Seems odd that they go from not being near one another at all (to partially cover) to suddenly being perfectly lined up.
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The moon isn’t orbiting the Earth in the same plane that Earth is orbiting the sun(about 5 degrees off), so we only get eclipses when the line where those planes cross is pointing at the sun(twice a year), and only if the moon is also at one of those points at the same time(twice a month). The distance the moon passes above or below the sun looks like a sine wave, the points where it crosses are where the position is changing the fastest.
For another way of thinking about it, the diameter of the moon is about 3500km, the circumference of the moon’s orbit around the earth is about 2.4 million km. There’s an awful lot of places for the moon to be that aren’t between the sun and the earth.
The orbits don’t work like that. The moon isn’t spinning around the Earth once a day, Earth is spinning once a day while the moon orbits once every 27 days. There is no lead up to the moon passing between Earth and the sun.
Lunar eclipses are over in hours at the most. That’s how quickly these things move. They don’t start and fade over several days.
There are no solar bodies between us and the sun that are big enough from the perspective of the Earth to partially block out the sun. Venus would be the biggest besides the Moon, and it just looks like a dot as a star from Earth’s perspective. So sometimes it could pass between us and fhe Sun, but it’s so little we wouldn’t even notice.
And another fun fact, the Earth is the only planet in our solar system with a moon that is the right size for the planet’s distance from the sun to both appear the same size in the sky.
As for why the moon doesn’t block out the Sun all the time as it’s orbiting the Earth once a month, the plane made from the moon orbiting the Earth is a few degrees off from the plane made by the Earth orbiting the moon. So normally the moon will be a little bit higher or a little bit lower than it would need to be to block out the sun. Sometimes it’s close enough as it passes between the Earth and sun to partially block out the sun for a partial eclipse. In even more rare instances, it will be fully in front and completely block out the Sun for a total eclipse.
The sun’s apparent motion across the sky is 27 times faster than the moon’s apparent motion. There’s no “overtaking” that covers a number of days; it’s like a race car passing a horse and buggy.
They don’t line up very closely and are not in the same plane, and the angle changes quite rapidly, so the conditions are only right for a short time. Hours not days, and minutes in any given spot on the surface.
The moon rotates the earth in 27 days (roughly), or slightly more than 13 degrees of arc per night.
Take your hand and hold it out at armslength. Make the devilhorns sign with your fingers as far between them as you can. That’s about 15 degrees of arc, so move your index finger so that it’s not quite so wide and instead pointing straight. That should be about 13 degrees between the outer edge of your finger tips. That’s how far the moon has traveled since the previous night. Much too far away to block the sun.
Imagine you’re sitting in an office chair in a dark room and someone is pushing you in circles around a glow in the dark ball
You also have a yo-yo that you’re spinning around your head on its string, but you’re not spinning it horizontal, you’re spinning it at an angle. That angle does not change as you get pushed around the room.
Only occasionally will the yo-yo be directly between you and the glowing ball, and only for a very brief amount of time. There is no lead up to it, it just crosses and is gone.
This is what is happening during a solar eclipse.
This video might help you understand it better.
The other answers don’t seem to be addressing your main question, which is “why does an eclipse start and stop so suddenly, instead of coming on slowly over the course of days?”
The moon’s speed in its orbit around earth is around 1km per second. The moon’s width is 3474 km. That means it takes 3474 seconds, or less than an hour, for the moon to move its own width.
The width of the sun as seen in the sky is approximately the same as the width of the moon. So, to get from one side of the sun to the other, the moon just needs to move one moon-width, which we calculated takes less than an hour.
To calculate the exact time an eclipse lasts from a point of view on earth, you need to factor in other things like your movement on the surface of earth as it rotates and the movement of the earth itself, but the basic answer to your question is “the moon is going really fast, and it doesn’t take very long for it to move one sun-width in the sky, so it can go from not overlapping, to overlapping, to not overlapping again very quickly.”
I think OP is asking why doesn’t the moon & sun line up over a number of days, in a similar way to the Manhattanhenge event that slowly aligns over days until it aligns perfectly on one day.
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