Grey line
The zone between day and night travels around the earth once a day. For Morse on the lower bands it is the most interesting place on the map.
Where it is what right now
| Place | Solar time | Sun elevation | State |
|---|
Why twilight matters so much
Sunlight builds up the D layer of the ionosphere at roughly 60 to 90 kilometres. It does not reflect, it absorbs - and the lower the frequency, the more. That is why during the day 80 metres barely reaches the next county.
After sunset the layer decays within minutes. The higher F layer stays and keeps reflecting. In that window - and again before sunrise - the same band suddenly carries across continents. That is the grey line.
The trick: if both stations sit on the grey line, the entire path between them runs through twilight. Then the absorption is missing along the whole route, not just at the ends. That is why DX hunters on 80 and 40 metres look for exactly this line.
What the map shows
The solid line is the terminator: there the sun sits exactly on the horizon. The two dashed lines bound the zone where it stands between six degrees below and six degrees above - the span in which the D layer is built up or torn down.
The zone is deliberately a band and not a line. A hairline boundary would be geometrically pretty and useless on the air; the work happens in the zone.
Above the map you can see where the sun is currently overhead. That point moves one degree west every four minutes, and the grey line moves with it.
Solar time, not clock time
The table gives solar time, not the clock at the place. It is derived from longitude: 15 degrees is one hour. It can differ from the local clock by up to two hours - through daylight saving, wide time zones and countries like China, which runs a single zone across 60 degrees of longitude.
That is on purpose. For the question "is the sun up there" solar time is the right quantity, and a time zone database would be half a megabyte for an answer nobody needs here.
Frequently asked questions
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How long does the window last?
At middle latitudes roughly 30 to 60 minutes around sunrise and sunset. At high latitudes it is considerably longer because the sun sits at a shallower angle - and in summer it does not set at all up there, so the grey line disappears entirely.
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Does this apply to 20 metres and above?
Hardly. The D layer absorbs low frequencies strongly and high ones almost not at all. On 20 metres and above the F layer decides, which means solar activity - see space weather. The grey line is a lower-band matter.
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Why does solar time differ from my clock?
Because time zones are political and the sun is not. Your clock follows a zone that can be 15 degrees wide, plus daylight saving. Solar time here follows longitude - it says how the sun stands, not what the clock says.