That’s what I meant–the earth rotates in a day is not a terrible misconception. On the other hand, your last sentence there needs a little fixing up. It’s not the mean solar day that can vary by 32 minutes, it’s not even the length of a day that can vary that much. It’s the time of the crossing that can vary so much, right? The length of the solar day would only have to vary by plus or minus a minute or two.
From day to day, that’s right. Today is June 24. The solar day is lengthening by 12-15 seconds a day right now. By late July it will stabilize and begin to shorten again. The effect is cumulative though – a solar day in early November is ~17 minutes shorter than the mean solar day, while a day in mid-February is about 15 minutes longer. The maximum rate of change occurs around the December solstice, about 30 seconds a day.
This comes from the fact that sun moves faster through the celestial sphere at around perihelion than at aphelion, and because the true sun moves faster than the mean sun at around the solstices, and slower than the mean sun near the equinoxes. If you have doubts, check your sundial – you will find that the sun crosses the meridian about 17 minutes before local noon on Nov 3-4, and at about 15 minutes after local noon on Feb 10-11.
>> solar day in early November is ~17 minutes shorter than the mean solar day
No, I believe that is mistaken. The Sun’s meridian crossing may be 17 minutes off local solar mean time but I do not believe the period between consecutive meridian crossings can be as short as 23 hrs 43 mins. Can you support what you said?
I do have my doubts. You may have to retract that statement about the general public. 
The solar day is getting shorter right now. For my location, I calculated it as 24 hours and 12.72 seconds. It’ll be a little more than a tenth of a second shorter tomorrow than it is today. I don’t think the length of a solar day is more than thirty seconds from 24 hours–on January 2, probably.
What I was trying to say is that the time between meridian crossings on Dec 25 & 26, say, is ~30 seconds less than that between the 26 & 27. No day is more than about 30 seconds longer or shorter than the mean solar day. But the cumulative difference is as much as 17 minutes.
Looking back over what I wrote I can see I left the wrong impression, for which I apologise.
jeez, you can’t joke about anything any more…
in all seriousness, RM, I’m a bit puzzled. From the links you supply, it appears that the earth is as smooth as a billiard ball, but it’s not a perfect sphere - one of your links said that the equatorial diameter is 24 miles greater than the polar diameter.
What would be the techical term for that? Is it close enough to being a sphere that geologists call it a sphere? Or would it be called something else?
[If you could dumb down the math, I’d appreciate it - I couldn’t understand the math in your article.]
yup. if you are standing at sea level at the equator you are farther from the center of the earth than if you are standing at the top of Everest.
That’s what I thought you were trying to say. That is wrong, too–those days in December are some of the longer ones, but the length of day from day to day differs by less than a second.
It’s known as an oblate spheroid, and is mostly a result of the rotation of the earth, causing the equator to bulge. Some of the looser billiard ball specifications would actually allow the earth to pass both the smoothness and roundness test.
Some of the deformation is due to the movement of the material within the earth–and geophysicists would like to understand how that works. It causes bumps in the gravity field which causes deflections of the satellite orbits–which allow us to calculate where the bumps are, so we can more accurately predict satellite orbits. Those bumps are important, but they are only on the order of a hundred meters or so.
The pear-shape of the earth was an approximation from the earliest days of the space program, about forty years ago. Notice, a pear shape is symmetrical about the axis–we didn’t have enough data in those days to determine anything but the symmetrical fields, because the earth rotated beneath the satellites, smearing out the results. Now, we know a lot more about the shape of the gravity field, but even back then, that pear-shape only added a few tens of meters to the overall shape. Nothing like the 24 miles.
I’m going to stick with my guns, here. I used December as an extreme example, since the sun’s right ascension changes fastest near perihelion, but it can be seen right now also. Go outside and look at the position of a shadow at exactly noon (or 1pm, daylight time). If you do this today, and do it again in two weeks, you’ll find that the shadow doesn’t hit the same spot until close to three minutes later. This change requires a difference between the true sun and the mean sun of 10-12 seconds per day.
This is the equation of time, illustrated quite well at the analemma. This table of sunrise/transit/sunset is for Griffith Observatory in L.A., but it shows quite well how quickly the time of meridian crossing changes. This is also why the earliest sunrise of the year precedes the longest day by about a week, and the latest sunset occurs about a week afterward.
