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Telescope House September Sky Guide

Telescope House Hosted by Bresser UK September 2023 Sky Guide 

September heralds the Autumnal Equinox in the Northern Hemisphere and the Spring Equinox for those in the Southern Hemisphere. This year, these phenomena take place on September 23rd, a day when, briefly, day and night approach equal durations. The term Equinox” has its roots in Latin, with Equi” denoting equal and Nox” meaning night. However, the true balance of day and night on this date varies based on one's location; there are limited spots on Earth where the 23rd of September witnesses a genuine equilibrium. Importantly, September 23rd signifies the moment the Sun transitions into the southern celestial hemisphere. This leads to longer nights in the Northern Hemisphere and shorter ones in the Southern Hemisphere. While many who dont particularly delve into astronomy might lament the diminishing daylight in the Northern Hemisphere, enthusiasts who follow this Sky Guide likely view it differently. For those of us stargazing in the upper Northern latitudes, the descent into winter brings its own rewards.


As ever, there's a lot to see in skies above us this month...


The Solar System


The Moon


As August was a month which featured a so-called Blue Moon (two full Moons in one calendar month), we begin September with the Moon at just past Full phase. A resident of the zodiacal constellation of Pisces, the Moon rises on the evening of the 1st at just before 9 pm (BST). Common sense dictates (though we will mention it anyway) that the beginning and ending of September will not be the most prime opportunities for observing, fainter deep sky targets, due to the well-illuminated Moons presence throughout the night. The moon will transit at just past 3 am (BST), the following morning, and will be flanked to the west by Neptune and Saturn and higher to the east, by Jupiter.


The next couple of days finds the Moon tracking through Pisces into Aries, where it will have a reasonably close encounter with Jupiter on the evenings of the fourth and fifth. The following evening, 6th September, finds the Moon at Last Quarter in Taurus, after which, the moon will begin to descend towards the Sun, displaying a crescent phase. This time of year, is analogous to the Moons High Spring Crescent phases” of springtime. But instead of the Moon appearing high in the sky at crescent phase in the evenings, at this time of year, due to the acute rising angle of the ecliptic plane, the Moons separation from the horizon in the northern hemisphere in the morning sky approaches maximum and leads to extremely favourable conditions for observation.


As the Moon crests over the topof the northern ecliptic, through the constellations of Gemini and Cancer over the next couple of days, the rapidly diminishing crescent appears to almost lie on its back” in relation to the horizon. This view of the Moon is commonly enough seen from the equatorial and tropical regions of the planet, where the ecliptic plane tends to run almost right overhead, but much more of a rarity for those of us resident in higher latitudes.  Ancient cultures often referred to this view of the Moon as boat-like. The most famous example of this is the Egyptian legend of the sacred moon-boat of the scribe god Thoth. When the Moon appeared in the sky on its back” in crescent phase, it was said that this was Thoth going about his heavenly perambulations in his boat.

The Moon crosses over the border into Leo on the morning of the 12th September and can be found almost in line with the dazzling planet Venus at sunrise.  By this time, the moon will be displaying a 6.9% illuminated crescent phase and will be progressively more difficult to find in the sky closer to dawn.


The Moon comes to New phase as it joins the Sun on the Leo/Virgo borders on 15th September. After which it will re-emerge as an evening target. The minutely illuminated crescent moon will pass the diminutive Mars in the early evening sky on September 16th. However, this conjunction will be unlikely to be witnessed by many – if any - observers, due to the proximity of both bodies to the Sun and the horizon.


The next few days will find the Moon traversing further eastwards, through the wide expanse of Virgo, into Libra and Scorpius, until it reaches First Quarter in Sagittarius on the evening of the 22nd.  Just as the Moons morning crescent phase appears high in the sky at this time of year for northern hemisphere observers, the evening crescent phase appears very low in the sky. This relatively poor separation from the horizon of the evening crescent Moon at this time of year will naturally have consequences for seeing conditions for those with telescopes. While always encouraging those with telescopes to make the most of observing our natural satellite, we advise to keep magnification sensibly low, in order to preserve the quality of view. The closer an object is to the horizon, the more one does battle with air currents and truncation of the atmosphere, leading to poor visibility.  And the larger the aperture of Telescope you have, the more efficiently it will resolve atmospheric turbulence. When you look at the Moon through a telescope in the northern hemisphere, at this time of the month, youll see what we mean!


The Moon then begins to climb out of the most southerly part of the ecliptic, passing through Sagittarius, Capricornus and on into Aquarius, where it will meet Saturn again on the evening of the 26th.


The Moon becomes Full on the Aquarius/Pisces borders on the 29th and ends the month the next evening on the Aries/Pisces borders, a little to the west of the very prominent Jupiter.






Mercury begins September in a poor position and unobservable for northern hemisphere observers. The solar systems smallest true planet is headed sunward and reaches inferior conjunction on 6th September.


As with anything Mercury-related, we dont have to wait too long for the situation to change from the dire, to the very much more favourable.  As mercury reemerges from the Sun into the morning sky from the latter part of the month, it becomes observable and rapidly brightens. As we mentioned in regards to the Moon in the morning sky, the area of the ecliptic (Leo) that Mercury finds itself in at present, rises at a very steep angle for observers in the northern hemisphere. This increases the separation of Mercury from the horizon, leading to a very favourable morning apparition. By the time mercury reaches maximum western elongation from the Sun on September 22, it will stand over 15° high due east at daybreak (from 51° north). By this time, the planet will be a visual magnitude of -0.3 and display a 7.2 arc second diameter disc, illuminated by just over 47%. 


