# Thread: math/astronomy question

1. ## math/astronomy question

am after;

The earth takes 4388 hours 17 minutes and 32 seconds to orbit halfway around the sun
It only take 4372 hours 57 minutes and 13 seconds to orbit the other half

I guessed at this time. I know it's wrong but there is a difference between the 2 half's. I don't know what it is. I want to be able to write the two sentences above with true values.

When the earth moves towards aphelion it is moving further from the suns gravity and slows down

When the earth moves towards perihelion it is moving closer to the suns gravity and speeds up

I have posted this question on other forums. I get all kinds of answers except what I asked for

Again

I want to write the two above sentences with true values. (The earth takes 4388 hours ......)
does anybody know?

2. It depends greatly on which two halves you're looking at, how you define "half", etc. Let's say you take "half" to be half of the arc-length distance traveled by the earth around the sun. Then you compare traveling from the perihelion to the aphelion (leg 1) to traveling from the aphelion to the perihelion (leg 2). By symmetry arguments, you can say that these two legs will take the same amount of time, which is half of a year. You're certainly not going to have the approximately 16 hour difference exhibited by your numbers.

Is this what you're looking for?

3. As I find out more, I can see that my question was ambiguous.

I thank you for your patience.

To explain what I'm after, I'll first show you what I'm going to do with it.

I want to introduce astronomy to young children. I am making a simple drawing to show this difficult concept to kids as young as 3rd graders.

I can see now that where I cut the orbit in half determines the difference between the two halves.

Where the half is divided, I'm not so concerned about. Now that I learned a bit more I'd like the halves in two places.

See the attachment

Perhaps one of the halves can be divided on the black line and the other can be on the green or purple line

As I said, I'm not concerned where the half's are but I would like it where I get the biggest difference between the halves

I know the this changes for each year but I don't want to get that accurate or complicate my drawing so that a 3rd grader can't read it.

4. I would like it where I get the biggest difference between the halves.
In that case, you'd better take one half of the orbit, as measured by arc length, to be the half, in looking at your picture, obtained by drawing a vertical line and splitting up the elliptical orbit into right and left halves. The left half will be traversed much more quickly than the right half, because the earth will be moving more quickly over the left half. It's approximately from one solstice to the next, but not quite, it looks like.

5. Originally Posted by rookie37 As I find out more, I can see that my question was ambiguous.

I thank you for your patience.

To explain what I'm after, I'll first show you what I'm going to do with it.

I want to introduce astronomy to young children. I am making a simple drawing to show this difficult concept to kids as young as 3rd graders.

I can see now that where I cut the orbit in half determines the difference between the two halves.

Where the half is divided, I'm not so concerned about. Now that I learned a bit more I'd like the halves in two places.

See the attachment

Perhaps one of the halves can be divided on the black line and the other can be on the green or purple line

As I said, I'm not concerned where the half's are but I would like it where I get the biggest difference between the halves

I know the this changes for each year but I don't want to get that accurate or complicate my drawing so that a 3rd grader can't read it.

There is one thing I would like to know: What do you mean by a half of an elliplse, other than that part which lies on one side of the major axis?

(if you are refering to the position with respect to the distant stars you need to remember that the Sun also moves)

You might want to read anout Kepler's "Astronomia Nova" on Wikipedia

CB

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