2 ms·

I hadn't thought of this part. I don't think that's the only limitation, though. Even with a spherical planet, no atmosphere, and 0-diameter sun, there might a

I hadn't thought of this part. I don't think that's the only limitation, though. Even with a spherical planet, no atmosphere, and 0-diameter sun, there might at most be a circle on the planet that experiences equal day and night. The planet, after all, is still rotating around the sun as it is rotating around its axis. It doesn't stop rotating around the sun at precisely the moment that the Earth's axis is orthogonal to the Earth-sun gravitational vector. Hence the day and night lengths are continuously changing, not changing once per day. So I imagine that there is at most a path/line/circle on the Earth's surface that has equal night and day by this measure, and at a minimum, a point, but I'm not mathematically inclined enough to calculate this. Edit: Same line of reasoning: https://news.ycombinator.com/item?id=37629367 https://news.ycombinator.com/item?id=37629367

shkkmo 3y ago
That circle (or "longitudinal line") would only share the same length of day and night if it were measured from the center of the Sun crossing the horizon. This is because of another point glossed over the the article. The length of time it takes the sun to traverse the horizon varies and gets longer the further you are from the equator during the equinox (or the further from perpendicular the suns apparent path is with the horizon). This means that on the equinox, days nearer the poles are longer than days at the equator because we measure days as when any part of the sun are over the horizon. This becomes clear when you think about how at the equinox the north and south pole are in the middle of a week or two long sunrise/sunset.