The Planets

Mercury

Innermost Mercury is lost in the solar glare until early October, when it re-emerges in the morning sky.

Venus

The Phases of Venus
The phases of Venus and the evolution of its apparent diameter. As the planet gets closer to Earth, its diameter increases yet its phase shrinks. TBGS Observatory

Venus starts the month low in the east during morning twilight. It reaches its greatest brilliancy, magnitude -4.8, on September 21. Viewers around latitude 40° north see Venus rise less than two hours before the Sun at the beginning of the month but about three and a half hours before at month’s end. It is almost 30° high an hour before sunrise on the last morning of the month.

Venus’ greatest brilliancy always occurs when the planet is showing a crescent phase. It then appears truly dazzling. Such an occasion late in the fall of 1887 brought widespread public speculation that the Star of Bethlehem had returned once more.

The planet’s disk shrinks from about 52” to 34” during September, while its phase thickens from approximately 10-percent to 34-percent lit. The narrow crescent will be easily visible in a small telescope or even in good, firmly braced binoculars.

Because Venus is so brilliant, you may be able to glimpse it right at sunrise or even after. Do not look at the Sun, but instead find Venus before sunrise one morning and note its location with respect to objects on the horizon. The next day you may be able to use the same landmarks to locate Venus shining serenely from beyond Earth’s blue sky.

Finder map (early September) – 30 minutes before sunrise, looking east.
Finder map (mid-September) – 30 minutes before sunrise, looking east.
Finder map (late September) – 30 minutes before sunrise, looking east.

Mars

Syrtis Major
This image of Mars taken by the Viking orbiter in 1980 shows the large, dark expanse of the extinct shield volcano Syrtis Major. USGS Astrogeology Science Center

Mars rises about two hours before the Sun as September starts but three hours before the Sun as the month ends. It shines at magnitude +1.8, so you will likely need binoculars to pick it out of the twilight glow, low above the eastern horizon.

The orange-gold world lies about 228 million miles (367 million kilometers) from Earth around mid-month. Coupled with the planet’s small physical size, this large distance renders Mars tiny in the eyepiece, just 4” across. Only the largest backyard telescopes will show surface features with any clarity.

Under good conditions, an amateur telescope will show Mars’ polar ice caps and the main dark areas. These were once thought to be seas but now are known to be regions where winds in the tenuous atmosphere have blown away the red, dusty material that covers most of the planet, exposing the darker layers below.

The two most prominent dark surface markings are the Syrtis Major in the equatorial region of the planet and Acidalia Planitia in the north; the Syrtis Major is V-shaped, and much the easiest to see. Do not expect to be able to make out a lot on Mars the first time you look through your telescope. Observing takes lots of practice, and you will find that many hours of experience will make an enormous difference to what you can make out.

Finder map (early September) – 30 minutes before sunrise, looking east.
Finder map (mid-September) – 30 minutes before sunrise, looking east.
Finder map (late September) – 30 minutes before sunrise, looking east.

Jupiter

Jupiter Imaged by Hubble
Gas giant Jupiter is the solar system’s largest world, with about 320 times the mass of planet Earth. This sharp Hubble image was taken on April 21, 2014. NASA / ESA / Amy Simon

During the first-half of September, Jupiter, king of the planets, is swallowed up in the glare of the even mightier Sun. Around September 15, Jupiter starts rising due east early enough before sunrise to become plainly visible about 20° lower left of Venus.

Jupiter will continue to rise earlier as Earth’s motion around the Sun carries us toward opposition with the giant planet in early March 2016. At that point, we will fly between Jupiter and the Sun, and they will be on opposite sides of our sky. Until then Jupiter will brighten further, because the distance between us is steadily decreasing.

The planet is 53 light-minutes from Earth on September 15, but this will shrink to only 37 light-minutes by March next year. Jupiter will then blaze at magnitude -2.5. On January 8, 2016, Jupiter prepares for that climax by starting its retrograde (westward) motion against the stars.

The orbit of Jupiter, like that of all the planets, is elliptical instead of circular. At perihelion (closest approach to the Sun) Jupiter comes within 4.95 astronomical units of the Sun. At its most distant point, called aphelion, Jupiter is 5.46 astronomical units from the Sun. The average between perihelion and aphelion is called the semi-major axis; Jupiter’s semi-major axis is 5.2 astronomical units.

