The anomalistic year is slightly longer than is the sidereal year thanks to apsidal precession. The tropical year, which is the basis of our calendar, is slightly shorter than is the sidereal year thanks to axial precession. The Earth currently reaches perihelion a couple of weeks after the December solstice. This is not always the case. The date of perihelion passage increases by about a day every 60 years thanks to the different values of the anomalistic and tropical years. When perihelion occurs is a key driver of when glaciations ("ice ages") start and end. Hot summers are anathema to glaciations; snow can't accumulate over the years if it melts during summer. Northern hemisphere summers are short but brutally hot when perihelion passage occurs between mid spring and mid autumn. The last glaciation ended about 12,000 years ago, when perihelion occurred in mid to late spring.
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