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Has an earth year always been 365.25 days?
Planetary Sci.
I remember hearing a very reputable source... a guest peddling a book on Coast to Coast AM... say that several thousand years ago the earth year used to be shorter, closer to 360 days, and at one point a comet came close to the earth and caused the earth to slow down. And evidence of that is all calendars from different cultures around the world switched from 360 days to 365 days around the same time. I feel stupid writing that out, but I thought it was an interesting theory at least and never heard it debunked (or repeated elsewhere). I somehow doubt any part of it is true though, even the calendar thing. Has anyone heard that theory before?
And even if that isn't true, has a year always been the same length of time? Does the earth's speed around the sun change over time at all?
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No, it has been considerably different from 365.25 days in the past, but the reason is not that the orbital period has changed significantly but rather that the length of a day has.
The rotation of the earth is slowing, due to the influence of the moon, and this causes days to get longer. There were more days in a year in the past, but this was due to the days being shorter.
This is a significant effect over geological time scales; at the time of the moon's formation a day lasted as little as 2-3 hours; by 620mya a day had extended to 21 hours.
http://www.abc.net.au/science/articles/2012/11/28/3642932.htm
It is not however a significant effect over historical time scales, the length of the year and day were not significantly different thousands of years ago, no.
It's a lot easier to change rotational period than orbital period, incidentally, even major impact events have little effect on orbital period.
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At no point in human history has the length of the period of the earth going around the sun changed in that manner. That's a backward rationalization trying to explain how ancient 360-day lunar calendars were actually "correct."
To my knowledge, the period of the earth around the sun is pretty reliable, but it is still subject to gravitational affects from other bodies in the solar system.