There are many, many things wrong with the curiously popular "theory" that the world is flat. One that seems particularly timely right now is the equinox dilemma, given that the September equinox occurred just last weekend (September 22, to be exact) – so LiveScience investigated.
During the September equinox, the Sun crosses south over the celestial equator, marking the beginning of autumn and bringing with it shorter days and longer nights. (The opposite occurs in March when the Sun crosses northwards.) Here's the thing: This seasonal cycle kind of relies on the Earth being round (or globular) and having what is called an axial tilt, which makes it an awkward conundrum for anyone who believes the blue planet is disc-shaped.
So, how exactly do flat-Earthers explain the September (or, indeed, the March) equinox?
For starters, flat-Earthers would suggest shrinking the size of the Sun.
Conventional wisdom would tell you that the Sun has a radius of around 695,700 kilometers (432,289 miles), though this may be an underestimate. It would also tell you that it is 149,600,000 kilometers (92,957,130 miles) away from the Earth. Well, forget that. Flat-Earthers believe (or at least profess to believe) that the Sun is just 4,828 kilometers (3,000 miles) – which would make it very near and very tiny. To put it into perspective, the moon has a 1,737-kilometer (1,080-mile) radius and is 384,400 kilometers (238,955 miles) away from the Earth, 159 times further away from us than flat-Earthers say the Sun is.
On the Flat Earth Society Wikipedia page, the author describes the Sun as like a "spotlight" moving in circles around the North Pole, which flat-Earthers believe sits directly in the Earth's center. When it's directly over your head, it is (apparently) daytime. When it's not, it is nighttime.
To explain the seasons, some flat-Earth theories claim the Sun moves in a spiracular fashion (see diagram below). When its orbit is close to the North Pole, the northern hemisphere experiences summer – the days are longer and the nights are shorter. The reverse is true when the orbit is further away from the pole. On the day of the equinox, the Sun's orbit fits perfectly in line with the equator so that the hours of day and night are equal on both sides of the equator.
The argument doesn't hold water at the best of times. As LiveScience and YouTube user Wolfie6020 point out, it would mean that the Sun never really "sets" as such because it would never fully dip below the horizon.
But it is especially hard to explain during the equinoxes when half the Earth's surface is cast in sunlight at any one time and both sunrise and sunset are directly due east and due west everywhere except the poles. For this to happen on a flat-Earth, it would require some very interesting physics (and bendy light) – as YouTube user Flat Out explains in the video below:
But will this explanation convince prominent flat-Earthers like the rapper B.o.B that the Earth is not actually flat? Probably not.