Comments on: Another orbit? Why, you don't look a rotation older than 4.56 billion years!

By: Matt B.

Matt B. — Wed, 29 Feb 2012 23:09:29 +0000

Careful. The Gregorian year is not the same as the tropical year, it’s only a calendrical approximation of the tropical year, and in fact differs by a day about every 1,190,827 years. I think Russia is the only country not using that calendar (perhaps the former SSRs use it too), and theirs hasn’t technically differed yet.

By: A Year and a Day — aren't quite that simple | ***Dave Does the Blog

A Year and a Day — aren't quite that simple | ***Dave Does the Blog — Thu, 02 Feb 2012 18:09:26 +0000

[...] seemingly simple notion turns out to be fairly complex when you look at it in detail.Embedded Link Another orbit? Why, you don’t look a rotation older than 4.56 billion years! | Bad Astronomy | Dis… Astronomy | new year | In what is becoming an annual January tradition celebrating my laziness, [...]

By: Neil Haggath

Neil Haggath — Thu, 05 Jan 2012 21:54:29 +0000

#43 Tony:
1. The planets’ orbits are all in roughly the same plane, as a consequence of the way in which the Solar System formed. It formed from a huge cloud of gas, which condensed under gravity and began to rotate. That rotation gradually flattened the cloud into a disc. It’s all about conservation of angular momentum.
2. Who says they can’t detect the wobble of the Sun? Of course they can – but there’s no need for anyone to measure it, as we know the orbital periods of the planets very accurately.
3. I’m sure there are plenty of explanations of the tides on the internet – including a very good one somewhere by Dr. Plait himself, if you care to search for it.
You seem to have a serious misunderstanding of gravity! Your phrase “as far as the Moon’s gravity reached” is totally meaningless! Firstly, it’s a complete fallacy to say that any one body “exerts a gravitational pull on” another. What actually happens is that every two objects – in this case, the Earth and Moon – exert a mutual gravitational force on each other, which is directly proportional to the product of their masses, and inversely proportional to the square of their distance apart. So “as far as the Moon’s gravity reaches” is in fact to infinity, but it becomes weaker by the inverse square law – at double the distance, the attraction is reduced by a factor of four, etc.
Now for your point about the Apollo spacecraft. You’re thinking of the point at which the Moon’s gravity balanced that of the Earth. Each spacecraft was boosted by its rocket to the Earth’s escape velocity, about 25000 mph, then the rocket cut out, and the spacecraft was left coasting. So it was being constantly decelerated by the Earth’s gravity, and at the same time, accelerated by that of the Moon. As the Earth’s gravity, at its surface, is six times that of the Moon, it follows that it predominated for most of the way, so the spacecraft slowed down. At a point roughly 5/6 of the way to the Moon – when the spacecraft had slowed down to a mere 2200 mph – the gravitational acceleration due to Earth and Moon became equal. Up to that point, the spacecraft was being slowed down by Earth’s gravity; after that point, it was being accelerated towards the Moon, as the Moon’s gravity now predominated.
On the return journey, the exact opposite happened; the spacecraft was boosted by its engine to the Moon’s escape velocity, then was slowed down by the Moon’s gravity until it reached that point of balance, then was accelerated towards the Earth after passing that point.
So it was nothing to do with “as far as the Moon’s gravity reaches”; it was to do with when it predominated over that of the Earth.
The Moon has everything to do with the tides! The Earth and Moon exert a mutual gravitational pull on each other. If you consider the water in the oceans as a separate body from the solid body of the Earth, then there is also a mutual gravitational pull between that body of water and the Moon. This causes a “bulge” in the body of water, which is elongated in the direction of the line between the Earth and Moon, and compressed in the direction at right angles. Think of it like a sphere being stretched into an ellipsoid – so the bulge occurs on the opposite side of the Earth, as well as on the side facing the Moon. As the solid Earth rotates daily, it rotates with respect to the bulge; places on its surface facing the Moon, and those 180 degrees away, pass under the bulge, causing high tide, while places 90 degrees away pass under the narrowest part of the ellipsoid, causing low tide. Hence we have two high and two low tides per day.
That’s a very brief precis. Please do a web search for a more detailed explanation; it’s bound to be in Wikipedia, and as I said, you can find it somewhere in Dr. Plait’s blog.
Hope this helps.