Finder map (mid-September) – 30 minutes before sunrise, looking east.
Finder map (late September) – 30 minutes before sunrise, looking east.

Saturn

Saturn is the bright yellow “star” in the southwest during early evening. The ringed planet shines at magnitude +0.6 at midmonth, a little brighter than 1st-magnitude Antares, 12° to the southeast. Despite its proximity to the luminary of Scorpius, Saturn actually lies among the background stars of eastern Libra the Balance.

Saturn’s disk, which measures 16” across this month, tends to be bland. Yet careful scrutiny with a 6-inch or larger telescope reveals subtle atmospheric details. The most obvious are a couple of dusky bands and the darker polar regions.

Saturn’s rings currently span 37” and can never be described as bland or subtle. The rings tilt 24° to our line of sight, which affords excellent views. You should have no trouble seeing the dark Cassini Division, which separates the outer A ring from the brighter B ring. On nights with low atmospheric turbulence, the innermost, dusky C ring also comes into view.

Any telescope will show Saturn’s brightest moon, Titan, throughout its 16-day orbit of the planet. Glowing at 8th-magnitude, it passes due north of Saturn on September 7 and 23 and due south on the 15th. 4-inch or larger instruments will also reveal 10th-magnitude Tethys, Dione, and Rhea, all heavily cratered worlds that orbit Saturn in periods between two and five days.

Finder map (early September) – one hour after sunset, looking southwest.
Finder map (mid-September) – one hour after sunset, looking southwest.
Finder map (late September) – one hour after sunset, looking southwest.

Uranus

Uranus is an easy target. The giant planet is rising in the east as Neptune climbs higher in the southeastern sky. Look for it in the faint constellation Pisces, in the same binocular field as 5th-magnitude Zeta Piscium.

On September 1, Uranus lies 0.5° (the diameter of the Full Moon) due south of Zeta and about half that distance northwest of 88 Piscium, a 6th-magnitude star. The planet’s westward motion relative to the background stars during September carries it to a position 1.2° southwest of Zeta by month’s end.

Uranus is not one of the five classic naked-eye planets, but observers under dark skies should still be able to glimpse the first “discovered” planet without optical aid. It remains at magnitude +5.7 or brighter all year; around opposition October 12 it will be magnitude +5.6.

If you use a telescope at high magnification, on a night with good seeing conditions, Uranus appears as a tiny featureless disk with a pale greenish hue. The planet’s 27 known natural satellites are beyond easy reach of most amateur telescopes, for visual observations.

However, if you are equipped with one of the very large-aperture telescopes that have become common nowadays you will find it possible to glimpse the largest and brightest of the moons. Titania (magnitude +13.9) and Oberon (magnitude +14.1) will be the easiest because they attain the greatest separation from the glare of the planet.

Finder map – field width 15°, stars to magnitude +8.

Neptune

Neptune reached opposition and peak visibility on the American evening of August 31. During September, it remains visible almost all night, reaching its highest point around midnight local daylight time.

You can find Neptune with binoculars or a telescope in Aquarius the Water-Bearer, roughly midway between 4th-magnitude Lambda and 5th-magnitude Sigma Aquarii. The planet glows at magnitude +7.8 and its blue-gray disk appears 2.5” across, just big enough to be resolved under good conditions.

Denser than the other gas giants, Neptune probably has ice and molten rock in its interior, although rotational data imply that these heavy materials are spread out rather than concentrated in a small core. The atmosphere is swept by winds moving at up to 2,300 feet (700 meters) per second, the fastest found on any planet. At the equator, the winds blow westwards (retrograde) and beyond latitude 50° they become eastwards. Temperature measurements show that there are cold mid-latitude regions with a warmer equator and pole.

Neptune’s 14 known moons include Nereid, with the most eccentric orbit of any planetary satellite, seven times as distant from the planet at its farthest compared with its closest approach; and Triton, the only large moon in the solar system with a retrograde orbit, which is an orbit in the opposite direction to that of Neptune’s.

Finder map – field width 15°, stars to magnitude +8.5.

