Skywatcher's Guide: August and September 2018


Stars and Constellations

In August we can still see part of the spring sky at the beginning of the night after sunset. The bright star Spica in the constellation Virgo is low in the west-southwest. A little higher in the west is even brighter Arcturus in the constellation Boötes. The easily recognizable Big Dipper (Ursa Major) is also visible in the northwest. You can use two stars in the end of the bowl to find Polaris, the north star, which is the end of the handle of the Little Dipper (Ursa Minor). Next, high in the middle of the sky we see the summer constellations, with the three bright stars of the Summer Triangle being the most prominent. Vega is the highest and brightest of the three, with Deneb below towards the east and Altair to the southeast. Then Scorpius with the bright star Antares is visible to the south-southwest, and the "teapot" of Sagittarius is nearby towards the south. The summer Milky Way is prominent this time of year stretching all the way across the sky from south-southwest to north-northeast. Next, the fall sky is beginning to rise in the east at the beginning of the night. The "great square" of Pegasus is low towards the east and Andromeda is adjacent to the northeast. Cassiopeia is a little higher in the northeast, and appears as a "W" this time of year.

In September, Boötes is now low in the sky towards the west-northwest.  The Big Dipper is also very low in the northwest.  Polaris and the Little Dipper are of course still in the North.  Now Scorpius is low in the southwest, and we can see the Sagittarius "teapot" in the south-southwest.  The Summer Triangle is still up in the middle of the sky, and the summer Milky Way is still a prominent streak across the sky.  Next, Pegasus and Andromeda are higher in the east, and you might be able to find the Andromeda Galaxy. Cassiopeia is higher in the northeast and is starting to rotate to a "3" orientation. Finally, below that you may see Perseus along the horizon at the beginning of the night.

Interesting Stars Visible in August and September (during observatory hours)

Name / Designation Apparent Magnitude
(lower = brighter)
Arcturus -0.05 36.7  
Vega 0.03 25  
Capella 0.08 42  
Altair 0.76 17  
Spica 0.98 262  
Markab 1.25 140  
Deneb 1.25 3230  
Regulus 1.36 77 means "Little King"
Polaris 1.97 431  
Alpheratz or Sirrah 2.07 97  
Mirach 2.07 199  
Algol 2.09 93 variable star
Denebola 2.14 36.2  
Enif 2.38 670  
Almak 2.1 / 5.0 & 6.3 355  triple star system w/ 64 yr orbit
Albireo 3.2 / 5.8 & 5.1 390 / 380 possibly a triple star system
Eta Cassiopeiae 3.5 / 7.4 19 480 yr orbit

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Solar System

Mercury will be passing between us and the sun in early August, but will reemerge in the morning sky later in the month. It will then pass behind the sun in mid September.

Venus remains visible in the evening sky after sunset, reaching its highest point in mid August.

Mars is now coming up before sunset and can be found in the constellation Capricornus.

Jupiter remains in the constellation Libra in the southwest after sunset, but sets earlier each night.

Saturn remains prominent throughout August and September near the teapot of Sagittarius.

Jupiter Great Red Spot Transits during August and September (when the Flandrau dome is open)

Note: The GRS is visible on the disk of Jupiter for 50 minutes before and after meridian transit time.

Date Meridian Transit Time
08/02/18 07:41 PM
08/04/18 09:20 PM
08/09/18 08:29 PM
08/16/18 09:18 PM
09/07/18 07:36 PM
09/14/18 08:25 PM

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Calendar of Night Sky Events

Date Event


Last Quarter Moon.


Mercury at inferior conjunction. — Between us and the sun.


New Moon.


Peak of Perseids meteor shower.


Venus at greatest eastern elongation. — Visible after sunset.


First Quarter Moon.


Full Moon.


Mercury at greatest western elongation. — Visible before sunrise.


Last Quarter Moon.


Neptune at opposition. — Best time to see our farthest planet.


New Moon.


First Quarter Moon.


Mercury at superior conjunction. — Passing behind the sun.


Earth at Southward Equinox. Beginning of our Fall.


Full Moon.

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Deep Sky

There are many deep sky objects we can see since the summer Milky Way is high in the sky.  There are many open star clusters that can be seen with only binoculars scanning this part of the sky.  For example we have the Butterfly Cluster (M6) and Ptolemy's Cluster (M7) near the tail of Scorpius.  Further north there is the Wild Duck Cluster (M11) in the faint constellation of Scutum between Sagittarius and Aquila.  There is also the asterism of the Coathanger between Aquila and Cygnus in the fainter constellation of Vulpecula.  Next, heading towards the west we can see the Coma Star Cluster in the constellation of Coma Berenices, which is even visible naked-eye.  The Pleiades (M45) will be visible later in the night, after midnight.

There are a several globular clusters we can see as well, as the center of our galaxy is the highest it gets for the year.  Near the bright star Antares in Scorpius lies the globular cluster M4.  In Sagittarius we also have the Teapot Cluster (M22).  Of course we have the famous Hercules globular (M13) high in the east.  Also, M15 is visible in the east near the head of Pegasus.

