Artema’s Universe

Barrycenter of a hollow sphere

The gravitational barrycenter is a fascinating thing. It creates a point between to massive objects in which their joined revolution is centered. The earth and our moon has one 1,100 kilometers beneath the earth’s surface. Pluto and Charon have one situated between the planets. If you were to have an imaginary hollow sphere made out of a very thick and massive material, and you were stuck inside, where do you think you would come to rest? Would you fall to the center? Would you fall to the edge? As it turns out, the mass of the side to which you are closest has a pull identical to the far end of the sphere’s mass. As it is farther away, its gravitational forces are lessened in perfect synchronicity. This is another of the many miracles of pi. Complete equilibrium throughout the entire structure. You would feel weightless as the gravitational pull of all sides counteracts itself.

Continue reading Barrycenter of a hollow sphere…

Tides: More than the ocean waves

The average person upon hearing the term tides would imagine the ocean waves receding and advancing on the beach. They would be wrong. This is only an affect of the tides produced by the gravitational disturbances related to the moon and the sun. You might think that the sun, being our closest star, and holding a volume of about 24,000,000,000,000,000,000 cubic kilometers (24 quadrillion km3) of hydrogen, would create great tidal forces. This is true, but at our distance of 149,600,000 kilometers average (1 AU) its effects contribute to only 10% of earth’s tides. The moon, while only having a volume of 385,847,000,000 cubic kilometers (385 billion km3) of rock, actually attributes to the other 90% of the tidal forces felt on earth. This is because our moon is only 384,400 kilometers away. The locality of a much smaller body affects the earth far greater than such a large and distant body as the sun.

This leads to the question of how these tidal forces create the motion of the ocean. To picture this you have to understand the way the gravity on earth holds things together, and the resulting layers of matter. To start you have the inner and outer cores. These rotate within the earth at a speed approximately 550 miles per hour faster than the earth itself rotates. You then have the mantle, above that is the crust, above that is the oceans, and above that is the atmosphere (which has many layers itself). These, over billions of years, have been sorted through gravity and the rotation of the earth. The abundance of liquid water on earth added to the earth’s malleability, speeding up the process of separation.

The tides most easily affect the atmosphere. From our perspective on earth we cannot see this. The space shuttles and other non-earthly observatories have noted tremendous bulging of the earth’s gasses, creating an oblong near egg shape preceeding the moon’s revolutions along the equator. Water on earth’s surface affect is minor; to the point it will recede and advance only in terms of a several meters. The earth’s crust actually bulges as much as a 30 centimeters as well. The question remains, how does the gravity from our moon and sun play with earth’s surface?

The gravitational pull between earth and our moon actually creates a point of equilibrium, called a barrycenter. At this point no gravity is felt. I’ll explain this more in another post, but the earth is 12,756 kilometers in diameter, and our barrycenter is revolving at only 1,100 kilometers below the earth’s surface. This creates a literal vacuum at the point between the earth and the moon, and our atmosphere and oceans are the only matter which can significantly fill the void, which creates the bulge. Now there approximately two tides per day, so from where does the second bulge come? On the opposite side of the globe we have an area of slightly lower gravity, so the oceans fill that void as well.

If you remember I said that the ocean’s and atmosphere’s bulge preceeds the revolutions of the moon, rather than following it. This is caused by the rotation of the earth, pulling them slightly ahead of the moon’s pull. The matter in these bulges, as with any matter, has gravity of its own, and it pulls the moon as well, giving it more orbital energy, which in turn pushes the moon away from the earth at a rate of 2 centimeters per year. You might ask if the bulge can pull the moon, does the moon pull the bulge? As the bulge preceeds the moon due to rotation, and it is being pulled backwards, does this slow the rotation of the earth? Yep. We lose 0.002 seconds every day due to this.

The aforementioned effect of negative rotational acceleration due to the tides has caused the moon to have one side always face the earth. The same effect will one day have the earth’s most massive side always facing that side of the moon. Someone on the other side of the world may never see the moon! Billions of years ago the moon’s rotation was not locked with the earth, and was much closer. This would have been a sight, as the moon would take up a much larger portion of the sky, and you could see all sides! The earth’s days at this time would also be much shorter, in terms of 18 hours or so.

The moon revolves around the earth about once every 27 days. This lunar cycle controls countless things, including the menstrual cycle of countless animals, among other interesting things.