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Twilight Sights For Summer Nights

by Geoff Chester, USNO Public Affairs | 25 May 2021

by Geoff Chester, USNO Public Affairs | 25 May 2021

Globular Cluster Messier 3 in Boötes, imaged 2020 April 19 from Alexandria, Virginia
with an Explore Scientific AR102 10.2-cm (4-inch) f/6.5 refractor and a Canon EOS Rebel T2i DSLR.

The Moon wanes from Full to Last Quarter this week, skirting the southern horizon before climbing northward among the rising constellations of autumn.  Last Quarter occurs on June 2nd at 3:24 am Eastern Daylight Time.  Look for the Moon about five degrees below Saturn in the pre-dawn sky of the 31st.  On the following morning she will be a similar distance below bright Jupiter.  

As we mentioned last week, observers in the western United States will see at least parts of a total lunar eclipse on the morning of the 26th.  Here in the Washington area it won’t be much of a show as the Moon enters the Earth’s umbral shadow shortly before she sets.  Save your early-morning activities for the planetary conjunctions at the week’s end.

It’s a bit hard to believe that we are approaching the shortest nights of the year, but as the Sun’s declination change begins to slow as he approaches the solstice on June 21st that is exactly the case.  Here in Washington we will see just 5 hours and 40 minutes of darkness between astronomical twilights this week, while at the time of the solstice we’ll experience 5 hours 13 minutes between dusk and dawn.  That leaves precious little time for skywatchers to enjoy the fruits of the night.  However, we can cheat a little bit if we don’t mind looking at brighter objects in earlier times of twilight.  There are actually three types of “official” twilight, each defined by the Sun’s apparent depression below the horizon.  Most of us are familiar with “Civil Twilight”, which begins or ends when the center of the Sun’s disc is six degrees below the horizon.  This is the time when most places require car headlights to be turned on, and colors become difficult to discern with the naked eye.  “Nautical Twilight” is the time when the Sun is 12 degrees below the horizon.  At this time mariners navigating with sextants cannot distinguish the horizon over the open ocean and must rely on artificial horizons to obtain a position.  “Astronomical Twilight” occurs when the Sun is more that 18 degrees below the horizon and no longer contributes any ambient light to the sky.

So what is there to see in those times between nautical and astronomical twilight?  For most observers in urban and near-urban areas this is about as dark as the sky is going to get, so you can start exploring brighter stars and “deep-sky” objects.  The current sky abounds with a number of these targets.  Last week we described the bright double stars Mizar, Algieba, and Porrima in the constellations of Ursa Major, Leo, and Virgo, respectively.  This week we will focus on the constellation Boötes, which hosts a number of interesting double stars.  The first of these is Epsilon Boötis, popularly known as Izar.  To the naked eye Izar is the second-brightest star in the constellation and lies about 10 degrees northeast of the bright star Arcturus.  In a modest telescope the star resolves into a yellow and blue pair nestled very close together.  The duo’s discoverer, F.G.W. Struve, gave it the name “Pulcherrima” when he found them in 1829.  Some 25 degrees northwest of Arcturus is the second-magnitude star Cor Caroli, part of the diminutive constellation of Canes Venatici, the Hunting Dogs.  It, too, is a fine double star that will easily resolve in a good spotting scope.  If you trace a line between Arcturus and Cor Caroli and point your telescope about 1/3 the distance from Arcturus, your eyepiece should reveal a diffuse glow.  In a six-inch telescope this glow will begin to resolve into a myriad of tiny stars, which become more numerous with increasing aperture.  This is Messier 3, a deep-sky object called a globular star cluster.  About 150 of these objects circle the hub of our Milky Way galaxy, and we are entering the time of year when they are most visible.

Venus and Mercury are still holding court low in the west as civil twilight ends.  Venus should be easy to spot, but Mercury may be more of a challenge.  That said, on the evenings of the 27th through the 29th the two planets will be about a degree apart.  They will be closest on the evening of the 28th, when just half a degree separates them.  Use binoculars to locate Mercury. 

Mars continues to plod through the stars of Gemini with Venus hot on his heels.  The red planet spends the week within a few degrees of Pollux, the eastern pair of the Gemini twins.  Next week he will form a line with Castor and Pollux as he leaves Gemini for the obscure constellation of Cancer, the Crab.

The giant planets Saturn and Jupiter are still firmly entrenched in the pre-dawn sky.  You will find them above the southeast horizon about an hour before sunrise.  Watch the waning Moon glide by them on the mornings of the 31st and the 1st.  They should make for an attractive “photo op”.


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

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