I heard about time dilation is somewhat proportional to gravity, and that gravity force may vary by latitude and elevation.
So I am wondering, is there a significant time "speed" variation between if you're at the south/north VS at the equator? Is there also a significant variation whether you're at sea level or at the top of mount Everest?
By significant I mean at least 1 second per decade.
You are correct, time runs more slowly the deeper you are in a gravity well i.e., the closer you are to the surface of the earth. High-tech clocks can detect the difference in tick rate between sea level and the top of a bell tower, which means they could measure the tick rate difference between 30,000 feet and sea level- but it is far too small in any case for humans to notice. A search on the references furnished in the comments above will tell you exactly how much.
Regarding the difference between time at the pole and at the equator, this would occur too because the earth is not perfectly spherical but again the difference is miniscule.
Quoting: "[i]t is different but extremely insignificant." (below):
I see your point in that the sum of dilation is very low, however, I respectfully disagree that it is at all insignificant. You are correct that the sum of dilation, or time slippage, is insignificant, however, as “Hoagie” pointed out with the example of the ISS, we’re also looking at a reversal effect, which is absolutely significant. As noted, the difference between the ISS’ relative velocity (higher velocity = slower time) outweighs Earth’s diminished gravitational effect (further away from Earth = faster time). According to the gravitational effect by itself, the ISS should experience faster time, but it’s clocks actually run slower due to its velocity. To distinguish the two opposite effects, we can first use an example of gravitational effect without the interference of velocity. Here, we can measure the difference between a clock at sea level (closer to source of gravity = slower time) and a clock at the top of Mt. Everest (further away from the source of gravity = faster time). According to “PM” (above), over the entire history of the planet (~4.6 billion years), the clock at sea level would be ~39 minutes behind a clock that’s been sitting at top of Mt. Everest for the same period of time (well, fixed at that elevation until the Indian and Asian continents crushed together and thrust the Himalayas into existence). Bringing the effect of relative velocity into the equation by using the ISS as an example, you correctly noted that the ISS’ velocity (higher velocity = slower time) outweighs the opposing gravitational effect (further away from the source of gravity (Earth) = faster time), thus, its clocks run slower because it’s velocity outweighs the effect of Earth’s gravity which, absent velocity, would make the ISS’ clocks run faster. Although the amount of time slippage is insignificant, the existence of the reversal that exists due to the velocity is significant because it flips what would otherwise be reality upside down: Astronauts return home slightly younger than their companions who have never ventured into the high altitude of space, nor conquered Everest, whereas native populations in the Himalayans who also don’t venture into space grow slightly older than those companions, and are, in fact, even older in comparison to those same Astronauts.
