Between each day and night are three phases of twilight. The astronomical, nautical and – the brightest – civil twilight. Wait a minute, not every night. Ok, let me stop right there. That isn’t true for all places on our planet. Let’s take Oulu (surprise, surpise) for example, located at 65°N, summer nights are bright as daylight. The twilight phases I mentioned above become more and more important.
Being the darkest twilight zone, the sun is between 12° and 18° below the horizon. While faint stars and other objects are then tricky to observe, auroras are easily visible. The first day without proper night (pitch black that is) here in Oulu was actually yesterday, April 9th, 2015. Astronomical twilight is then the darkest phase for about two weeks.
That’s when the sun is between 6° and 12° below. It is bright enough to see the horizon. Auroras are partly visible then; more exact: the sun needs to be around 10° below or lower. Going towards summer here in Oulu, on May 17th will be no more nautical twilight.
That’s right, the sun is then between 0° and 6° below. Outdoors it’s bright enough for almost anything, no point to talk auroras. In Oulu these are our brightest nights and we have about 73 of them.
Going further North, the nights will be brighter and brighter. Breaking through the Arctic Circle, the sun will be visible throughout the night – unless the horizon is covered by mountains, etc.
Ok, ok, enough with the numbers :) If you are interested in your local twilights, head over to www.timeanddate.com , that’s what I did.
Last aurora season I caught the last ones on April 25th at 1.30am. As you can see with the bright skies towards North it was very very faint. This year I’ll problably sleep :)