For nebulae, we have several in the plane of the galaxy, one of which is the Swan Nebula (M17) in Sagittarius, also known as the Omega Nebula.  There is also the Lagoon Nebula (M8) nearby and the North America Nebula (C20) further north in Cygnus.  For planetary nebulae we have the Ring Nebula (M57) in Lyra, the Dumbell Nebula (M27) in Vulpecula, and the Blue Snowball (C22) in Andromeda.

And now the galaxies:  Although the Big Dipper is getting lower in the sky, you may still be able to find some of the galaxies in this part of the sky.  We have the Whirlpool Galaxy (M51) and the Pinwheel Galaxy (M101) near the handle, and the Cigar Galaxy (M82) and Bode's Galaxy (M81) near the bowl.  The spectacular Andromeda Galaxy (M31) is now coming up in the northeast, along with the nearby Triangulum Galaxy (M33).

Interesting Deep Sky Objects to Observe during August and September (during observatory hours)

Designation Name Apparent Magnitude Apparent Size Distance

Messier 45





open cluster

Messier 31

Andromeda Galaxy


3° x 1°


spiral galaxy

Messier 33

Triangulum Galaxy


67' x 42'


spiral galaxy

Messier 3

(in Canes Venatici)




globular cluster

NGC 7293 Helix Nebula 7.3 16' 450 planetary nebula
Messier 27 Dumbbell Nebula 7.4 8' × 6' 1,250 planetary nebula

NGC 7009

Saturn Nebula




planetary nebula

Messier 81

Bode's Galaxy




spiral galaxy

Messier 57 Ring Nebula 8.8 1' 2,300 planetary nebula

Messier 82

Cigar Galaxy





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Frequently Asked Questions

What is an opposition, and why is it special?

This is perhaps one issue late, as we just had a very close opposition of Mars in July.  But there are several planetary oppositions every year, and some of them are more significant than others.  In general, an opposition is when the object in question is opposite from the sun in the sky, which is usually when the object is closest to us as well.  That means it is the best time of year to see that particular object because it will look bigger and brighter than usual.

First, I should mention that Mercury and Venus never have oppositions.  They are always closer to the sun than we are, so they can never be opposite from the sun in the sky.  The best time to view these planets is when they reach maximum elongation, which is the biggest angle they can make with the sun from our perspective.

Mars has the most significant oppositions. Because its orbit is only about 1.5 times the size of ours, that means its distance varies from 0.5 to 2.5 AU (astronomical units = our average distance from the sun), or a factor of 5 from its closest to its farthest point.  Oppositions happen roughly every 26 months, but not all oppositions are the same:  Mars's orbit is fairly elliptical, so oppositions can actually range from 0.4 to 0.7 AU (a factor of 1.8 difference) depending on where it and we are in our orbits.  This year's opposition was a very close one, the closest we've had since 2003.  The next very close one will be in 2035.

Jupiter is the next planet out, with an orbit about 5 times the size of ours. This means its distance varies between 4 and 6 AU, or a factor of 1.5, with oppositions happening roughly every 13 months.  So it's not quite as variable as Mars, but still noticeable.  Jupiter's eccentricity is not as great as Mars, either, so oppositions don't vary from one to the next as significantly. (The difference is only a factor of 1.1.)

The other planets further out don't really have a noticeable difference in distance, but they can have a noticeable difference in brightness.  There is a phenomenon called the opposition effect, which makes things look brighter than expected when they are close to opposition.  At most angles, we are able to see shadows around objects (clouds, rocks, mountains, etc), which decreases their overall brightness.  But at opposition, the shadow is directly behind the object from our perspective, so we lose this dimming effect.  For example, Saturn only has a 1.2 difference factor in distance, but it has a 1.7 difference factor in brightness. (The rings contribute a lot to the opposition effect in this case.)

If you have any questions you'd like me to answer in the next issue of SWG, please let me know.  I'm also happy to take suggestions or comments, and also pictures if you'd like to send them.  Happy viewing!

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  • Cornelius, Geoffrey. The Starlore Handbook: an Essential Guide to the Night Sky. San Francisco, CA: Chronicle, 1997. Print.
  • Ottewell, Guy. Astronomical Calendar 2012. Raynham, Mass: Universal Workshop, 2011. Print.
  • Ottewell, Guy. Astronomical Calendar 2013. Raynham, Mass: Universal Workshop, 2012. Print.
  • Ottewell, Guy. The Astronomical Companion. 2nd ed. Raynham, Mass: Universal Workshop, 2010. Print.
  • Astronomy Magazine. February 2013. Volume 41, No 2.
  • Astronomy Magazine. February 2013. Volume 41, No 3.
  • Sky & Telescope. February 2013. Volume 125, No 2.
  • Sky & Telescope. March 2013. Volume 125, No 3.

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