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The Different States of Twilight

by Geoff Chester, USNO Public Affairs | 09 May 2023

by Geoff Chester, USNO Public Affairs | 09 May 2023

Markarian's Chain, the Heart of the Virgo Galaxy Cluster
Imaged 2022 May 29 from Mollusk, Virginia with a 10.2-cm (4-inch) f/6.5 Explore Scientific AR102 refractor
and a ZWO ASI183MC CMOS color imager.

The Moon wanes in the morning sky this week, skimming the southern horizon before gradually turning northward as she passes into the rising autumnal constellations.  Last Quarter occurs on the 12th at 10:28 am Eastern Daylight Time.  Look for yellow Saturn five degrees to the northeast of the Moon before dawn on the 13th.  

We have reached the point in the year when the length of the astronomically dark night reaches a mere six hours.  We use the term “twilight” to describe the afterglow of the rising or setting Sun, and there are three degrees of twilight that are tabulated in our almanacs.  “Civil Twilight” is the time that the center of the Sun’s disc is six degrees below the horizon.  At the latitude of Washington that is generally half an hour after sunset or before sunrise.  This is the time when objects can be clearly seen, but we begin to lose color perception.  “Nautical Twilight” is the time when discerning the horizon at sea becomes difficult, and occurs when the center of the Sun’s disc is 12 degrees below the horizon.  “Astronomical Twilight” is the time when residual glow from the Sun is no longer visible, the Sun is 18 degrees below the horizon, and generally occurs 90 minutes after sunset or before sunrise.

The few hours of true darkness mean that it is time for the May sky-watching campaign for the Globe at Night citizen science program.  As with April’s campaign, the target constellation for May is Leo, the Lion, which you will find just west of the meridian at 10:00 pm local time.  Leo is led by the bright blue-white star Regulus, which sits at the base of a semi-circle of stars that form an asterism called The Sickle.  Leo’s hindquarters are marked by a right triangle, whose eastern apex is marked by the star Denebola.  Once you locate Leo, use the Globe at Night web app to record your observation of the number of stars that you see from your location.

Leo leads the spring constellations into the overnight hours.  These constellations lack the bright stars that are typical of the winter and summer skies, and from a dark sky site you will quickly understand why this is the case by the absence of the Milky Way.  At this time of the year we are looking directly out of the plane of the Galaxy through a relatively sparse veil of stars toward the vast emptiness of intergalactic space.  The majority of the “deep sky” objects visible to our telescopes shift from galactic star clusters and glowing gaseous nebulae to faint smudges of light betraying distant galaxies.  If you use a modest-aperture telescope to “sweep” the area of the sky bounded by Leo, the Big Dipper, and the bright stars Arcturus and Spica, you will spot dozens of these wisps of light wafting through the eyepiece.  While not as impressive as the bright star clusters or the Orion Nebula that graced the winter nights, these soft glows each represent a distant version of our Milky Way seen over a vast gulf of space and time.  The light from many of these galaxies began its journey toward us at around the same time as the dinosaurs became extinct some 60 million years ago!

Brilliant Venus is now reaching her highest position relative to the horizon for the current evening apparition.  She dominates the western sky as evening twilight fades to darkness.  She is now crossing the stars of Gemini, seemingly in hot pursuit of Mars.  By the end of the month she will be about ten degrees west of Mars, and throughout June she will slowly inch closer to the red planet as both objects move through Cancer and Leo.  However, Venus won’t quite catch up to the ruddy world, and during June and July they will both start inching closer to the Sun.

Mars gradually passes the Twin Stars of Gemini as the week progresses, and by the end of the week moves into the dim constellation of Cancer, the Crab.  His tiny disc is little more than a small pink dot in the telescope, and the increasing gulf between Earth and the red planet has caused him to fade by three full magnitudes from his brightest appearance last fall.

Golden Saturn should now be relatively easy to find in the pre-dawn sky.  You will find the ringed planet in the southeastern part of the sky.  Saturn gets a visit from the waning crescent Moon on the morning of the 13th.  Once Luna departs, Saturn should stand out among the faint stars of autumn’s rising constellations.


Commander, Naval Meteorology and Oceanography Command | 1100 Balch Blvd. | Stennis Space Center, Mississippi 39529

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