Dundee Astro - Sky Notes

At the beginning of October the Moon is at broad gibbous phase just two days past Full. On the evening of Sunday 1st, the Moon will appear immediately above the bright planet Jupiter.

Last Quarter is on Friday October 6th when the half-illuminated Moon will rise in the north-east late in the evening and shine to the right of the stars Castor and Pollux in the constellation of Gemini.

In the early hours of the morning of Tuesday 10th, the waning crescent Moon will lie to the upper left of brilliant Venus; on the following morning, Wednesday 11th, the Moon will be to the lower left of the planet.

New Moon occurs on Saturday October 14th and the narrow young crescent Moon may be spotted low above the south-west horizon after sunset from Thursday 19th onwards.

First Quarter is on Sunday October 22nd, with the half Moon shining low in the south as the sky grows dark. A few nights later, on Tuesday 24th, the gibbous Moon lies to the lower left of the planet Saturn.

Full Moon falls on the evening of Saturday October 28th; the Moon rises in the east just before sunset, and sets in the west shortly after sunrise the following morning. In the first part of the evening there is a small partial eclipse, when the Moon passes through the upper edge of the Earth’s shadow – see below.

The Full Moon of October is traditionally known as the ‘Hunter’s Moon’, and this year it will also appear just to the right of the planet Jupiter.

The waning Moon continues to rise soon after sunset for several nights following Full so will remain visible in the early evening sky. On Sunday 29th, the gibbous Moon will be to the left of Jupiter, and on Monday 30th will lie below the Pleiades star cluster.

2023 is rapidly heading towards winter, and this means darker skies, longer nights and being able to observe some phases of the Moon at rather better times of the day (or night). It also means that the irritating British Summer Time ends on 29th of October and gives us an hour of earlier darkness.

The Moon starts off October at 16.7 days old, rising at 19:15 UT and transiting at 02:22 UT at an elevation of 49⁰. This means that it is reasonably high by late evening with the terminator having crossed about 1/3 on Mare Crisium and to the south, just touching the eastern wall of Langrenus and Petavius. Being so close to the terminator it’s unlikely you will see anything within the craters except shadow which will probably hide the distinct fracture in the floor of Petavius.

As last quarter Moon is favoured for elevation in autumn, the Moon starts to rise relatively early towards last quarter and there are opportunities to view it at more reasonable times. Transits are in the early hours but, for example, the 17.8 day old Moon on the 2nd October reaches 30⁰ elevation in the east by 23:00 UT so is reasonably observable. Atlas and Hercules are nicely placed close to the terminator towards the north, and moving southwards, the terminator is just at the west edge of Mare Crisium. Further south yet, the crater Rheita and the associated Vallis Rheita are right on the terminator and will be worth a look. For the next few days, the Moon continues to rise only slightly later each night and by last quarter on 6th October it is still low in the ENE but accessible. Alternatively it can be viewed in the early morning as it transits at 60⁰ at 06:50 UT. You can follow the Moon in the early mornings as it progresses towards new on the 14th October.

Following new, the Moon returns to the sky at a low elevation and on the 16th October, the 2 day old Moon sets at 18:19 UT so is not readily accessible. Things become worse towards first quarter on the 22nd when the Moon reaches only 9⁰ at transit at 19:51 UT. By the 24th the 10 day old Moon claws its way back to an elevation of 20⁰ at a transit time of 21:39 UT. On that evening, Sinus Iridum is dramatically placed on the terminator with Plato to the east showing some shadow across its floor which is well displayed with a +6⁰ libration. At the terminator to the west of Copernicus, watch for the appearance of the rayed crater Kepler which will emerge fully by midnight. Looking further south at the same time, the eastern edge of Gassendi will make an appearance as will the Rimae Hippalus at the south-east side of Mare Humorum.

The 25th October sees the 11 day old Moon transiting at 22:30 UT at 27⁰ elevation and by mid-evening you will see Aristarchus emerging from the terminator followed by the source, ‘Cobra’s Head’, of Vallis Schroteri. By 23:30 UT, the Valley will have emerged together with the crater Herodotus. Don’t miss Prinz, to the north-east, a half buried crater with numerous delicate sinuous rilles nearby. Look southwards towards Mare Humorum, close to the terminator, and Gassendi, a crater with a complex and rugged floor. To the north of Gassendi is that wonderful bay, Letronne, the flooded remains of a basalt filled crater. Just a bit further north is the 21km crater Flamsteed, not of great interest, but look for the ring of low ridges and hills named Flamsteed P which mark another long buried crater.

