Live on the seacoast and have a grandson who has for a few years now been fascinated with the tides. Now that he is older and is starting to grasp the relationship of the sun-earth-moon and the concept of gravity which causes the tides he is much curious with that also.
He asked me three questions the other day that frankly have never occurred to me. I told him I did not know the answer, but I knew a place that would be a good place to start. So please the astronomy types help me out here and thanks to you in advance. My apologies if I do not quite use the correct terms here, but I think you can grasp the questions.
1
So the earth orbits the sun, and the moon orbits the earth. At what time in the moon’s orbit is the moon “leading” the earth as the earth orbits the sun, and at what time in the month is the moon “behind” the earth?
2
Does the orbit of the earth speed up and slow down because of that concept? In other words sometimes in the month the moon is in front of the earth and therefor the moons gravity should be pulling the earth, and at other times the moon is behind the earth and should be holding back the earth a bit.
3
Similarly at what time of the year is the earth in “front” of the sun in the suns revolution around the galaxy and at what times is the earth behind ?
Thank you
During a full moon, the Earth is between the moon and sun. During a new moon, the moon is between the sun and the Earth. On question #3, the plane of the solar system is inclined about 60 degrees to the plane of the galaxy. It may therefore not make much difference which is the trailing edge and which is the leading edge.
The moon is moving in it’s orbit in a counter-clockwise direction when observed from above the north pole. So from an observer in the northern hemisphere this is a leftward movement (easterly) against the background stars (confusing since the Earth’s spin sends everything from east to west). The Earth is also moving counter-clockwise in it’s orbit around the Sun, moving to the right for the same observer at noon. The New Moon occurs when the moon is between the Earth and the Sun, so from the New Moon to Full Moon, it is “behind” the Earth. And from Full Moon back to New Moon, it is “ahead”.
Earth and the Moon orbit about their barycentre (common center of mass), which lies about 4600 km from Earth’s center (about three quarters of the radius of Earth). Wiki link so there is a “wobble” in the Earth’s motion around the Sun.
Regarding (2), you can think of it as, rather than the Moon orbiting the Earth, both the Earth and Moon orbit a point called the barycenter of the system. Because the Earth is so much more massive than the Moon, the barycenter is deep within the bulk of the Earth, but not at the exact center, so the Earth’s orbiting around this point looks like a little wobble, rather than what one would normally thing of as “orbiting”.
When the moon is in ahead of the Earth in its orbit, the Earth is at the back of its wobble, and when the moon is behind the Earth, the Earth is at the front of its wobble, so there is indeed a small change in velocity as it wobbles back and forth.
Of course the size of this change in velocity is much, much smaller than the mean orbital velocity of the Earth.
At sunrise you’re on the “front” of the Earth in the direction we’re travelling around the sun, and at sunset you’re on the back. If you look up in the sky at dawn and see a half moon, then it’s “in front” of the Earth on its plane of motion, and if you look up at sunset and see a half moon then it’s behind us.
The moon induces a wobble so we are affected by it much the same way a person throwing a hammer in the olympics wobbles while spinning the hammer.
The easiest way to think about this is that most of the large things in the Solar System rotate and revolve in the same direction. Suppose you were way above the North Pole of the Earth (little o) and the Sun o—O. It’s high noon right where the dash is. The top of the o is to the east for those people experiencing noon. The sun sets in the west so the Earth must be spinning counter-clockwise. In this same picture, the Earth orbits the sun counter clockwise and the moon orbits the Earth coutner-clockwise. (The moon also rotates on its own axis counterclockwise, but it does so at exactly the same rate as to keep the same face towards earth.)
So when the moon is below the Earth in my o—O picture, it is preceding the Earth in its orbit around the sun. When it is directly below the Earth, you have a quarter moon. And since it is moving towards the sun, it’s the last quarter moon (the one that precedes the new moon.
Roughly speaking the center of mass of the Earth-Moon system could be said to orbit the sun (this center of mass is inside the Earth). This center of mas is moving at a constant orbital speed (*wrong but I’ll get back to that). When the earth is between its last quarter and next first quarter it is moving backwards relative to the Earth-Moon center of mas so is orbiting slower than the Earth. Between the first quarter and the next last quarter, the Moon is orbiting faster than the Earth.