Pluto

Global View of Pluto
A global view of Pluto, taken by the New Horizons spacecraft from a distance of 280,000 miles (450,000 kilometers). NASA / JHUAPL / SwRI

Pluto wanders far from the ecliptic, where the major planets reside. It lies north of the Teapot asterism in Sagittarius, following a number of years in Ophiuchus and Serpens. Look for it as soon as full darkness falls, due west of magnitude +3.5 Xi2 Sagittarii at the following distances: 36’ (September 1); 41’ (September 15); and 42’ (September 30).

At magnitude +14.2, Pluto is hard to spot visually even under the best conditions. You will likely need at least a 10-inch scope, although a smaller telescope with a CCD camera attached will also work. Take images a few nights apart, and Pluto’s motion relative to the background stars will betray its location.

On July 14 this year, NASA’s New Horizons spacecraft has reached its closest point to Pluto, about 7,750 miles (12,472 kilometers) above the surface. The probe sent back breathtaking photographs of Pluto in high resolutions, which reveal a varied surface with frozen plains and ice mountains. Rising to an estimated 11,000 feet (3,500 meters) the mountains are likely composed of water ice. They are also likely young, with an estimated age of 100 million years or so.

After passing by Pluto, New Horizons now continues further into the Kuiper Belt. Mission planners are searching for one or more Kuiper Belt Objects (KBOs) approximately thirty to sixty miles in diameter for flybys similar to the spacecraft’s Plutonian encounter. As maneuvering capability is limited, this phase of the mission is contingent on finding suitable KBOs close to New Horizons’ flight path.

Coarse finder map – field width 10°, stars to magnitude +8.5.
Fine finder map – field width 1°, stars to magnitude +14.5.

The Deep Sky

High overhead on early fall evenings, in a busy region of the Milky Way near Deneb, is a rather ordinary double star. With a small telescope at 60x you will see 61 Cygni as a bright warm white primary with a slightly dimmer secondary, also warm white, lying about 30” to the south-southeast.

61 Cygni is more interesting for another of its characteristics. Like Barnard’s Star, 61 Cygni has an unusually high proper motion, causing it to move visibly against the more distant stellar background over the course of relatively few years. At present, 61 Cygni has a proper motion of more than 5” per year.

Since 1838, 61 Cygni also holds a grand place in astronomical history. That year, it became the first star to have its parallax measured. Using the Fraunhofer heliometer at Königsberg Observatory, German astronomer Friedrich Wilhelm Bessel determined that 61 Cygni has a parallax of 0.31” (very close to the modern value of 0.29”), implying that the star was 10.3 light years from Earth (today’s accepted value is 11.41 light years). The Royal Astronomical Society awarded him its Gold Medal for this achievement, which marked the first step towards accurately measuring stellar distances.

The very high proper motion of 61 Cygni results from three factors. First, the star actually moves very fast, clipping along at 67 miles (108 kilometers) per second relative to the Sun. Second, 61 Cygni’s motion is almost perpendicular to our line of sight. Third, the star is very close to us (it is in fact the fourth nearest star that is visible to the naked eye for mid-northern latitudes, after Sirius, Epsilon Eridani, and Procyon A).

For backyard observers whishing to monitor 61 Cygni’s proper motion over the course of several years, the second chart below shows the pair’s path from 1900 to 2100. Notice how the two components of 61 Cygni, A (magnitude +5.20) and B (magnitude +6.05), are passing either side of the 11th-magnitude star GSC 3168:590.

Finder map – field width 35°, stars to magnitude +6.5.
Map – 61 Cygni’s proper motion from 1900 to 2100.

Asteroids

The term asteroid means “starlike” and describes exactly how asteroids appear in a telescope; they are tiny points that never show any surface details. The brightest asteroid, 4 Vesta, reaches visual magnitude +6.5 and can be seen with the naked eye under ideal conditions. Hundreds of others reach at least magnitude +10 and are easy targets for amateur telescopes. 15 Eunomia is one of them.

This rock, 220 miles (350 kilometers) across, was discovered in 1851 by the Italian astronomer Annibale de Gasparis and named after one of the Horae, personifications of order and law in Greek mythology. It is the largest of the stony (S-type) asteroids, and somewhere between the 8th-to-12th-largest main-belt asteroid overall, containing 1-percent of the mass of the asteroid belt.

Starlike asteroids stand up to moonlight and urban sky glow better than deep sky objects or fuzzy comets, making them excellent targets for viewing any clear night. Eunomia appears high in the east in late evening, at the border between Andromeda and Pegasus, within easy reach of a 4-inch telescope.

