What’s Up! September 2021

A monthly look at astronomical events in the sky and on Earth 

Compiled by John Rowland

Well, here we are again at the start of another astronomical season. Autumn can offer quite pleasant observing opportunities as the ground still retains some of the residual heat built up over the summer and the evening temperatures don’t drop too low. In addition, September and October are two of the most atmospherically stable months, with less rain, less cloud and a higher proportion of nights with good seeing.


So what should we give our time to this month? Unquestionably, the primary targets are Jupiter and Saturn. Whilst rather low, this is the month to concentrate on them. And for those who would like to tick off a couple of more challenging objects, Neptune and the asteroid Pallas (see star chart below) come to opposition in September. Finally – and notably in the first half of the month when the Moon is out of the way – there are many deep sky objects worthy of your attention now the skies get astronomically dark by 10pm.


Oh, and while Jupiter is on our minds, you may be interested in an excellent program, made three years ago but with all the findings from the Juno mission. It’s a Horizon production called Jupiter Revealed and viewable on BBC iPlayer but only until 7th September. The images are stunning, the graphics excellent and most importantly it tells us what we have never known before: what is at the heart of this extraordinary planet. Thoroughly recommended.


Please note that all 4-figure times given in this article are in BST.


September Objects

The Sun

Although the Sun has shown some signs of life recently, it’s still a little like Rip Van Winkle! It’s worth checking spaceweatherlive.com to see whether anything’s going on.


The Moon

September is just about the worst month to observe the Moon because during the phases that traditionally give the best views (early crescent to mid-gibbous) the Moon doesn’t climb higher than 10° above the horizon. This month’s new moon is at 0152 on the 7th. For those wishing to spot the earliest crescent moon, the best (but challenging) day will be Wednesday 8th after the Sun sets at 1932. (See the Crescent Moon Watch page for more details.)


The Planets

The comments for Mercury, Venus and Mars are exactly the same as last month, namely:


Mercury is too low at sunset to be observable this month.



Venus is catching up with us following its journey from behind the Sun but due to the low angle of the ecliptic at this time of year, Venus is only about 10° above the western horizon at sunset. This situation will continue right through until the end of the year.



Mars is on the far side of the Sun and completely unobservable this month. We’re going to have to wait until its next opposition in December 2022 before it presents itself once more.


Jupiter and Saturn

Both these gas giants reached opposition last month and although quite low in the sky, are now very conveniently placed for late evening observation. They are due south and highest in the sky at about 2330 at the start of the month, 2230 on the 15th and 2130 on the 30th. At these times, Jupiter climbs to 21½° altitude (which is far from ideal but high enough for decent views), whilst Saturn climbs to 17°.


Jupiter must surely be THE most satisfying and entertaining planet to observe. For starters it’s the largest surface-featured planet in the telescope eyepiece. At an angular diameter of 48 arc seconds it only requires a magnification of x40 to make it look as large as the Moon does to the naked eye. This means that on a night of good seeing, even the most modest telescope of reasonable quality will reveal at least two of its famous cloud belts. Its other notable feature is of course its four Galilean moons, who perform a never-ending waltz round their parent. If you know the right time to look, you can literally see them appearing and disappearing as they pass in and out of eclipse by the shadow cast by Jupiter itself. For those with larger telescopes of high optical quality, one can even see the shadows of the moons on the face of Jupiter as they pass between it and the Sun.

Here are the dates and times (to the nearest 10 minutes for transits) of some notable events.


Date & Time Event
6th 1940 – 2150 Transit of Io’s shadow
7th @ 2141 Europa appears out of eclipse to east of Jupiter
9th @ 2122 Ganymede appears out of eclipse to east of Jupiter
13th 2140 – 2350 Transit of Io’s shadow
14th @ 2103 Io appears out of eclipse to east of Jupiter
20th 2330 – 0140 Transit of Io’s shadow
21st @ 2258 Io appears out of eclipse to east of Jupiter
23rd 1840 – 2110 Shadow of Europa transits alongside Great Red Spot
29th 2000 – 2210 Transit of Io’s shadow
30th @ 1921 Io appears out of eclipse to east of Jupiter
30th 2110 – 2350 Transit of Europa’s shadow


Saturn, whilst being smaller and displaying only subtle cloud belts is without question the most beautiful object in the night sky. Its rings never fail to generate “Wow”s from people who see it for the first time though a telescope. The diameter of the disc of the planet is only 18.5 arc seconds but with the rings, the overall diameter is about 85% that of Jupiter. Saturn has 52 satellites but one of them, Titan, at 8th magnitude, is easy to see even with a small telescope. It can however appear as far as 5 ring diameters from Saturn.


