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Cyberseeker
2010-Mar-13, 10:36 PM
Hi, Im new here so I dont know if this has been asked before.

As we know, the earth takes 365.242 days to get around the sun. Has this ever been different? In other words, is there anything to suggest that the earths speed has been slowing?

Thanks

violentquaker
2010-Mar-13, 10:50 PM
Hi, Im new here so I dont know if this has been asked before.

As we know, the earth takes 365.242 days to get around the sun. Has this ever been different? In other words, is there anything to suggest that the earths speed has been slowing?

Thanks

Due to the peculiarities of orbital mechanics, slowing down the earth would actually shorten the year. It seem likely that there have been some changes to earth's orbit in the last few billion years, but I'll leave it to the folks with more letters after their names to go into the details if they are known.

EDG
2010-Mar-13, 11:34 PM
Depends what you mean by "slowing". The Earth's taken more days to go around the sun in the past, but that's because the Earth's days were shorter (the moon slows down Earth's rotation period around its axis over geological time due to tides).

But you're talking about units of time that are independent of our rotation (eg. seconds), then the earth's orbital period around the sun hasn't changed appreciably. I guess solar tides should be pushing the Earth's orbit outward (so the orbital period would be getting longer), but the effect is miniscule.

sabianq
2010-Mar-13, 11:53 PM
Depends what you mean by "slowing". The Earth's taken more days to go around the sun in the past, but that's because the Earth's days were shorter (the moon slows down Earth's rotation period around its axis over geological time due to tides).

what??

how does the rotation of the planet on its axis have anything at all to do with the orbital distance from the sun?

it does not..

sabianq
2010-Mar-14, 12:15 AM
the distance from the sun determines the length of the year.

here is an example
Mercurian Year: A year on Mercury takes 87.97 Earth days; it takes 87.97 Earth days for Mercury to orbit the sun once..

its axis rotation peroid has nothing to do with its orbital peroid around the sun

Each sidereal day on Mercury takes 58.65 Earth days; it takes Mercury 58.65 days (2/3's of its year) to rotate around its axis once.

Venus rotates on its axis the opposite direction of the Earth. This means that on Venus, the Sun rises in the west and sets in the east.
its day is equal to 243 Earth days.. almost an earth year and more than one venus year for the planet to rotate on its axis once..

its year is equal to 225 earth days..

Nowhere Man
2010-Mar-14, 12:17 AM
I do hope you're joking. In the past, the day was less than 24 hours long, therefore it took more days to fill one year. The year was the same length in, say, hours as it is today (or as near as makes no difference), but the day had fewer hours. Thus, more days were required to make a year.

Fred

sabianq
2010-Mar-14, 12:33 AM
as a planets orbit gets closer to its parent star the year gets shorter..

sabianq
2010-Mar-14, 12:38 AM
I do hope you're joking. In the past, the day was less than 24 hours long, therefore it took more days to fill one year. The year was the same length in, say, hours as it is today (or as near as makes no difference), but the day had fewer hours. Thus, more days were required to make a year.

Fred

the length of a day it the time it takes to spin on its axis and the year it the time it takes to orbit the star..

the langth of the day and the year are not connected to each other..

EDG
2010-Mar-14, 12:43 AM
the length of a day it the time it takes to spin on its axis and the year it the time it takes to orbit the star..

the langth of the day and the year are not connected to each other..

I think you're completely misunderstanding what I'm saying.


It currently takes about 365.25 days for the Earth to orbit the sun.

A few hundred million years ago, during the Devonian period on Earth, it took about 400 days for the Earth to orbit the sun. But that wasn't because the Earth's orbit was bigger, it was because the length of the day back then wasn't about 24 hours - it was about 21.9 hours. This is because tidal interaction with the moon is slowing down the earth's rotation over time (therefore, day length was shorter in the past). Therefore, there were more days in a year back then than there are now, even though the actual chronological length of the year (as measured in seconds or hours or minutes) wasn't longer.

You have to be careful when talking about "days", because they're not fixed in length (over geological time). The answer to "have there ever been more days in a year?" is "yes", but it's not because the year has actually been longer in the past - it's because the days were shorter. The number of 24 hour days are the same, but the earth wasn't rotating once every 24 hours back then.

Nowhere Man
2010-Mar-14, 12:51 AM
Oh, dear, you're not joking. Think about it. I specified that the year measured in hours has been the same length* for quite some time -- for a few billion years, at least. The length of the day in hours has changed, gotten longer, over that period. Thus, by the wonder of long division, more days per year back in the "good old days."

edit to add: *Assuming that one defines the hour as 3600 seconds, and not 1/24 of a solar day.

Fred

loglo
2010-Mar-14, 12:53 AM
Getting back to the OP, does anyone know how the glancing blow received by the Earth during the creation of the Moon is supposed to have affected the orbit? Since a Mars sized mass was added to the system as well as some inertia I would guess that some change in orbit would have occurred. I can't see any other mechanism to change the orbit after the Moon was formed.