Even though it will reach opposition only next month, on October 3, Eunomia is big enough that it remains reasonably bright for several months. It starts September at magnitude +8.4, but brightens to +7.9 by month’s end.

The chart below shows all background stars as bright as Eunomia. This will help you identify the asteroid using basic pattern-shapes. If you are not sure which dot of light is Eunomia, sketch the field including four or five stars, then come back in a night or two to see which one moved.

Finder map – field width 15°, stars to magnitude +9.

Comets

Australian amateur astronomer Terry Lovejoy discovered comet C/2014 Q2 Lovejoy one year ago, on the night of August 17. His name is already familiar to many stargazers around the world, as a pioneer of early digital SLR imaging and discoverer of no less than five comets.

The comet brightened to roughly magnitude +4 in January this year and became one of the brightest comets located high in a dark sky in years. Observers throughout the world managed to spot it with the unaided eye, even from mildly light polluted suburban locations.

Despite receding from both the Earth and Sun, Comet Lovejoy is still hovering around magnitude +10.5. It clips through the constellations Bootes, Hercules and Corona Borealis and, as the month begins, lies 2° northwest of 5th-magnitude Chi Herculis. A 6-inch telescope should reveal the comet as a small fuzz ball roughly 4’ across, not unlike a globular cluster.

For the best views, find a dark-sky site and observe around midmonth when the Moon’s additional light pollution does not interfere. In midevening, Comet Lovejoy lies well up in the northwestern sky. It is heading south, however, so it sets earlier and dips lower towards the horizon with each passing night.

Finder map – field width 20°, stars to magnitude +8.

Discovery Image of C/2015 F4 Jacques
Discovery image of C/2015 F4 Jacques, obtained with an 11-inch (0.28 meter) f/2.2 astrograph at SONEAR Observatory in Brazil. C. Jacques / E. Pimentel / J. Barros / SONEAR

As the sky darkens on September evenings, a solar system visitor lies nearly overhead for much of the Northern Hemisphere. Comet C/2015 F4 Jacques sweeps slowly within the borders of the constellation Lyra, glowing with the combined light of an 11th-magnitude star. It should be in the reach of a 6-inch telescope if you observe from a location with dark skies, far from the glare of the city.

At the beginning of the month, F4 Jacques is only 1.5° southwest of the 4th-magnitude semiregular variable star R Lyrae. As September progresses, the comet describes a graceful arch around the star, and by the 30th it lies 3° northeast of its initial position.

Do not expect a spectacular sight when you have the comet in your eyepiece’s field. C/2015 F4 Jacques appears only 1.5’ across, with a small and faint central condensation and no hint of a tail. Try to use a range of magnifications to see all of the comet’s features – while one magnification may give you the best overall view, the extremes can be revealing.

Finder map – field width 10°, stars to magnitude +9.

Meteors

Starting about August 28, the Aurigids remain active until September 5. Except for four impressive showings in 1935, 1986, 1994 and 2007, the shower has always produced only a few meteors per hour. Although no predictions for unusual activity have been made for 2015, and the near-full Moon will be in the sky too, the Aurigid shower is still well worth a try.

The Aurigids reach maximum on September 1, when less than five meteors per hour are expected, even when the radiant is high in the sky. The radiant (area from which the meteors appear to emanate) rises in the northeast about 10 P.M. local daylight time and is nearly overhead around 5 A.M. Most Aurigid meteors are quite Perseid-like, in that they are swift and often leave very nice trails in their wake.

The Aurigids come from Comet Kiess (C/1911 N1), a hurtling ice ball of unambiguous long-period character, which takes around 2,000 years to orbit the Sun. The comet last appeared in 1911, and after examining its movement, astronomers have determined that prior to that year Kiess swung by the Sun some time in the 50 years or so around 1 A.D.

Map – Aurigids radiant position.

By the time the Aurigids fade away, around September 5, another minor shower is in full swing. The September Epsilon-Perseids appear to radiate from a point in Perseus near 2nd-magnitude Algol. For North American observers, the peak arrives the evening of September 9, with ZHRs averaging 5.