By the 26th, the terminator has revealed the 41km diameter crater Marius, south of Aristarchus and west of the rayed crater Kepler. The floor of Marius is flooded with basalt and there are a few small craterlets. The walls of Marius show terracing but otherwise it has no outstanding features. However, what is interesting about Marius is the large area surrounding it called the Marius Hills which is a field of lunar domes. Lunar domes are of volcanic origin, some having been formed by upthrusts from magma chambers below while others show a summit pit which probably allowed lava flow. Lunar domes are generally about 200 – 500m in height with diameters of various sizes. The LRO satellite has also imaged a ‘hole’ in the area which may be the entrance to a lava tube and is believed to be about 85m deep. Don’t expect to see this feature, though!

The large dark basin, Grimaldi appears near the terminator on the 27th. It is due west of Flamsteed and has a flat, featureless floor, but is unmistakable because of its low albedo. It is an impact feature about 173km in diameter but doesn’t really show the features of a crater as any walls it originally had are now heavily eroded.

Full Moon falls on the 28th when the Moon transits at 47⁰ at just after midnight. Libration is not particularly high in latitude or longitude so I suggest having a look at the variations in ‘colour’ of the various maria. Look at the contrast between Mare Tranquillitatis and Mare Serenitatis. Also look at the variations in the large area of Mare Imbrium which has the additional variables of the rays extending from Copernicus and the ejecta blankets formed by Archimedes, and to a lesser extent by Aristillus and Autolycus.

The transit elevation of the Moon rises on the last three days of October and by 31st is 60⁰. The Moon is then 17 days old and the terminator passes through the middle of Mare Crisium. Transit time is 01:47 UT but the Moon is high enough earlier in the evening to allow a good look round.

Please make a note of anything you observe relating to the Moon and we can all compare notes at the DAS meetings.

Mercury shines at magnitude -1.0 low in the east at dawn at the start of October, rising about an hour-and-a-half before the Sun. However it slowly sinks a little lower each morning and will disappear into the glow of sunrise by mid-month. Superior conjunction, when Mercury lies on the far side of the Sun, is on Friday 20th and the little planet will remain hidden in the twilight for the rest of the month.

Venus is at its greatest elongation from the Sun on Monday 23rd and appears as a brilliant ‘morning star’ of magnitude -4.6 in the eastern sky throughout October, rising over four hours ahead of Sun.

Mars sets just minutes after the Sun and is invisible in the bright twilight sky.

Jupiter rises in the north-east in the early evening in September and shines brightly at magnitude -2.9 high in the south in the early hours of the morning.

Saturn is low in the south-east as the sky grows dark and reaches its maximum altitude in the south of just 21o late in the evening, shining at magnitude 0.6.

Uranus is in Aries, about 9o to the east of Jupiter and at magnitude 5.6 is an easy binocular object.

Neptune lies in southern Pisces, and binoculars will show it as a faint ‘star’ of magnitude 7.8.

The ‘Summer Triangle’ of Deneb, Vega and Altair is still prominent on October evenings, standing high in the south as the sky grows dark. To the lower left of the Triangle is the small but eye-catching diamond shape of Delphinus, the Dolphin.

The orange star Arcturus is now low in the north-west with the kite-shape of Boötes the Herdsman stretching above. The little semi-circle of Corona Borealis, the Northern Crown, sits to the left of Boötes and beyond that is Hercules. The most distinctive part of this rather faint constellation is the central group of four stars commonly known as the ‘Keystone’.

In the south is the zodiacal group of Capricornus the Sea-Goat. To the left of Capricornus is another rather faint zodiacal constellation, Aquarius the Water Carrier; this autumn the planet Saturn appears as an additional bright ‘star’ in Aquarius.

Below Aquarius and low above the south-eastern horizon is the white star Fomalhaut, the most southerly of the first-magnitude stars visible from Britain. Fomalhaut just clears the horizon for a few hours on autumn evenings.

The Great Square of Pegasus, the winged horse, is high in the south-east. This pattern is traditionally shown upside-down on the sky, with the horse’s neck and head leading off to the lower right of the Square; the star Enif marks Pegasus’ nose. The stars of Andromeda run off from the opposite corner of the Square, towards the bright stars of the hero Perseus. Above them are Cassiopeia and Cepheus, Andromeda’s mythological parents.

Low in the north-east are the first of the winter constellations – the V-shaped head of Taurus the Bull and the little cluster of the Pleiades or Seven Sisters, along with Auriga the Charioteer and its bright star Capella. Out to the right of the Pleiades, in the constellation of Aries, is the bright planet Jupiter.

Centred on the Pole, or North, Star are the circumpolar stars – those that remain above the horizon all night long, all year round. The most famous of these are probably the seven stars that make up the pattern of the Plough or Big Dipper, two of which point the way to the Pole Star.

The Milky Way runs from the north-east past Capella into Perseus and Cassiopeia. It then passes almost overhead through the Summer Triangle and down into the south-west.