Uranus and Neptune

Uranus is still a morning object and best seen after its opposition in November.  Neptune reaches opposition on the 14th so is well placed for observing, but you might like to wait until mid-October, when it’s highest in the sky (30+°) earlier in the evening (about 2300).


Deep Sky Objects

The first half of this September provides excellent deep sky viewing, before the Moon lightens the sky.


All the deep sky objects listed in August’s What’s Up can still be seen. In addition I would recommend the following two objects. Both can be seen in the south at about 2300 on the 1st, 2200 on the 15th, and 2100 on the 30th. You will find them north of Jupiter as shown on the following sky map.


Fig.1 SkyViewCafe September 14th at 2200



M2 is an exceptionally fine globular cluster. Its magnitude is 6.3 making it just discernible to the naked eye on a clear, dark night. It is 16 arc minutes in diameter, roughly half that of the Moon. It looks like a fuzzy patch in binoculars, partly resolvable into individual stars with a 6-inch telescope, and quite magical in a 12-inch.



Another globular, and even better than M2. Its magnitude is 6.2  and its diameter 18 arc minutes. It has a particularly condensed centre and is worth reading about in Wikipedia before you observe. There is an impressive HST image of it in that article.


Man’s Space Activities


Spotting the International Space Station

The ISS is visible on many nights during September. The Heavens Above web site gives all the details.  The link is preset to return the data for sightings from York. Use the left and right chevron buttons to change the date range. All times are BST.


General Space News

There’s always something of interest going on in space. See the space.com web site for the latest news. And for a detailed timetable of projected launches and other space-related activities, the Wikipedia web site is an excellent place to start as it has links to each project/mission mentioned.


Last Month’s Challenge


Fig. Mystery Crater (August 2021)

The challenge was to identify the crater. The text of the challenge was “Some body (one of four) obviously made an impression here. If you can identify it, let us know by using the Leave a Reply facility as your messenger.” The clues were the reference to messenger (spacecraft to Mercury and messenger of the gods) and impression. The crater is called Renoir, it’s on Mercury and is one of four craters named after Impressionist painters. Congratulations to Dave Armeson for being the first to identify it. And congratulations to Andrew Downie for using the clues in the text to come a close second.


This Month’s Challenge – Name that Star

The challenge this month is to name a star.

The star is included in the Wikipedia List of Brightest Stars (which is 93 stars long, all brighter than mag 2.5).

The star you are looking for is the only one in that list whose name contains four particular letters.

Those four letters are also the first letters of the first names of four people famous in the world of astronomy or spaceflight.

Those four people are shown below.



Fig.2 Person 1 of 4


Fig.2 Person 2 of 4


Fig.2 Person 3 of 4


Fig.2 Person 4 of 4

If you get the four letters right, there is only one star that contains those four letters. If you only know one or two, then below are images involving four more astronomers. but this time, the initial letters of their first names are all letters that are NOT in the star name. If you know one or two of those, you could combine “does contain these but doesn’t contain those”.


Fig.3 Person 1 of 4


Fig.3 Person 2 of 4


Fig.3 Person 3 of 4


Fig.3 Person 4 of 4

Good luck!
Please send your answers to us by using the Leave a Reply facility below. Your post will not be published until the 15th, when all responses will be published along with the winner and of course, the answer.

All general comments about this What’s Up are welcome and may be published immediately after moderation. But if they give or hint at the answer to the challenge, they will be published on the 15th.

Thanks for your interest, and I wish you clear skies and good viewing. Stay safe!

John Rowland

6 thoughts on “What’s Up! September 2021

  1. The name of the star is “Alpheratz”, which is also known as “Alpha Andromedae”.

    The “first letters of the first names of four people” are: E.R.P.L from the following list: Edwin Hubble P1, Robert H. Goddard P2, Percival Lowell P3, Lewis A. Swift P4

    The “initial letters of their first names are all letters that are NOT in the star name” are: C.D.W.I from the following list: Caroline Herschel P1, David Howard Levy P2, William Johnson McDonald P3, Isaac Newton P4.

    Thanks for the Challenge, John. I waited for 10 days and since no one had guessed it, I thought I’ll have a go at it, but happy to withdraw it if someone else has identified it?

  2. Thanks for the challenge John.
    I finally worked it out!!! I see I am not the 1st but enjoyed the search.
    Never got Robert Goddard or David Levy.

  3. Actually, P3 of the NOT list features Otto Struve, the observatory’s first director. I wouldn’t have made finding the name of P3 so easy as for it to be the first name of the surname McDonald shown in the image.
    So I declare Andrew Downie to be the winner because even if he made the same mistake as Qamar, his answer doesn’t show that. He just gave the right answer, short and sweet. Sorry Qamar!

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