These differences are never large enough so that the Earth or Moon is actually moving “backwards”. In fact if you plotted the motion of the moon alone, it is always concave towards the sun. It’s just an almost circle with little scallops on it.
*The orbital speed of the Earth-Moon system is not actually constant as the orbit is an ellipse not a circle. The Earth is closest to the sun in January and is orbiting faster then, but what I said about first to last quarter and vice-versa is still true.
I don’t know the answer to your galaxy question. If someone doesn’t give it to you, I’ll find out.
The moon goes around the earth in the same direction that the earth orbits the sun- counterclockwise, as seen from above the North Pole. When the moon is at first quarter (a half moon, halfway between new and full), it will be behind the earth in orbit around the sun. When it’s at last quarter (the half moon between full and new) it will be ahead of the earth.
Yes it does. In any system where you have two bodies in orbit, they both move around a common center of mass. Draw a line between the centers of the two bodies, and the center of mass will be along that line but closer to the larger body. How much closer will depend on the relative masses of the two bodies. Looking for this kind of wobble in the motion of a star is one way that we find planets around stars other than the Sun.
Still further info: Apparently the plane of the ecliptic (the path of the earth around the sun) is at almost a right angle to the plane of the galaxy’s bulge (and plane of rotation) so there is really no time of the year in which the Earth is gong faster or slower than the Sun around the center of the galaxy. I assume this may change during the time it takes the Sun to orbit the galaxy, but that cosmic year is over 200 million years long so we’ll have a while before the story needs changing.
The plane of the Earth’s orbit around the Sun and the plane of the Sun’s orbit around the Galactic Center are not the same; they’re off by about 60°. So the path of the the Earth around the Galactic center would look something like a corkscrew (one that’s been very stretched out.) Because the angle isn’t exactly 90°, though, the Earth is “ahead” of (and “below”) the Sun for about half the year, and “behind” (and “above”) the Sun for the other half. I think the dividing line is around mid-February and mid-August, but this is astonishingly hard to look up to confirm, and I welcome corrections.
I got a lot of information here, as I knew I would, and I will do more research using what I have gained here and continue to have interesting conversations with the little man. Thanks again
The solar apex, the direction of the Sun’s path through interstellar space, is near the constellation Hercules, basically toward the bright star Vega… So the earth is headed there too …
So when you can see Vega/Hercules at its highest in the sky, thats the time of year earth is in front of the sun’s movement.
Around the beginning of July, the earth is in front of the sun, as at midnight, Lyra and its bright star Vega will be up the dividing line between east and west…
the sun, the earth, and Vega, are in the closest “line” of the year.
In the northern winter, you won’t see Lyra at all.
Isn’t the solar apex the direction that the sun is heading relative to the local stars? That would not necessarily be the direction of the caused by the galaxy’s rotation.
Yep - another interesting outcome of this is that meteors are more prevalent after midnight, because you’re then on the leading edge of the Earth’s movement around the Sun, so the best time to see meteors is after midnight. Meteors are that prevalent up until noon, but you simply can’t see them after dawn, so the best time to see them is midnight to dawn.
I’m not an expert on this at all (and actually learning quite a bit from these answers), but it seems to that on this one, we can approximate the answer: no effect.
The mass of the earth (& moon) is such a small part of the total solar system mass, given the sun itself and the gigantic gas planets like Jupiter & Saturn, it would have only an imperceptible effect on the speed of the solar system. Even if ALL the planets were lined up on the front or back side of the sun, I don’t think the masses are sufficient to make a noticeable difference. And since some of the planet orbits take decades; it could be hundreds of years before most or all of the planets are aligned this way (if it ever happens).
I recall seeing pictures of the surface of the sun, with a solar flare projecting out from it into space, and then pictures of the earth (to scale) inserted alongside that flare – this flare was several earths high from the surface of the sun. And the sun itself covered the whole bottom of the picture, and yet from the curvature obviously you were only seeing a small portion of the sun’s surface. Your grandson would probably a picture like that, and it would probably help him get some idea of the comparative sizes. (I think it was on the Astronomy Picture of the Day site.)