Van Rijn
2010-Mar-14, 12:55 AM
the length of a day it the time it takes to spin on its axis and the year it the time it takes to orbit the star..

the langth of the day and the year are not connected to each other..

In the case of Mercury, they are (spin-orbit resonance).

If you're using one as a unit of measure for the other, they are.

You have to be careful about the details.

Van Rijn
2010-Mar-14, 12:59 AM
Solar mass loss would increase the diameter of the Earth's orbit a bit over geological time, but it's a small effect. The solar tidal effect would be even smaller.

EDG
2010-Mar-14, 01:16 AM
Solar mass loss would increase the diameter of the Earth's orbit a bit over geological time, but it's a small effect. The solar tidal effect would be even smaller.

Not that the sun has actually lost much mass over it's (so far) 4.6 billion year life so far. At the end of its life though (when mass loss accelerates during the red giant phases in about 5-6 billion years), it might mean the difference between Earth surviving the red giant phases or being consumed by the expanded sun.

filrabat
2010-Mar-14, 01:42 AM
As we know, the earth takes 365.242 days to get around the sun. Has this ever been different? In other words, is there anything to suggest that the earths speed has been slowing?

Like others said, it depends on what you mean by days. If by days, you mean "the time from one sunrise to the next sunrise, then yes the earth did perform more rotations per (I think) 31,558,149 seconds (AFAIK, the second is an absolute measure of time, as decided by the scientific community, but correct me if I'm wrong).

If by "earth's speed" slowing, you mean the revolution around the sun, then that's not something I know about (though others on BAUT might). Even so, there are probably small subtle changes in the Earth's orbit constantly - some of which are theorized (but not proven) to cause Ice Ages. This undoubtely creates some miniscule amount of change in the revolution period of the Earth. Still, I doubt it's enough to substantially alter the number of seconds in earth's year (unless you are talking about at least millions of years, if not tens or hundreds of millions of year).

sabianq
2010-Mar-14, 02:33 AM
there are estimates that the Earth's rotation is slowing at about 1.4 milliseconds per solar day per century.

The length of time it takes the Earth, at the present time today, to rotate once on its axis is 86,400.002 seconds compared to 86,400 seconds back in 1820. The rotation has slowed roughly only by 2 milliseconds since 1820. that corrosponsd to the 1.4 milliseconds per solar day per century.

if you calculate back to 4.5 billion years, that shows that the day/night rotation was 63,000 seconds shorter 4.5 billion years ago compared to the than the present 86,400 second rotational peroid it is today.

in hours, that comes to about 6.5 hours per day/night cycle which means that a year still takes 31.5 million seconds to orbit (same as today) but comes to 1346 (6.5 hour) day/nights in a year..

is that what you mean?

Cyberseeker
2010-Mar-14, 02:37 AM
Like others said, it depends on what you mean by days.

What Im talking about is calendars. For example the ancient Egyptian calendar with its 365 days. Yes, it was a wandering calendar we know, but would it have wandered as much as if it was in use now?

What Im asking is, do you think that within thousands (not millions) of years the orbit may have been say, 365.15 or 365.20 or whatever. Is that a possibility?

EDG
2010-Mar-14, 02:45 AM
is that what you mean?

Well, I don't know about the specific numbers, but generally yes. Days were shorter in the past, but the year was the same length, so there were more days in a year. I don't know how much more clearly that can be explained.

Spaceman Spiff
2010-Mar-14, 03:01 AM
What Im talking about is calendars. For example the ancient Egyptian calendar with its 365 days. Yes, it was a wandering calendar we know, but would it have wandered as much as if it was in use now?

What Im asking is, do you think that within thousands (not millions) of years the orbit may have been say, 365.15 or 365.20 or whatever. Is that a possibility?

In a word, 'no'.

Van Rijn
2010-Mar-14, 03:15 AM
What Im asking is, do you think that within thousands (not millions) of years the orbit may have been say, 365.15 or 365.20 or whatever. Is that a possibility?

As previously noted - No.

There would have only been small changes in the diameter of the orbit over billions of years. The length of the day would have changed significantly over billions of years, but not over thousands.

astromark
2010-Mar-14, 08:11 AM
This has been a fine example of people talking at each other and not to...
Which is a shame of a sham... Fortunately the information has been given and conclusions explained sufficant to have the understanding unwound... whew...:)
So the length of Earths year has not changed. The rotational velocity about its own axis has. So its unwise to count the years length by the unit 'days'.
There is an interesting aside to this for me... As planet Earth has evolved has its mass changed significantly. Is the Solar mass changing, and if either is changing does this effect the Earths year length ? But I will make the point that its not by much so very little over a long time...

grapes
2010-Mar-14, 01:15 PM
As we know, the earth takes 365.242 days to get around the sun. Has this ever been different? In other words, is there anything to suggest that the earths speed has been slowing?Just so we don't get further confused, by an already confusing subject, I have to point out that it takes the earth 365.256 days to get around the sun, about twenty minutes longer.