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Stars come out during twilight this past Saturday night in time exposure taken about 11:15 p.m. Photo: Bob King

The other night while anticipating the aurora I kept looking north for signs of that familiar green glow, but twilight seemed to last forever. The sky was too bright right on through to 11:30 p.m.; only by 11:45 had it grown dark enough for that first blush of electrified air to  show.

Twilight does last a long time at higher latitudes in the northern hemisphere around the time of summer solstice, which happens Wednesday the 20th at 7:09 p.m. (CDT). For Duluth, Minn., the sun sets at 9:06 p.m. this week followed by almost three hours of twilight. Once night begins, hurry up and enjoy it, because morning twilight commences at 2:20 a.m. Night-sky gazing for sky watchers in the northern U.S. this time of year always means staying up late. Not so for those living in the southern states.

The sun’s shallow-angled path seen from Duluth, Minn. this time of year means it doesn’t drop to 18 degrees below the horizon until 3 hours after sunset. The farther north your latitude, the shallower the sun’s angle and longer twilight becomes. Illustrations created with Chris Mariott’s SkyMap software

In San Diego, Calif., latitude 32 degrees north, the sun sets about 8 p.m. and twilight’s over an hour and 40 minutes later at 9:40 p.m. What gives here? Why do southerners get an extra helping of night?

Twilight is the time between sunset and night (and night and sunrise) when sunlight illuminates the upper atmosphere, spreading a soft, indirect light across the sky and landscape below.

The beginning of true night, when the sun is far enough below the horizon that its rays no longer brighten the atmosphere, occurs at the end of astronomical twilight. This is defined as the moment sun is 18 degrees (almost two outstretched fists) below the horizon. You may also have heard of civil twilight, which begins when the sun is 6 degrees below the horizon. During civil twilight, the first stars come out and there’s still enough light to kick a ball around in the front yard with your kids.

The sun’s steeper path seen from San Diego means it drops below the horizon faster, making twilight much shorter.

At higher latitudes in northern spring and summer, the sun travels in a lower arc toward the horizon at sunset, intersecting it at a very shallow angle. It takes several hours for it to dip the requisite 18 degrees below the horizon for true night to begin.

In San Diego, the sun’s path is more steeply inclined to the horizon; it plummets the required distance in little more than an hour and a half. Near the equator the time is even less. Twilight lasts only an hour and 15 minutes in Bogota, Colombia this week.

North of the Arctic Circle during the summer months, the sun approaches the horizon but then climbs back up without setting. Credit: Free Image Archive

The opposite is true the further north you travel. Inky night never happens from mid-June to mid-July north of about 48 degrees latitude north. In places like International Falls, Minn. and Bellingham, Wash. there’s always a faint blush dusk low in the northern sky all night long.

Head up to the Arctic Circle at 66.5 degrees north and the sun doesn’t set at all on the summer solstice. Not only is there no night, there’s no twilight either. Continue north to the pole and the sun stays up for six months in a row.

I can handle staying up late OK, but by late August, sky watchers in Minnesota and Wisconsin feel a sense of relief when twilight ends by 10, leaving enough time for both star-gazing and shut-eye. Since that also coincides with the decline of the fearsome mosquito, being outside at night becomes a little more relaxing.

Article by astrobob www.astrobob.areavoices.com    Read Full Article