Where Is the Line Between Day and Night? Moving at a Thousand Miles an Hour
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The boundary dividing the lit half of a world from the dark half is a moving line whose shape, speed and sharpness say a great deal about the body it runs across.
What the line is
Any body lit from one side has a lit half and a dark half, and the great circle separating them is the boundary in question. On Earth it sweeps around the planet once a day, moving westward at roughly sixteen hundred kilometres an hour at the equator and slowing towards the poles. Standing on it means standing at sunrise or sunset. It is not fixed relative to the surface, and the surface rotates through it, so the line is a position in space rather than a place on the ground.
Why the shape changes
Its path across a flat map varies over the year:
- •It runs pole to pole only at the two equinoxes
- •At other times it tilts, cutting across the polar regions
- •At midsummer it leaves one polar cap entirely in daylight
- •And the other entirely in darkness
- •The tilt equals the tilt of the planet's axis
- •Which is why day length varies with latitude and season
Why it is not sharp
On Earth the transition is gradual rather than abrupt, and two separate effects widen it. The atmosphere scatters sunlight into the region beyond the geometric boundary, which is what twilight is, and that adds up to well over an hour of usable light at temperate latitudes. Refraction also bends light around the curve of the planet, so the Sun is still visible when it is geometrically below the horizon. A body with no atmosphere has neither effect, which is why the boundary on the Moon is a hard edge with black shadow immediately beside bright ground.
Why it is not a straight line
Seen on a flat world map the boundary is a curve rather than a straight line, and the reason is worth pinning down because it confuses people. The division is a great circle on a sphere, and any great circle other than the equator or a meridian appears as a curve when a sphere is flattened onto a rectangle. Nothing is bending. The same is true of the shortest flight path between two cities, which also looks curved on such a map. Seen from space the line is straight in the sense that matters, being the edge of a plane cutting through the centre of the planet.
What observers use it for
The line is where the most information is, which is why it gets deliberate attention. Looking at the Moon, the region along it shows the longest shadows and therefore the clearest relief, so craters and mountains that vanish at full moon stand out sharply there, and observers plan sessions around which features are near it. Spacecraft imaging of other worlds is timed for the same reason. Long distance radio operators exploit a related effect, since the ionosphere behaves unusually along the boundary and signals can travel exceptional distances by following it around the planet.
The takeaway
The great circle dividing lit from unlit sweeps westward around Earth at about sixteen hundred kilometres an hour at the equator, running pole to pole only at the equinoxes and tilting by the planet's axial tilt at other times. Atmosphere blurs it into twilight, and an airless body like the Moon shows a hard edge instead. Shadows are longest along it, which is where observers look.