earth's orbit and seasons

The earth’s orbit is an ellipse, not a circle. So there are two points a year when the entire earth is farthest away from the sun, and two points when the earth is closest to the sun.

I know that the earth rotates on an angle, so that one hemisphere is closer to the sun at any given time. But shouldn’t the small tilt of the rotation not compare to the large variation in revolution? In other words, shouldn’t the entire earth be in winter twice a year at the far ends of the ellipse?

You’ve got it backwards. It’s a rather large tilt (percentage-wise) of approx 22.5 degrees, and a rather small orbital variation of (damn, where did I put that astronomy book?) of (mumble-mumble) miles.

Anyway, the tilt’s effect on the length of the daylight hours and the angle of that the sunlight hits have a much larger effect on the weather.

Ugly

Even if the orbital eccentricity were the deciding factor in the seasons, and not the axial tilt, there would still only be one Winter and one Summer per year. You’re thinking (I believe) that the Earth orbits the Sun in an ellipse, with the Sun in the Center. This is not so. Rather, the Earth orbits the Sun in an ellipse, with the Sun at one Focus. Looking at a picture of an ellipse, like, say, this one, you will see that there is one point where the curve is closest to the Focus. This is called the perihelion, and in the picture I provided, it’s to the right. Similiarly, there is only one aphelion, to the left.

I used to wonder that, too… Do us all a favor, next time you see an author of elementary school science books, shoot on sight. Don’t ask questions first, he’d be sure to get the wrong answers. Most of what you learned in grade-school science is just plain wrong-- Not sort of true, not approximately true, just wrong.

I don’t have my books here, either, but the actual difference in distance between closest and farthest is about 1%. Interestingly also, the point of closest approach (perihelion) comes on January 12, right in the heart of winter in the norther hemisphere. This does have some slight effect, and since the southern hemisphere’s seasons are the reverse of the northern, this means that seasons are slightly more extreme south of the Equator.

So… I would conclude quite safely that since there are obviously not two winters in one year, then the assumptions are wrong. Or is it that the Earth takes Two years to orbit the sun… hey! I just discovered a new theory! :slight_smile:

BTW, perihelion was january 3 and aphelion will be july 5

I think the OP was answered (it’s the tilt of the Earth, not the distance from the sun). But Chronos raises an interesting topic (at least to me :)), what science topics were you incorrectly taught in school?

From other people, I’ve heard things taught like “stars don’t move” (false), “there are no negative numbers” (what???), and of course, there’s the misunderstanding that “evolution is just a theory”. What are our kids being taught?!? It would be nice if people could say “I don’t know…let’s check the library”.

Phobos:

This is a sore point with me. I was disgusted by my Science teachers in grammar school, because they got things so completely wrong. For example:
– Mosquitos give you malaria by laying eggs in your blood
– A robin has a red breast because you can see its blood through its skin (and feathers, presumably)
– An airplane stays up because it “floats” on a cushion of air.

It was for this reason that I gave up on my Science teachers in school and got the facts on my own.

The thing is, they did a GREAT job on grammar and on math, and some of the teachers clearly knew their science (just not the ones teaching it!)

I plan to kep a close eye on my daughter’s education, when she gets old enough to enter the local school system.

How about when answering Phobos’s question, give the approximate year. Maybe we can see a “dumbing-down” trend in textbooks through the years.
For myself, I will have to think much to find something I was taught wrong in school because I did most of my science learning from books other than standard textbooks.

How about that load of crap about the Earth being round??? What an idiot my science teacher was. And then this-

What? It is? Oh.

I have to go.

Perihelion falls on or about January 2nd and we are about 3 million km closer (152E+06 -v- 149E+06) then than at aphelion, which is July 3rd or 4th this year. If you were to look at the solar system from above, you would be hard pressed to tell the Earth’s orbit from a circle.

I remember a group of Ivy League grads were interviewed, and most were able to identify the Earth’s tilt as the primary forcing on seasonality, but even they fell into the old saw and thought that summer was when one hemisphere is “closer” to the sun due to the tilt. Keep in mind that the change in distance due to the tilt is at most a few parts per million.