By: Tony Favazza

Tony Favazza — Wed, 04 Jan 2012 23:23:10 +0000

OK most of all this seems to make sense about orbits and gravity and all of that. No one explains why there has to be a solar plane that the planets follow. If these scientists can detect the wobble of other stars caused by gravity of orbiting planets. Why can they not find a wobble of our sun? It seems to be much closer. What is up with the tides anyway? When I was about 17 years old NASA sent men to the moon. This trip took about 3 days. There did not seem like much going on for those 3 days. At the beginning of the third ray I can remember Walter Cronkite making a big deal about the spaceship passing from earth’s field of gravity into the moon’s field of gravity. From that point on the spaceship would be falling towards the moon. If that is as far as the moon’s gravity reached how does the moon’s gravity have anything to do with the tides? If you look at the tables for tides you will find that high tide comes twice a day. So if I am standing on the beach and the high tide is occurring, it is also happening on the other side of the earth. I’m not real smart, but how does the moon have anything to do with the tides on the opposite side of the world?

By: Neil Haggath

Neil Haggath — Wed, 04 Jan 2012 12:44:16 +0000

#40 RobT:
I don’t know what your latitude is, but…
Sometime in about the 1960′s, the UK tried exactly what you suggest, keeping British Summer Time ( GMT + one hour ) all year round. The result was that around the winter solstice, it didn’t get light in the morning until about 9 a.m., so children had to walk to school in the dark.
So the idea was dropped, and we reverted to the twice yearly change.

Since you brought up daylight saving time; that was first done in Germany after the end of the Second World War. It was the idea of their famous Chancellor… Dr. Adenauer.
Sorry. I’ll go now.

By: avout

avout — Tue, 03 Jan 2012 21:20:10 +0000

Interesting about the word for ‘day and night’, I have thought of this before. Since I’m Swedish I’ve found the lack of an english word for 24h somewhat strange. In Swedish we have the word ‘dag’ = day, and the word ‘dygn’ = 24h. Both of these are common words used pretty much all the time..

Gott nytt år!

By: RobT

RobT — Tue, 03 Jan 2012 20:07:51 +0000

Phil said
“First, I will ignore a few things. For example, time zones. These were invented by a
sadistic watchmaker, who only wanted to keep people in thrall of his devious plans.”

Keeping with the tongue-in-cheek tone of that sentence, here’s my response:

I think time zones are better than the alternative; there’s a reason they were standardized (thank you Sir Sanford Fleming). If every city kept their own time based on their solar day, like they used to do before the railroads, then timetables would be off – try having a set time for trains, planes or TV when everyone’s time is off by a few minutes. Or if everything was referenced to GMT or UT then everyone would have to do conversion to their own time anyway. Try catching a TV show when it starts at 8:12 “in your region”.

Now, if we could only get Daylight Savings Time as the standard and get rid of the stupid time change twice a year. Why DST? I like it better here in the northern climes when the sun doesn’t set at 16:30, as it does in the winter; 17:30 would be much nicer. That way most people aren’t driving to and from work in the dark.

Interesting article, either way.

By: Gwif

Gwif — Tue, 03 Jan 2012 19:02:53 +0000

Don’t forget that other cultures have different years as well. For example, the Islamic year is based on the moon – 12 lunar months. (354 or 355 days)

By: Jaycee

Jaycee — Tue, 03 Jan 2012 19:02:06 +0000

Phil, this paragraph of yours has an error in it:

“Wait, that doesn’t sound right. You’ve always read it’s 365.25 days per year, right? But that first number, 366.256, is a year in sidereal days. In solar days, you divide the seconds in a year by 86,400 to get 365.256 days.”

By: Slim

Slim — Tue, 03 Jan 2012 16:55:53 +0000

@11. Alan(UK)

If the Sun is going around the Earth, there is still going to be a difference of about one day, for the same reason. In that system, in one day, the sun has moved about 1° relative to the fixed stars. So the Earth has to rotate (or rather, the crystal spheres had to rotate) a bit more to have a full day. Just as in the real sun-centered system. They probably said that we had to be at the center, because otherwise why isn’t there any observable parallax in the stars? They didn’t realize just how far away the stars really are.

Actually, I was thinking about how we measure a year not too long ago, and I realized that this must be why real astronomers don’t use the term light-year even though it makes a lot more sense than a parsec. But if we want to be precise, an AU (used to calculate a parsec) is a lot better defined than a year.