Changing Paleoclimates and Mass Extinctions

The Astronomical Model

by Donald L. Blanchard


      The Coincidence:
The Milky Way Galaxy
The proposed model derives from a very curious cosmic coincidence -- the fact that the plane defined by the Earth's orbit (the Plane of the Ecliptic) very nearly intersects the center of mass of our galaxy. In other words, the line of intersection between the plane of the galactic lens and the Plane of the Ecliptic passes not only through the center of mass of the Solar system (by definition) but also through the center of mass of the galaxy. If this truly is a coincidence, then the same alignment will not recur until the Solar System has transversed one half of an orbit around the galaxy, or ½ of a Solar 'year' (estimated to be between 200 - 400 million years). In the absence of perturbing forces, the conservation of angular momentum should maintain the Plane of the Ecliptic, and the line of its intersection with the galactic plane, at a constant angle relative to a reference point external to the galaxy, while the line between the center of the Solar System and the center of the galaxy rotates through 360°. (See Figure 1.)

Figure 1

Figure 1 - The 'Coincidence' Condition

showing the line of intersection of the Plane of the Ecliptic and the plane of the Galactic Lens in its current position, passing through the center of the Galaxy. Line A - A' represents the line of intersection of the two planes one quarter of a Solar 'year' from now.

Viewed from above the plane of the Galaxy.
(Not to scale.)

A Modest Proposal:
Rather than accept that this alignment is a coincidence and only occurs twice in a Solar 'year', the postulate presented here is that some perturbing force does exist, and that the Plane of the Ecliptic changes continuously through a full 360°, so that it always passes very nearly through the galactic center. (See Figure 2.) The rate of change predicted is .00338 arc seconds per year, or less than 7 arc seconds in the 2,000 or so years since ancient Greek astronomers first catalogued the sky. This is probably well below the threshold of perception, even with modern astronomical instruments.

Figure 2

Figure 2 - The 'Perturbation' Condition

Line A represents the line of intersection of the Plane of the Ecliptic and the plane of the Galactic Lens one quarter of a Solar 'year' from now, rotated 90°, and still passing through the center of the Galaxy.

Viewed from above the plane of the Galaxy.
(Not to scale.)

The Mechanism:

The cause of the proposed perturbation remains unknown, although one proposal for a possible mechanism is presented below. It appears (based on my very limited research) that very little is actually known about Galactic dynamics and stellar orbits. It is not known, for example, what the actual size of our galaxy is, nor how far our Sun is from the galactic center. Nor is it known with any degree of accuracy how long it takes the Solar System to complete an orbit, the calculation of which requires not only knowledge of the distance to the galactic center, but also the total mass of the galaxy, another unknown.

One possible mechanism for a perturbation of the Plane of the Ecliptic is the very slight differential in gravitational attraction between the Earth and the galactic center. As the Solar System orbits around the galactic center, the conservation of angular momentum of the orbiting Earth tries to draw the line of intersection between the Plane of the Ecliptic and the plane of the galactic lens farther away from the galactic center. This causes the gravitational attraction of the galaxy to be slightly oblique to the Ecliptic. This attraction is greatest when the Earth is at that point in its orbit that is closest to the center of the galaxy: during the Northern hemisphere summer. In Northern hemisphere winter, the Earth is on the other side of the Sun from the galactic center, ever so slightly farther away, and its gravitational pull is diminished. Thus the slightly greater attraction when the Earth is closest to the galactic center, applied at an angle to the Earth's orbital motion, tends to draw the Earth a little bit closer towards alignment. (See figure 3.) Admittedly this difference in attraction is small, but it has millions of years to operate.

Figure 3

Figure 3 - The Perturbational Force

showing the slight difference in gravitational attraction between Earth and the galactic center when the Earth is on opposite sides of its orbit. The perturbing force is oblique to the Plane of the Ecliptic, causing a rotation of the orbital plane.
(Earth shown in position for the month of July.)

Viewed from above the plane of the Galaxy.
(Not to scale.)

In evidence that the gravitational attraction is not negligible is the fact that the Earth's orbit is not circular; the Earth is farthest from the Sun just when it is closest to the center of the galaxy. If differential gravitational attraction does cause perturbation, one would expect that the alignment of the Ecliptic and the galactic center would not be exact, and in fact it is off by just a few degrees. All of the other planets in the Solar System orbit on planes very nearly parallel to the Earth's, but not exactly the same. This also should be expected, as the amount of 'misalignment' required for stability should be a function of the relative mass of the planet and the number of planetary orbits per orbit of the Solar System. The direction of other planets' spin axes, on the other hand, varies to a much greater degree; Uranus's poles lie only 8° away from its orbital plane.

The earth's obliquity (currently 23½°) is measured from the Earth's axis of rotation to a line perpendicular to the Plane of the Ecliptic. The Earth's axis of rotation is gyroscopically stabilized. It can precess (wobble), but precession doesn't change the angle of obliquity, which remains at 23½°. However, if the Plane of the Ecliptic changes, the Earth's obliquity must also change. That change in obliquity is what is postulated in this article as the cause of cyclic climate changes and major extinction events.

At an obliquity of 0°, the Sun would be directly overhead on the Equator all year long (under which conditions there would be no seasonal change). At an obliquity of 90°, the Sun would sit directly over the North Pole in July and over the South Pole in December. This would undoubtedly produce a radically different climate distribution than we currently enjoy.

Cycle Timing and Extinction Events:
The time interval calculated for climate changes caused by variations in obliquity are derived from major extinction events, postulated to be caused by radical and sudden (on a geological time scale) changes in climatic regimens. The major extinctions associated with the postulated climate changes are the End-Cretaceous (K-T) at 65-66 Ma (Million years ago), the End-Permian / Guadaloupian at 248-256 Ma, and the Late Ordovician (Ashgill) at 438-448 Ma. These are the three greatest extinction events known, and the intervals between them are approximately equal, averaging about 190 million years, using the earlier dates given. An even earlier (and more poorly known) extinction event occurred among the acritarchs (a calcium-secreting algae) in the late Precambrian, at about 650 Ma, fairly close to the same 190 MYr interval. (Dating of extinction events is somewhat problematical, and some events appear to have occurred over a several million year span, leading to considerable uncertainty on the earlier events.)

The change in obliquity from 0° through 90° and back to 0° again represents only 1/2 of a Solar orbit of the galaxy, so the estimated interval for a full Solar 'year' is therefore about 380 MYr. This is somewhat higher than most published estimates, most of which range between 200 and 250 MYr, but considering the paucity of raw data on which the estimates are based, 380 MYr seems an eminently reasonable number.

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File created on June 1, 1997.
Last updated November 2004.