The next few days sees Mercury climbing in visual magnitude, as pulls further around its orbit and increases its illumination, as seen from Earth.


By the time we get to the end of the month on the morning of the 30th, Mercury will be -1.0 magnitude and stand an altitude of just over 13° high (again, as observed from 51° north), as the Sun rises. This apparition of Mercury in the morning is one of the most favourable of the year, so those early risers are encouraged to make the most of it.



Venus has emerged rapidly from mid-Augusts inferior conjunction and starts September as a resident of Cancer.  A morning target, or nearest neighbour presents a visual magnitude of -4.4 and displays an 11% illuminated disc, of just under 50 seconds diameter. Venus will stand just under 18° high in the east as the Sun rises on the morning of the 1st (as observed from 51° north).


Venus has a much wider orbit than Mercury, and as a consequence, makes its way through the skies at a much more leisurely pace.  By mid-month, the planet will have increased its illumination to just under 24%, but will have shrunk somewhat to just over 40 orc seconds diameter. However, despite shrinking inside, Venus has gained slightly in brightness from the months beginning and is now -4.5 magnitude. The reason for this increase in brightness, despite the planets apparent shrinking in size, is that its illuminated area is slightly larger at this point in time than it was at the beginning of September. 


By the time we reach the end of the month, Venus has remained static in brightness at dazzling -4.5 magnitude, while decreasing size to just over 32 arc seconds diameter. The planet is now illuminated by around 36%, which means that its illumination area increase has kept pace - and indeed balanced out - its decrease in size.




As mentioned previously, Mars is an incredibly disappointing target at present. On the evening of the 1st of September, Mars is to be found in Virgo at a magnitude of +1.7 and an apparent size of just 3.8 arc seconds. As Mars is situated in a very shallow setting part of the ecliptic, at present, it is very easily lost in the glare of dusk.

As the month progresses Mars tracks closer to the Sun, decreasing any chance of finding it in the evening sky significantly. Amazingly, the Red Planet is, at the end of September, still over a month and a half away from superior conjunction (the opposite side of the Sun, as seen from Earth). It will be sometime after reemerges from this before it is a worthwhile target for observation again.







Where is Mars is languishing at present, Jupiter is brightening in anticipation of early Novembers opposition and arguably the planetary highlight at present. The planet is currently to be found in Aries at a magnitude of -2.6 and an apparent size of 44 arc seconds, as seen on the morning of the 1st. 

Jupiter will rise at a little after 10 pm on the evening of the first, transiting at a little before 5:30 am and setting at just before 1 pm the following afternoon (all times, BST).


As the planet is now firmly a northern celestial hemisphere target, its presents those of us in the northern hemisphere with a very favourable opportunity for observation. At transit point on the 1st (5.26am BST), the planet stands over 54 1/2° high due South (as seen from 51° north). This separation from the horizon is significant, as it puts the planet in an area of sky that is much less unencumbered by atmospheric turbulence and truncation. Naturally, this only applies to northern hemisphere observers. Those observers in the higher latitudes of the southern hemisphere have had the benefit of Jupiter in a favourable position for them for the past six years, but solar system dynamics now favour us northerners”.  However, its worth mentioning that for observers in the equatorial and tropical regions of the Earth, this Jovian hemispherical change is somewhat moot, as the ecliptic tends to run close to overhead throughout the year.


Jupiter reaches static point in the sky around early September (4/5th) and afterwards begins its retrograde path through the sky, which outer planets always do in anticipation of opposition. Weve covered the mechanics of retrograde motion in previous sky guides, but for those unfamiliar with the situation, this is where a planet appears to move backwards” in its path through the sky. This is not because the planet has reversed direction, rather that the Earth has caught upwith the outer planets position in the solar system, on its faster interior orbit, making the outer planet appear to move backwards in relation to its proper motionthrough the sky. The easiest analogy to use is that of occupants in a car moving overtaking another car. The car that has been overtaken would appear to move backwards from the perspective of the occupants in the overtaking car, even though both cars are headed in the same direction. This is a rather simplified - but effective - way of describing retrograde planetary motion.


By the time we get to mid-month, Jupiter has increased its brightness to -2.7 magnitude and now displays an apparent size of just under 46 arc seconds. The planet will now rise as a little after 9 pm, transiting at around 4:30 am and sitting at just before noon, the following afternoon.



Fast forward towards the end of the month and Jupiter has gained fractionally again in brightness to -2.8 magnitude and now displays a 47.6 arc second diameter disk.  The planet will now rise at around 8 pm, transiting at a little before 3:30 am the following morning and setting a little before 11 am (again, all times BST).

For those up early enough, there are some Jovian transit highlights to observe from Europe during September (all times BST):


On September 9, sees a Great Red Spot (GRS) and grazing transit of Jupiter’s South Pole by Callisto, peaking around 6:16 am. Callisto transits are fairly rare at present, as the moon is the furthest orbit out of Jupiters Galilean satellites (orbiting it once every 16.69 days) and its orbit barely intersects the Jovian disk currently.


September 13, finds a mutual GRS and Io transit, with both transits visible at around 6:16 am.


September 19 find a decent mutual GRS and Europa transit peaking at around 1:15 am.


September 22 sees a mutual GRS, Io and Io shadow transit, peaking at around 12:15 am.


September 26, sees a GRS, Europa and Europa shadow transit peaking at around 2:15 am.


September 29 at around 1:16 am sees a transit of Io, the GRS and Ganymede. 


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