The radiant area is well on-view all night from about 10 P.M. local daylight time for mid-northern locations. A thin waning crescent Moon, which rises around 4 A.M. local daylight time, will not interfere at all.

For a while, the September Epsilon-Perseids were included with the Delta Aurigids. They are normally dim and few, but in 2008 the shower produced an unexpected outburst of swift, bright fireballs. A similar event occurred five years later, in 2013, around UT midnight on September 9-10.

No one knows the source of the September Epsilon-Perseids. Whatever the parent is, probably a comet, its orbit must be highly eccentric. As astronomers (both professional and amateur) continue to gather data on this shower, orbital parameters will become more accurately known, possibly leading to a match.

Map – September Epsilon-Perseids radiant position.

Some meteors do not belong to any known shower. These are the sporadic meteors, caused by random bits of comet debris spread throughout the inner solar system. They appear randomly across the sky all year long.

In this month’s night sky, careful observers can expect around ten sporadics per hour during the morning hours and four during the dark evening.

Observing Aids

Northern Hemisphere’s Sky – This map portrays the sky as seen near 40° north latitude at 9 P.M. local daylight time in early September and 8 P.M. in late September.

Southern Hemisphere’s Sky – This map is plotted for 35° south latitude. It shows the sky at 8 P.M. local time in early September and 7 P.M. in late September.

Visibility of the Planets – The table provides general information about the visibility of the planets during the current month.

Phases of the Moon – This Moon Phase Calendar shows the Moon’s phase for every day in September.

Jupiter’s Moons – The diagram shows the positions of Galilean satellites on each day in September at midnight.

Sky Events

September 1 – 5 A.M. EDT: The Aurigid meteor shower is at peak activity. 1 P.M. EDT: The Moon is 1.1° south of Uranus.

September 4 – 6 A.M. EDT: Mercury is at greatest eastern elongation, 27.1° east of the Sun.

September 5 – 2 A.M. EDT: The Moon is 0.5° north of Aldebaran. 5 A.M. EDT: Venus is stationary. 5:54 A.M. EDT: Last Quarter Moon. 11 P.M. EDT: Asteroid 9 Metis is at opposition.

September 9 – 5 A.M. EDT: The Epsilon-Perseid meteor shower is at peak activity.

September 10 – 2 A.M. EDT: The Moon is 3° north of Venus. 7 P.M. EDT: The Moon is 5° south of Mars.

September 13 – 2:41 A.M. EDT: New Moon. 2:52 A.M. EDT: A partial solar eclipse is visible from southern Africa, Antarctica and parts of the Indian and Atlantic Oceans.

September 14 – 7:27 A.M. EDT: The Moon is at apogee, the point in its orbit when it is farthest from Earth.

September 15 – 2 A.M. EDT: The Moon is 5° north of Mercury. 2 P.M. EDT: Dwarf planet Ceres is stationary.

September 17 – 9 A.M. EDT: Mercury is stationary.

September 18 – 11 P.M. EDT: The Moon is 3° north of Saturn.

September 21 – 4:59 A.M. EDT: First Quarter Moon.

September 23 – 4:21 A.M. EDT: The equinox occurs; fall begins in the Northern Hemisphere, spring in the Southern Hemisphere.

September 24 – 1 P.M. EDT: Mars is 0.8° north of Regulus. 3 P.M. EDT: Pluto is stationary.

September 26 – 6 A.M. EDT: The Moon is 3° north of Neptune. 11 P.M. EDT: Asteroid 3 Juno is in conjunction with the Sun.

September 27 – 9:46 P.M. EDT: The Moon is at perigee, the point in its orbit when it is nearest to Earth. 10:47 P.M. EDT: A total lunar eclipse is visible from North America, South America, Europe, west Asia and parts of Africa. 10:50 P.M. EDT: Full Moon.

September 28 – 9 P.M. EDT: The Moon is 1° south of Uranus. 11 P.M. EDT: Asteroid 4 Vesta is at opposition.

September 30 – 11 A.M. EDT: Mercury is in inferior conjunction with the Sun.

The information provided on this page is accurate for the world’s mid-northern latitudes. Finder maps for the five naked eye planets are plotted for 40° north latitude, but can also be used from other latitudes close to 40° north. Except the two all-sky maps, all other maps can be used no matter the latitude. Local time (local daylight time during summer) represents the time of the reader.