Why Does the Same Side of the Moon Always Face the Earth?
The same side of the moon always faces the Earth. The “dark side” of the moon is not actually dark — it gets cycles of day and night just like most places on Earth — it is just so named because we never see it. The “far side of the moon” is a more correct term. The reason that one side of the moon is never visible from the Earth is because the moon spins once on its axis in precisely the same amount of time it takes to revolve around the Earth. If its rate of rotation were slightly different than its rate of revolution, we would eventually be exposed to the entire surface of the moon. However, these two intervals have been equal for all of recorded history, and probably for millions of years or longer. This otherwise bizarre phenomenon can be explained in terms of a subtle effect generated by gravitation and friction — tidal locking. Through their mutual gravitational attraction, the Earth and the moon create tidal bulges on each other. One bulge faces in the direction of the other bod
Dear Straight Dope: As every fourth grader (at least of my generation) is or was taught, the moon revolves once on its axis for every orbit around the earth. Thus, the same side of the moon is seen from earth at all times. OK, fine. But it recently struck me that it’s gotta be EXACTLY one r.p.o.; if it’s off by even half a footprint, before long we’ll be able to see the dark side of the moon for ourselves, rather that depending on Pink Floyd’s description. My question: Is this a cosmic coincidence? Or is there something regulating Luna’s spin? My first reaction is that maybe the moon’s mass is off center, and the earth’s gravity is holding the heavy side down. But then wouldn’t the moon revolve around its center of mass (rather than its center of volume) and thus appear to wobble? — Tim Snyder After I got done writing this up I discovered Cecil had already answered the question. But my feeling is that celestial mechanics is a topic of enduring interest. It’s not like because Shakespear
• Students will recognize the factors that contribute to the phases of the moon – The moon orbits around the earth once every 29.5 days. – The light we see from the moon is reflected light from the sun. – How much light (the part of the moon) we see depends on the positions of the earth, moon, and sun. • Students will identify the 8 different phases of the moon and the order they occur in. • Students will predict when and where the moon will rise on the horizon and know how that is related to its phase. • Students will know that the revolution and rotation of the moon occur at equal rates and result in the same side of the moon facing Earth. • Students will know that eclipses are due to rare alignments of the sun, moon and Earth. 6. Students will recognize the scale relationships between the sun, moon, and the Earth.
Dear Cecil: Why does the same side of the moon always faces the earth? It seems like quite a coincidence that it should rotate so perfectly in sync with us. Is there any slippage, so that parts of the dark side of the moon are slowly being revealed to us? — Trigby P., Northridge, California Cecil replies: You’re right to be suspicious about this, spud, but let’s clear up one thing first: there is no “dark side of the moon,” the popular expression notwithstanding. All of the moon is illuminated at some point during the month-long lunar day. It’s just that we can’t see when it’s high noon on the back side. As for the moon’s rotation, you’re right in thinking the timing is a little too neat to be coincidental. It was different once upon a time. Billions of years ago experts think the moon was much closer to the earth than it is now and rotated much faster, so that over time the entire lunar surface could be seen from earth. But “tidal friction” slowed the moon down. The earth’s gravity ca