Climate is affected more by the heat budget; when the n. hemisphere is tilted toward the sun (between March and September) the days are longer (more heat is absorbed) while nights are shorter (less time for the surface to cool down). At ~35 degrees north the difference in heat received on the summer solstice -v- winter solstice is about 50% (14.5 hours of daylight -v- 9.5 hours). There is also a greater intensity of sunlight in the summer, as sunlight enters the atmosphere at a higher angle (less scattering).

By comparison, the difference in sunlight received at perihelion and aphelion is ~4%.

Believe it or not, my first grade teacher taught us that if it weren’t for gravity, the people on the bottom of the earthy would fall off. She even drew a helpful diagram on the board, with one stick figure standing on top of the earth and another on the bottom. I remember this because it struck me as so obviously absurd, but the other children in the class accepted it without complaint.

My grade school teacher (in the late fifties) allowed another student to explain her theory of the match between the coasts of the western hemisphere and the eastern hemisphere. You know, they did look like they used to fit together. Luckily, our textbooks weren’t up to date on that.

actually, Earth’s pear-shaped nowadays - not been going to the gym enough, taking it easy in the orbit - it adds up.

I know that this was a joke, but it is a pet peeve of mine:
Is the Earth Pear-shaped?

Back to regularly scheduled programming. I just looked over this webpage Popular Misconceptions in Astronomy. It’s a pretty good site, but I feel uneasy saying that “The earth rotates in a period of one day” is a misconception. I’d prefer to call it an approximation–especially since the rotational time given at the site (23 hours, 56 minutes, 4 seconds) is also not exact.

I dunno, I never had a problem with the science teachers, just the science books. Certainly, I got plenty of answers of “I don’t know”, but then, I tend to ask tough questions. Besides, “I don’t know” is infinitely better than spouting off a wrong answer.
As for the shape of the Earth, I heard somewhere (here?) that the Earth’s deviation from a perfect sphere is less than the tolerances for a regulation billiard ball. I’ve never heard billiard balls describes as oblate or ovoid or any other oddball (sorry) shape.

Possibly here but it depends on who you talk to: Is the Earth really as smooth as a billiard ball?.

Science teachers can make the same sort of mistakes that textbook authors make. My son brought home a study guide developed by his middle school science teacher that listed two vocabulary words, lodestone and iodestone. He’d derived the second one from a couple bold entries in the text, and assumed the lowercase “L” in the second mention of lodestone was an uppercase “I”.

"Earth rotates in a period of one day." I have no problem with a statement such as this. This is why we define days differently. A sidereal day refers to Earth’s mean rotational period with respect to the stars (23h 56m 04s). Of course this is the mean rotational period and it varies due to gravitational effects of the Sun, the Moon, Jupiter, tectonics, tidal drag, etc… The mean solar day is the average time between meridian crossings (24h), and can vary by as much as 32 minutes (the Sun is about 18 minutes slow in November, and up to 14 fast in February: check out The Analemma).

It’d be nice if the general public weren’t ignorant of these things, but I’d be pretty happy if they understood seasons and the phases of the Moon…

A coupla other good URLs: Bad Science; and Bad Astronomy.

Not to hi jack or anything, but what if the earth were to slowly tilt the other way?
I mean, slowly enough for there to be no disaster.
What would happen to the climate? Or the deserts?

If the earth were tilted the other way, the seasons would be reversed. In the Northern Hemishere, summer would be December-February (approx), and winter would run from about June-August. If the Earth had no tilt at all, there would be no seasons.

It does, it’s called precession, or wobble (imagine the wobble on a spinning top). The period is ~21,000 years, so the tilt will be “the other way” (if you could call it that) in around 10,000 years. It’s effect is that the ‘cardinal points’ on Earth’s orbit (the solstices and the equinoxes) precess around the calendar. Ultimately, other than changing the time of year that the seasons occur, the effect is minor: if the summer solstice in the n. hemisphere were to occur in Dec./Jan., the longest day of the year would correspond to perihelion, and the longest with aphelion, resulting in marginally warmer summers and colder winters. This is the situation in the southern hemisphere now, but the effect is not as great since the southern hemisphere has a greater percentage of ocean, which moderates temperatures…