Darkness comes early now. I noticed it Saturday night when the sky rang with stars at 9 p.m. Two months ago the late-setting sun and long twilight held back the onset of night until after 11:30 p.m. September’s earlier sunsets and quickie twilights mean more nighttime viewing hours and the potential to get to bed at a reasonable hour. With the mosquitos nearly gone, late summer and early fall are the best times to explore the night sky.
The sun sets 1-2 minutes earlier and rises 1 minute later each day this month. Add it up and we’re losing about 3 minutes of sunlight each day … or gaining 3 minutes of darkness depending on your point of view. Twilight also reaches its minimum for the northern hemisphere. Here in northern Minnesota twilight length has shortened from 2 1/2 hours in July to just 1 hour 42 minutes and will remain there until next April.
Earlier sunsets and short twilights tip will soon tip the balance in favor of longer nighttime hours beginning the day after the fall equinox on September 22.
You’ve probably already noticed that the noonday sun is lower or farther south in the sky now compared to June. Every day since the summer solstice the sun has inched south and won’t stop until the winter solstice in December.
As each day clicks by, the sun’s rapid southward movement causes it meet the horizon sooner (set earlier) for northern hemisphere observers. As you might guess, observers in the southern hemisphere experience the opposite effect. A southward-moving sun keeps it up off the horizon a few minutes longer each evening, leading to later sunsets and longer days.
Not only does the sun’s rapid retreat to the south make for later sunsets, it also means shorter twilights. During twilight, sunlight continues to illuminate the upper atmosphere, dimly lighting the landscape and holding back the full brilliance of the stars. Once the sun dips to 18 degrees below the horizon, astronomical twilight is over and all traces of sunlight have left the sky. True night begins.
Since twilight length depends on how far the sun is below the horizon, the quicker it gets down to -18 degrees, the briefer twilight is. The shortest twilights are found at the equator (1 hour 10 mins), where the sun’s path is nearly always vertical to the horizon, and longest at the poles, where its path is nearly parallel to the horizon. Twilight lingers 6 weeks before the annual sunrise and after the annual sunset.
Twilight length varies according to the path the sun takes below the horizon. To understand why, consider that the sun’s position in the sky is defined by declination – the celestial equivalent of latitude – and right ascension, which is similar to longitude. After sunset, the sun continues to move along its line of declination. At the equator, those lines are perpendicular to the horizon, so once the sun sets, it quickly sinks to -18 degrees and twilight ends.
Away from the equator those declination lines intersect the horizon at an angle. To reach the required 18 degrees below the horizon, more time must pass. That means longer twilights.
Twilight is longer in summer than winter for the same reason. In summer, when the sun is much further north in the sky than in winter, the lines of declination curve upward below the horizon. This lessens or flattens the angle of the sun as it travels below the horizon, increasing the time it takes to dip to -18 degrees (twilight’s end).
In winter, the declination lines curve downward below the horizon, sending the sun to -18 faster than during the summer and putting an end to twilight in about an hour and a half.
Enjoy these idyllic nights, when darkness strikes a neat balance with daylight creating both pleasant temperatures and earlier nights.