A twilight saga

While composing yesterday's post on Golden Plovers, I found myself querying my own understanding of 'dusk' and 'dawn', and whether I ever use them correctly.

"The Lapwings flew back to the estuary just before dawn."

So Mr. Thornton, it must have been completely dark and you can have only heard them, because-

Dawn is, officially, the time from the start of twilight to sunrise (and dusk the time from sunset to the end of twilight).

And twilight is the illumination of the atmosphere while the sun is below the horizon- any time our sky has some light in it. There are three clearly defined elements to twilight- first-

  - astronomical twilight: this starts when faint astronomical objects, such as nebulae, disappear. (Of course, light pollution messes this right up and we struggle to see such things) This period lasts until-

  - nautical twilight: starts when sailors can pick out and use the horizon for navigation. This middle period lasts until-

  - civil twilight: when you start to distinguish 'terrestrial objects'. (This is the one we come upon most frequently with respect to civil law- street lighting times, when we must have car lights on, etc.) It ends with-

Sunrise, the instance the upper edge of the sun appears above the horizon. Technically, sunrise lasts for just a fleeting moment. All those photos we see labelled 'sunrise over the estuary', or similar, are wrong- the sun has already risen. (Yup, that includes all of mine. My bad.)

Does this all matter? Not really, unless, like me, you are looking to make sense of bird behaviour. As I first typed this, during dusk, a Robin was tac-ing in the garden. Civil twilight. As I proof-read this, said Robin had gone quiet, in nautical twilight. Knowing those definitions help make a more specific interpretation of the event. Must try harder- I cannot remember the last time I said 'twilight' though.

Back to those Lapwings, flying back to the estuary, just 'before dawn'.

It wasn't just before dawn, it happened a long time after dawn had started. I could make them out above the treeline. It was during civil twilight. With their far superior eyesight, the Lapwings would have been seeing everything extremely well by this point.

Oh, and just to add to the fun. The angle of the earth, at different times of year, increases the length of each of those twilight periods during the summer months, to such an extent that anywhere on the globe above 48.5 degrees north (that's us here in Blighty) has dusk run into dawn- twilight never stops, complete darkness never happens. No wonder many birds can remain active 'at night' in the late spring(!)

Around midnight on our marshes, light pollution always plays a role. Even on a new moon, reflected artificial light off low cloud means you can see reasonably well to walk the marshes. It hardly ever gets dark. Bird activity is affected.

Only once recently has a nocturnal walk around here gone truly dark, when a sudden fog descended during a ringing session; the light from the Medway Towns could no longer reach me and, unlike that grey fog we get used to around town under streetlights, this was black, really black. Navigating my way back around the creeks that night was a fun experience I have no wish to try again in a hurry.

Finally, before anyone asks- night lasts from when the sun disappears completely from view to when the sun first reappears- sunset to sunrise. Those Lapwings were flying at night, but they could be seen clear as day.

Happy solstice everyone.