# Homework Help: Moon falls off during ecclipse?

1. Aug 8, 2006

### NotMrX

During an ecclipse where the moon is between the Earth and the sun the moon experiences more gravitational force from the sun. How come the moon doesn't fall off into the sun?

2. Aug 8, 2006

### Chi Meson

Because it is not true. There is nothing about an eclipse that increases the effects of gravity. They are two unconnected phenomena.

3. Aug 8, 2006

### chroot

Staff Emeritus
The Moon's distance from the Sun changes by about +/- 400,000 km out of about 150,000,000 km, or about three-tenths of one percent.

The change in the gravitational force exerted by the Sun on the Moon as it goes around its orbit is almost completely negligible.

- Warren

4. Aug 8, 2006

5. Aug 8, 2006

### pallidin

As a side-note:
Source http://www.pbs.org/wgbh/nova/venice/tide_curiosities.html

Although tides are affected mainly by the moon's gravity, gravity from the sun also pulls on the oceans. In fact, the tidal force of the sun is almost half (46%) that of the moon. When the moon, sun, and Earth line up, we see either a full moon (if we're between the moon and the sun) or a new moon (if the moon is between us and the sun). At these times, the gravitational forces of the sun and moon work together, increasing the overall pull on the Earth and its oceans.

6. Aug 8, 2006

### pallidin

And here's a curious statement from http://www.mathpages.com/home/kmath405/kmath405.htm

The Moon Always Falls Toward the Sun

It's an interesting fact that the path of our Moon (with respect to the inertial rest frame of the Sun) always curves toward the Sun. This might seem surprising at first, considering that the Moon revolves around the Earth, but it's not hard to show that, nevertheless, the Moon always has a positive acceleration toward the Sun...

7. Aug 8, 2006

### chroot

Staff Emeritus
Well, duh... the Earth-Moon system is orbiting the Sun, so every part of it has a positive acceleration towards the Sun.

- Warren

8. Aug 8, 2006

### pallidin

You really make it hard at times for others to have an intelligent, progressive discourse while dealing with your emotional outburst of our "ignorance".
Too bad we are not as smart as you, but I am glad we have more manners in communication.

9. Aug 8, 2006

### Staff: Mentor

Sorry, but I have to admit I had the same "well, duh" reaction to the quote. I just figured that you were posting it to poke fun at it. You did call it curious, after all.

10. Aug 8, 2006

### Staff: Mentor

And after a little more thought, I think that the statement turns out to not be true. Huh. Think about it, the distance from the Earth/moon system is relatively constant over a short time interval, and the moon orbits the Earth. So for half of the moon orbit, the moon's distance from the sun is decreasing, and for half it is increasing.

So I guess I should have thought to myself, "Well, duh, that statement is obviously false." I guess I'm a little slow today....

EDIT -- Oh, it says always has a positive acceleration toward the sun. I think I should just go hide somewhere for a while.

11. Aug 8, 2006

### pallidin

Another interesting set of statements.
Source: http://home.netcom.com/~sbyers11/grav11d.htm

Gravity Anomalies via Solar Eclipse

This is a review,... and quote,... and graph of the Saxl and Allen gravity research work demonstrating and measuring the earth's surface gravity perturbations that occur during a solar eclipse. This section is to present the view that the gravity perturbations are caused by planetary gravitational shielding as described with this radiation and shadowing model of gravity. The radiation and shadowing model clearly predicts an increase in gravity within the umbra and penumbra of the eclipse shadow.

The Saxl and Allen work, Phy. Rev. D, 3:4: pg. 823-825 indicates a 5% increase in local surface gravity during the eclipse and states that this measured value is One Hundred Thousand Times (1X10^5) greater than the expected change in gravity computed according to the older theories. The paper also states: "Results of this order of magnitude have been consistently observed in Harvard over a period of 17 years."

12. Aug 8, 2006

### tony873004

I don't think it's that straightforward. Although the Jovian system is always accelerating towards the Sun, sometimes Europa is accelerating away from the Sun.

For the Earth / Moon system, its instantaneous acceleration towards the Sun is GM/r^2 =
6.67x10-11*1.989x1030 / 1500000000002 = .0059 m/s2.

The Moon’s instantaneous acceleration towards Earth is 6.67 x10-11 *5.97x1024/384000000^2 = 0.0027 m/s/s.

So the Moon’s minimum acceleration towards the Sun is 0.0059 – 0.0027 = 0.0032 m/s2.

But the same is not true for Jupiter’s Europa. The Jovian system’s instantaneous acceleration towards the Sun is 6.67 x10-11 *1.989 x1030 /779000000000 2 = 0.00022 m/s2.

But Europa’s instantaneous acceleration towards Jupiter is 6.67 x10-11 *1898.6 x1024 /6709000002 = 0.28135 m/s2.

So Europa’s minimum acceleration towards the Sun is 0.00022 – 0.28135 = -0.28113, which is the same as saying that it is accelerating 0.28133 m/s2 away from the Sun.

This is not only true for Europa, but for all moons in the solar system except for Earth’s moon.

13. Aug 8, 2006

### Chi Meson

From the site aforementioned:
"This table shows that our Moon is unique among all the satellites of the planets, is so far as it is the only planetary satellite whose orbital radius exceeds the threshold value, which means it is the one satellite on which the Sun's gravitational acceleration exceeds the host planet's gravitational acceleration. Consequently, it is the only moon in the solar system that is always falling toward the Sun."

This is the first time I've heard this. Very curious indeed and I loved the analysis shown on the page. I think this is a very good find palladin, and I think Chroot was a bit hasty.

Last edited: Aug 8, 2006
14. Aug 8, 2006

### DaveC426913

You know, that is interesting, although to be honest I don't quite understand it. (Anyone got any tiny words to explain it?)

Something of note that might be a relevant - if not critical - factor: Earth's moon is also the closest moon to the Sun in the entire SS; neither Mercury nor Venus have one. It is only logical that the Sun's gravitational acceleration on it would be higher than with any other moon in the SS.

15. Aug 9, 2006

### Chi Meson

Yes that is critical, and might be one explaination (of several) why the first two planets have none.

It is really comparing the ratios of (planet mass & distance to moon) to (sun mass & distance to moon). Since Mercury and Venus are so close to the sun, any satellite would have to be very close to those planets. And it is not that our moon is so very far from the planet; it is becuase, relative to Jupiter and beyond, the earth is essentially right next to the sun, so the ratio is more likely to be in the sun's favor.

Tangential factoid: most people don't realize that the sun's gravitational force on our oceans is greater than that of the moon. The reason that the moon's tidal effects are greater is that there is a greater difference in gravitational force on near and far side of the earth (due to the fact that the diameter of the Earth is a significant proportion [3%] of the distance to the moon).