|Watsisname||Date: Saturday, 31.08.2013, 09:38 | Message # 16|
Group: Global Moderators
|Ah, gotcha. It is certainly not constant, and the changes are mostly due to torque and tidal evolution with the Moon. Conveniently this topic is included in the first link I gave earlier, with discussion and charts for the axial tilt (also called obliquity) as a function of the moon's semi-major axis, and as time. In particular, check figures 5, 11, 17, 21, and 23, and §5.2. |
What we find is that over the very long term, the obliquity is increasing (so it was smaller, on average, in the past), but there is also variability across a range of timescales. For example, in the shorter term we see variations of ~2° with ~40,000 year periodicity -- this is a part of the Milankovitch Cycles, which are relevant to glaciations.
If we look at geologic evidence and go back farther in time things get more questionable. Indeed, there are some who propose that during the Proterozoic, the obliquity was very high (~55° or more!), with low-latitude ice and extreme seasonal variability (HOLIST hypothesis). This, in contrast to the view of global glaciation with lower obliquity (standard snowball earth hypothesis). Both sides argue the evidence supports their interpretation, and it is an ongoing debate. Relevant papers may be found here and here, and there are many others.
Now I have my own views as to which is the more plausible explanation but I encourage readers to arrive at their own conclusions.