Lemme say it again, since my earlier posts obfuscated a bit: The mean solar day of 24 hours is represented by the mean sun, which annually moves around the Earth in a perfect circle above the equator. Since the true sun moves around the earth in an ellipse around the ecliptic, it can be ahead of/behind the mean sun by as much as 30 seconds per day.
I think some of us here are confusing two very different things. Suppose today the meridian crossing of the sun happens exactly at 12:00:00 and everything is copacetic. Suppose tomorrow it happens at 12:00:10, then the difference between the two is 10 seconds. If the next apparent day is also long by 10 s we have accumulated 20 sec of time difference. At the end of 6 days the sun is one full minute behind and yet no day was off by more than 10 sec. The equation of time gives you the accumulated time difference not the length of any individual day. After one full cycle (year) the positive and the negative differences cancel out and you are back at the beginning.
Just to confirm, I checked the almanac and here are some meridian crossings for this year:
jan 3 12:04:15
jan 4 12:04:52 +37s
jan 5 12:05:10 +18s
You can see the first day is 37 sec over 24h but the accummulated time is about 4m30s. The second day is 18 seconds over 24h.
More meridian crossings for this year:
jul 03 12:04:16
jul 04 12:04:27 +11sec
apr 13 12:00:27
apr 14 12:00:12 -15sec
I am not going to do all 365 days but anyone claiming there is any solar day that deviates from 24 hours by more than a minute should provide the evidence. I do not believe it exists.
maybe thats why they started Christmas:
they were so happy that ther sun wasn’t disappearing that they celebrated! heh heh
Actually, in the Talmud (Tractate Avoda Zara 8a), there is a discussion about this very subject, and they come to the same conclusion as in your “heh heh”.
I agree with your point, but I’m suspicious of the data. I don’t think the length of the solar day changes that much from day to day. That the 52 in that second line may be a 42 in your almanac–or else the almanac made a mistake. If it were a 42, the length of the solar days are 24 hrs and 27 sec, and 24 hrs and 28 sec–which are still approximations. I believe the length of the solar day decreases in January.
He’s sticking with his guns, but seems to be shooting at himself.
The theory of gravitation is just a theory too. Theory is the end of the scientific process, emphasizing just is typically a “don’t piss off the religious parents” ploy. Like any theory it has its faults and assuming its the final truth is a misunderstanding not by the system, but by you.
If there’s anything really wrong with the system its that they teach science without eventually teaching the philosphy of science.
I refer you to my previous post, in which I claim that the solar day can vary by up to 30 seconds from the mean value of 24 hours. The numbers from sailor’s almanac confirm what I said.
jrepka I think what’s happening is that you are not expressin yourself clearly and you are confusing the rest of us.
So to clarify:
(A) every individual day (time between meridian passages) does not deviate from 24 H by more than 1 minute.
(B) the accumulated deviation, represented by the equation of time can add up to 17 minutes or whatever it is.
your quotes:
- the solar day can vary by up to 30 seconds from the mean value of 24 hours Correct
- Since the true sun moves around the earth in an ellipse around the ecliptic, it can be ahead of/behind the mean sun by as much as 30 seconds per day. Incorrect the value is 17 minutes
- No day is more than about 30 seconds longer or shorter than the mean solar day. But the cumulative difference is as much as 17 minutes Correct! Exactly
- Looking back over what I wrote I can see I left the wrong impression, for which I apologise. You’ve done it again!
I think we are all in agreement and you need to be more clear in your posts.
And then there was:
- a solar day in early November is ~17 minutes shorter than the mean solar day, while a day in mid-February is about 15 minutes longer. Big time incorrect
Richard–I said last night that this was not what I meant to say. I apologized for being unclear, and have stated several times, with references, that an individual day can vary by up to ~30 seconds. Do you still contend that the difference can be no greater than 1 second?
Was that a personal attack? Hmm… Anyway, I agree that the philosophy of science should be taught more. I also understand that a Theory is not the ultimate truth. A theory is the best explanation to fit the best evidence. It’s modified when new/better/conflicting evidence is found and accepted. Life evolving is a fact. The Theory of Evolution is an explanation of “how”. Ditto for gravity. My comment on “just a theory” pertained to those who write-off evolution because of the common connotation of the word theory as “guess”, instead of understanding the scientific definition.
ok, so we agree about the erath’s orbit, Now I have another question. I have seen in several threads posts that obviously were meant for other threads. How can that happen? I mean, you are reading a thread, you want to rspond, you click reply, you type and click post. How can it go to another thread? That’s what I wanna know.