September twilight countdown – earlier nights make happy astronomers

Twilight is that special time between night and day when sunlight reflects from the atmosphere and softly illuminates the ground below and sky above. With no air, the moon has no twilight. Credit: Bob King

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.

Each type of twilight depends on how far the sun is below the horizon after sunset or before sunrise. During civil twilight, there’s enough light to find your way around with ease and Venus is visible; in nautical twilight the horizon is indistinct and the first stars come out. True, dark night begins at the end of astronomical twilight. 10 degrees equals one fist held out at arm’s length against the sky. Credit: TW Carlson

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.

The sun’s position is shown at noon on the first days of summer, fall and winter in 2013 for the northern hemisphere. Notice how it drops southward beginning right after the summer solstice. Created with Stellarium

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.

Twilight length throughout the year at latitude 38.4 degrees, about that of St. Louis, Missouri. Credit: Todd Hunter / NRAO

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.

Looking west at sunset in mid-September from a location on the equator. The sun hits the western horizon and proceeds straight down, following its line of declination (vertical lines). Twilights are brief. Illustrations created with Chris Marriott’s SkyMap software

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, lines of declination hit the horizon at an angle. The Earth must spin a while longer (shown as a longer arrow) before the sun is 18 degrees below the horizon 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.

The lines of declination (vertical lines) curve upward below the horizon in the summer, making the sun take longer to reach 18 degrees below the horizon. We get longer twilights as a result.

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 sun’s path follows the more strongly downward-curving declination lines. It reaches -18 degrees much faster than in the summer, giving us shorter twilights.

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.

22 thoughts on “September twilight countdown – earlier nights make happy astronomers

  1. Interesting article. When I happen to be at twilight in an observing astronomical session, I like to take note of the times when stars and planets disappear at dawn, or appear at dusk, the time when there’s enough light to see landscape details etc. I too noticed we have shorter twilight in this month, and with faster changes: I saw an incredible pink clouds dawn a few days ago and it lasted very few minutes. BTW I sent you by email a picture of it, along with some astronomy pics you may find useful for the blog.

  2. Bob,

    Sorry if I’ve misread you, but you say: “In September, the sun intersects the horizon at a steeper, southward tilt compared to June. In an hour’s time, the sun dips farther below the horizon than it does during that same hour around the time of summer solstice”.

    However, the diagram right above that quotation shows the September path of the sun at a shallower angle than that of June’s, meaning, I would think, that September’s twilight would be longer than June’s.

    Later, you say: “At mid-northern latitudes in September, the rapid southward movement of the sun makes haste of daylight hours.” Yet your diagram for Duluth shows a ‘long’ twilight.

    Can you clarify?

    • Carol,
      Yes, I had to be careful with the term “steep” or “steeply” and constantly reference it to “southward”. The shallower angle is further toward the south in September compared to June’s path. The further south the sun sets in the northern hemisphere, the faster it gets out of the sky and the briefer the twilight.

      As for your second question, Duluth’s twilight is long RELATIVE to the tropics and the equator. We’ve got two things going on here – the sun’s seasonal angle to the horizon AND the latitude of the observer. Both affect twilight length.

      Does that help?

      • Got it! The lines curving upward and downward below the horizon cleared it up for me. I hadn’t thought of that before. Thanks Bob, for going to the trouble of rewriting part of the blog just for me!

        • Carol,
          I’m glad it makes more sense now. Your question helped me get to the bottom of why twilight length varies. In the process, it improved my and other readers’ understanding of the concept.

  3. Sort of. You say: “The further south the sun sets in the northern hemisphere, the faster it gets out of the sky and the briefer the twilight.” That would refer to a winter sunset, no?

    However, with the sun in a low zodiacal constellation in winter, wouldn’t the angle of its setting be a very shallow one, given the angle of the ecliptic, even more than your Sept. 10th illustration above? I still can’t see how that shallow angle would make the twilight shorter than the steep angle of the June sunset, when the ecliptic makes a much steeper angle to the horizon.

    Am I missing something? Sorry if I am! :(

    • Carol,
      Your excellent questions and persistence have led me to re-think and re-work a much better explanation for the variation in twilight length with the seasons. I’ve updated the blog – please take a look. Hopefully your question will be answered satisfactorily. Thank you!

  4. About 9:50 PM with Saturn setting, there are no longer any bright planets up. The Moon is the only bright object in our Solar System. Tonight will probably be cloudy anyway. But I still plan on going out for a drive, and a walk, just to enjoy the sights, sounds and smells of Earth on this warm late Summer evening.

  5. The longer nights are good but I don’t mind trading a little bit of darkness for a little bit of warmer temperatures;when it’s -10 deg F outside,I don’t care how dark it is,lol

  6. Apparently, if ‘twilight’ as counted as daylight, according to timeanddate.com on December 27th this year, day and night will be EQUAL length in London, England! Astronomical twilight starts at 6:02AM on December 27th, 2013 and ends exactly twelve hours later, at 6:02PM.

    Astronomical twilight starts at 6:02AM as well on December 28th, 2013 so the “night” (excluding twilight before and after) between December 27th/28th will be EXACTLY twelve hours long, contrary to what most people say with the longest nights being in December in the Northern Hemisphere!!!!

  7. It is just by coincidence that astronomical twilight happens to end exactly twelve hours after it started on December 27th in London, UK.

    • basically yes, coincidence. i mean as coincidental as anything is, anyway. some believe that everything is part of a grand plan, and nothing is truly coincidental, etc., but basically anywhere in the world at the same latitude as London would have that same amount of nighttime, so it’s not like London is unique or anything. go a certain amount N, and the amount of nighttime would change predictably according to latitude, and same going S. just happens to be that London is right at that spot on that particular day. others will get the 12 hours on different days, if at all. in the tropics this can never occur, i’m pretty sure, too much light!

      • Sean,
        Yes, the London twilight length is just a coincidence and would be the same everywhere at the latitude both north and south of the equator at many places on Earth. I think the writer just noticed it, thought it was cool and wanted to share it with readers.

  8. Good Morning AstroBob,
    My son had aprogram on his Samsung Galaxy that allowed us to go outside, point it at the sky and we could see all the constellations day or night that were in the sky at the moment, as you know with the naked eye the light pollution only allows an oberver see see a few start. We could turn around in circles and it would keep up and tell us what they were. Is there a program like that for a laptop? If so can you tell me the name of it? I really want to learn the consellations and this is a good start. As alwaysBob, thank you in advance… P.S. the star charts that are pics dont help because when you look at sky most are not able to be seen.

  9. i used to have this great restaurant parking lot nearby with pretty low views on all sides which, once the restaurant closed and it lights went off, was a great location to stargaze in an urban setting. a little under a year ago another adjacent building installed/started keeping on all night these bright lights which glare right into that parking lot, which i now no longer use. i still get the urge to shatter those lights. definitely have thought of shooting them. if only i had a b-b or something lol.

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