Skywatcher's Guide: August and September 2023

Skywatcher's Guide written by: Lucas Snyder (Flandrau Planetarium Operator)

Navigation

• Stars and Constellations
• Solar System
• Calendar of Night Sky Events
• Deep Sky
• Frequently Asked Questions

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)

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Solar System

Mercury is visible in the evening sky for most of August.  It passes between us and the Sun in early September and then will be visible in the morning sky later in the month.

Venus may be visible in the evening sky for the first few days of August but will be passing between us and the Sun before the middle of the month.  It will then emerge in the morning sky later in August and remain visible throughout September.

Mars is getting lower in the evening sky after sunset, moving from Leo into Virgo.

Jupiter comes up a little after midnight in early August, steadily getting earlier until it rises before 9pm at the end of September.  It is in the constellation Aries.

Saturn rises early in the evening for most of August, eventually coming up at sunset by the end of the month.  It is in the constellation Aquarius.

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Calendar of Night Sky Events

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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)

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Frequently Asked Questions

What are seasons like on other planets?

With our equinox coming up (or just past depending on when you read this), we are transitioning from summer to fall here in the northern hemisphere.  But perhaps this gets you curious to learn about seasons on other planets in our solar system.

Seasons on Earth are caused by our axial tilt.  Our planet spins like a top, but the top is not perfectly vertical.  It is tilted about 23° if we consider our orbit as the table.  That means when we're on one side of the Sun, the axis is tilted such that the north pole is aimed more towards the Sun and the south pole is aimed more away.  Then half a year later, it is opposite.  Because of this angle, the Sun shines almost overhead during the summer and much closer to the horizon during the winter, causing the big contrast in temperatures we see between these times of year.  The equinoxes are the halfway points between these extremes.

Comparing the axial tilts of the other planets, Mercury, Venus, and Jupiter are all nearly perfectly vertical and so have very little variation due to tilt.  Mars, Saturn, and Neptune are all within the same ballpark as Earth and so they experience similar variations to what we are used to here at home.  (Note: Saturn's rings block sunlight from reaching parts of the planet, which increases the difference between spring/summer and fall/winter.)  Uranus is the odd one out with a tilt of 82°, giving it the most extreme seasons of all the planets in our solar system.

If Earth had the same tilt as Uranus, the Arctic Circle would extend all the way south to Panama, meaning everywhere north of that would experience continuous darkness for some period around the winter solstice and continuous daylight for some period around the summer solstice.  The Tropic of Cancer would extend all the way north to the upper reaches of Canada and Greenland, meaning everywhere south of that would see the Sun pass directly overhead once in the spring and once in the summer.  So Tucson, for example, would go from being even hotter than it already is in the summer (I know, hard to imagine) to colder than Alaska in the winter.

Besides tilt, there is one other factor that can create seasons:  orbital eccentricity.  Earth's eccentricity is less than 2%, so it doesn't affect us much here.  Mars' eccentricity is about 9% and has a more noticeable effect.  It is closer to the Sun near the southern solstice and farther near the northern solstice.  Therefore the southern hemisphere experiences about 50 F° greater seasonal variation than the northern hemisphere.  Mercury has an even larger eccentricity (over 20%), but since its day is twice as long as its year, it's hard to compare to Earth.  Depending on your longitude on the planet, you may experience the warmest temperatures in the middle of the day, or you may experience them near sunrise and sunset and have "noon" actually be a bit cooler (though still too hot for humans).  Pluto is another good example, with eccentricity near 25%.  When it is farthest from the Sun, the atmosphere partially freezes, greatly affecting its pressure.  We've not yet observed Pluto when it is farthest from the Sun, but we've already seen the pressure change by a factor of 3 in just 10% of its orbit.

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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Date of publication: 2023