Twilight, Two Comets, and Twelve Constellations

Here is your guide to the sky for May 22 to 28, 2017 – a week with a bounty of celestial sights…

Sun – Earth – Moon

Sunrise this week is 4:57 am and sunset at 8:08 pm and we are only about a month away from the Summer Solstice, but what about twilight?  Astronomers, sailors, and civil governments define twilight a bit differently.  Each group uses the horizon (the place where the Earth and sky appear to meet) as a guide, but astronomers define it as when the Sun is 18 degrees below the horizon, which is when the sky does not have any residual light from the Sun to interfere with observations.  Sailors use 12 degrees and this is called “Nautical Twilight” when both the horizon and some stars are visible making celestial navigation possible.  And Civil Twilight is when the Sun is a 6 degrees below the horizon, and sometimes called Dusk or Dawn. All twilight definitions depend on the geometry of the Sun or solar elevation angles – so go ahead and enjoy!

New Moon takes place on Thursday at 3:45 pm which is also when it is at Perigee, so look for the Moon the first two days of this week in the morning sky during “twilight” or at the end of the week right after sunset. Monday in the predawn sky the waning crescent Moon is just below Venus, and for a challenge try finding it on Tuesday morning low in “twilight” and a few degrees from Mercury.

Planets in the Sky

Mercury is very low in the eastern sky just prior to sunrise this week, try spotting it on Wednesday with the Moon.  Venus rises in the east at 3:20 am and makes a beautiful pairing with the Moon on Monday morning. Reddish Mars is low in the west at sunset, while Jupiter shines brilliantly in the south at that time. Saturn in Ophiuchus rises in the eastern sky around 10:00 pm and telescopes of two inches or more in diameter will reveal its stunning rings.

Starchart for this week at 8:30 pm – courtesy of

Constellations – Twelve of the Zodiac

Most folks are familiar with their Zodiacal Sign which is based on the Zodiac Constellation of Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus, Aquarius, and Pisces.  Instead of horoscopes (which by the way are complete malarkey, hogwash, and “alternative facts” – see my article on Zodiacal Constellations on the Eye on Maine Skies page) try finding the visible zodiacal constellation in your sky.  (Yes – these are actually real!)  Starting in the sky after sunset, look west and find the two bright twin stars Castor and Pollux marking Gemini, sweep east and look for the backward question mark which marks the head of Leo the Lion.  Between it and Gemini you can find a few faint stars marking Cancer.  After finding these three, continue eastward and look for bright Jupiter which is just above Spica which marks Virgo, finally in the south east look for a faint diamond shape marking Libra.  These five constellation mark the position of the Ecliptic or plane of the Earth’s orbit and knowing them will help you find the planets which are always near this line being in the same plane.  Now if you happen to stay up later, say to midnight, you can find Scorpius and Sagittarius.  Scorpius looks like a fishhook in the southeast with its heart marked by brilliant red Antares, and the teapot shape group east of it is Sagittarius which ancients thought was half horse, half man, but really it is easier to see a teapot. If you stay up to 2:30 am or are just an early riser, try finding Capricornus – supposedly a water-goat which looks more like a faint boomerang and Aquarius the water bearer which really looks just like a random collection of stars.  If you are up to see Venus in the pre-dawn sky you might catch a part of Pisces.  So on a given night, from sunset to sunrise, you can see a good number of them.

Two Comets – Johnson and Tuttle-Giacobani-Kresac

Comet Johnson and comet 41P (Tuttle-Giacobani-Kresak) are both visible in binoculars this week. Start with the fainter of the two, comet Johnson which is in Bootes.  Look on the eastern side of the herdsman.  See the finder chart here.  It is about 8.5 magnitude but is sporting two tails.  Next look for 41P which is a whole magnitude brighter at 7.5 and is passing between Hercules, Lyra, and Aquilla.  Here is a finder chart for it as well.  Both should be easy to find…..

Our weather is improving and as clear skies have returned, so get out there and try finding the comets, exploring twilight skies, or checking out zodiacal constellations.  Happy stargazing and keep your eye on the sky!

Shawn Laatsch

About Shawn Laatsch

Shawn Laatsch is the director of the Emera Astronomy Center and Jordan Planetarium at the University of Maine. He started his astronomy education career in 1984 and has directed planetariums in university and science center facilities, taught undergraduate astronomy courses, and given numerous lectures around the globe. He serves as President (2017 & 2018) of the International Planetarium Society, Inc. the world’s largest organization of planetarium professionals. Shawn has a passion for sharing astronomy and stargazing with people of all ages.