Actually the idea that the day must begin on a STRAIGHT LINE is an illusion.
Now if you start the day at midnight, it might work, but biblically each day begins at sunset.
This means that even if you chose a straight line along one single longitude, you can have a ten hour difference in the time the sun actually sets along that line.
So on any given longitude, the sunset doesn't set in a straight line following along that longitude, it can take more than a 10 hour span!
On a June 21 (summer solstice) the sun sets
--at the equator at 6:00 p.m.
--At the Arctic Circle it sets at 11:30 p.m. (close to midnight)
--At the Antarctic Circle it sets at 12:30 (noon)
On a Dec. 21, it's reversed
--at the equator at 6:00 p.m.
--At the Arctic Circle it sets at 12:30 (noon)
--At the Antarctic Circle it sets at 11:30 p.m.(close to midnight)
So when people argue the "straight line" with the illusion that the darkness falls in a straight line along a given longitude, have a wrong concept-- they haven't considered the problem of the earth's tilted axis.