Yasser, Canada asked:
I am especially curious to know if there was some shift in the seasons a long time ago? If we imposed our calendar system to the time of the dinosaurs for example, would we still find the seasons similar to autumn occurring around October or would the season have occurred at some point earlier in the year?
We posed this question to Dr John Nudds from the University of Manchester:
John - To answer this question, we firstly have to understand why we have seasons today. We have seasons today for a very simple reason, simply because the Earth is tilted.
If you remember the globe on your geography teacherís desk, the rotational axis of the Earth actually tilts an angle of about 23 degrees. Now if you imagine our tilted Earth revolving around the sun, when the northern hemisphere is tilted towards the sun, the southern hemisphere will be tilted away. But when our Earth gets around to the other side of the sun, the northern hemisphere will be tilted away from the sun, and the southern hemisphere will be tilted towards it. In the northern hemisphere, we have our summer in June, while the southern hemisphere has its winter in June. While the tropical regions around the equator will remain in a pretty constant distant from the sun all the time and therefore, experience little seasonal difference. So thatís why we have seasons today.
Now, we have no reason to believe that this situation is ever any different to this. As far as we know, the Earth has always been tilted at this angle, so in the geological past, for example in the time of the dinosaurs, we can assume that the temperate areas of the globe experience the same four seasons that we experience today.
So to answer the question, if you impose our calendar on the Jurassic year, those dinosaurs living in temperate regions in the northern hemisphere as we do, experience their summer in June. Those living in the southern hemisphere experience their winters in June, and those living in the tropics experience little seasonal change. The one thing that was different however, is that the Earth is slowing down on its rotational axis. So, within the time of the dinosaurs, the days were actually shorter, but there were more of them in a year.
Diana - And a long year with more days in it would mean our calendar wouldnít settle perfectly over the dinosaur year. That said, they probably experienced the same annual temperature changes that we do today.
Earth's rotation about it's axis has significantly slowed over geological time: in the past days were shorter.
"... if you impose our calendar on the Jurassic year, those dinosaurs living in temperate regions in the northern hemisphere as we do, experience their summer in June. Those living in the southern hemisphere experience their winters in June, and those living in the tropics experience little seasonal change."