"The sky is rich in silence, even lonely within it, as long as the dawn lasts, the sky is but a crucible of pink and crimson fire...its million flames, struck from the sun that lives beneath the horizon."....VE6AB
The Grey Line...
All ham's who operate on HF, know about The "grey line," the band around the Earth that separates daylight from darkness.
Propagation along the grey line is very efficient. One major reason for this is that the D layer, which absorbs HF signals, disappears rapidly on the sunset side of the grey line, and it has not yet built upon the sunrise side.
ham radio operators can optimize long distance communications to various areas of the world by monitoring this band as it moves around the globe.
There is nothing that I like better, than hitting the backroads of southern Alberta, as the first hint of dawn breaks on the eastern horizon, while spinning the dial on my HF rig connected up to one of the gearmotor antennas built in my home workshop and mounted on my mobile.
Link showing the center of it all.....