SEPTEMBER 2018

September 2018

The Autumnal Equinox on Sept 23^rd at 01h55 marks the official start of autumn. Day and night are almost equal in length again, and in the northern hemisphere, autumn lasts for 89.85 days. The Earth - Sun distance at this time is 150 125 903 kms.

The Sun begins the month in the constellation of Leo, but on the 17^th at 02h, crosses the astronomical border which separates Leo from Virgo, in which constellation it remains till the month ends.

September is the best month to observe the ethereal Zodiacal Light during the early mornings when the Moon is not present in the sky and you are well away from light pollution. Look towards the east before the onset of morning twilight, around 03h30, and you should see a faint cone of light pointing southwards at a steep angle of 60°. This phenomenon is caused by the Sun illuminating the disc of fine dust, which is the remnant of solar system formation 4.5 thousand million years ago. The best days to observe this morning cone are from the 10^th to the 24^th of the month.

The Moon

The Moon is at apogee, its farthest from the Earth, on the 20^th at around 00h00 (diameter of the Moon - 29’32’’ arc) and at perigee (nearest to the Earth) at 01h00 on the 8^th (diameter of the Moon 32’52’’ arc). 

Last Quarter Moon is at 02h38 on the 3^rd in Taurus, near the bright star Aldebaran. At 01h00 the Last Quarter Moon passes just 6 arc minutes (0.109°) to the north of this star. This is one of the highest LQ Moons of the year (60 degrees), when it crosses the meridian at sunrise, (despite the Moon being south of the ecliptic at this time).

New Moon is on the 9^th at 18h02, in the constellation of Leo, passing just over 2° north above) the Sun.

First Quarter Moon takes place at 23h16 on the 16^th, in Ophiuchus, and is the lowest FQ Moon of the year, and may be seen at 15° altitude in the south just before sunset on this day.

Full Moon at 02h53 on the 25^th, is at the junction of the Pisces/Cetus/Aquarius border. This Moon is called the “Harvest Moon”, since before the days of mechanisation, harvest was a long process and the labourers would have to work well into the night. For several nights before and after Full, the Moon rises at about the same time each evening as the sun sets and so its light enabled the harvesters to continue their labours into the night. Look for the Harvest Moon rising almost due east at sunset at this time. 

Earthshine may be seen on the dark hemisphere of the waning crescent Moon from the 1^st to the 9^th, and it may also be glimpsed on the night hemisphere of the waxing crescent from the 10^th to the 15^th.

 The Planets

As September starts, Mercury rises 90 minutes before the Sun in the constellation of Leo, and is best seen above the eastern horizon in brightening twilight at around 04h when it is 5° above the ENE point on the horizon.  It should be possible to glimpse Mercury at this time with the unaided eye, when it has the appearance of a bright star at visual magnitude -0.87. The planet may be seen in this direction during the first week of September, but gradually sinks lower as it moves towards superior conjunction with the Sun on the 21^st.

During September Venus sets less than an hour after the Sun, and as the month progresses it becomes difficult to observe. However, in binoculars, Venus takes on the appearance of an extremely thin miniature crescent moon. During the first week of September the planet may be seen low (3° altitude), in the WSW sky at around 19h00, and it may also be possible to glimpse Spica, the brightest star in Virgo nearby in the same binocular field.

On the first of the month, Mars sets at 01h, but at the month’s end the planet is setting just before 00h, so the planet has now become an evening object as Earth leaves the planet behind. However Mars, on the Capricornus/Sagittarius border is still shining brightly with its ochre coloured light. During September the planet fades from visual magnitude -2.1 to -1.3. So for almost all of the month it is brighter than Sirius (the brightest of all true stars, after the Sun). There is a conjunction with the Moon at 04h00 on the 20^th, by which time the planet will have set. However the gibbous waxing Moon will lie 5° to the west (right) of Mars at 23h00 the previous night (19^th) as the two objects are setting in the SW.

Jupiter is an evening object in the constellation of Libra and may be seen low in the SW sky in the early evening. By the end of September the planet sets at around 19h00. Its appearance is that of a star shining brightly and steadily at magnitude -1.8. The waxing crescent Moon may be seen approaching Jupiter on the 13^th, when at 19h00, Jupiter lies 5° to the east (left) of the Moon, and the following night the 14^th, when the waxing crescent and the giant planet are 8° apart. On this latter occasion Jupiter lies 8° to the west (right) of the Moon. Using binoculars look out for Jupiter’s moons and their nightly dance around the planet.

Mid-month, Saturn culminates (crosses the south meridian) at around 19h00, when its altitude is around 12° (from Scarborough). Again, despite the favourable presentation of the northern surface of the rings towards Earth, the low altitude of the planet affects the clarity of the view through a small telescope. However modern techniques in imaging using software to ‘stack’ images of the planet produce excellent results. Saturn’s largest moon Titan is at its greatest elongation east of the planet on the 2^nd and 18^th and western elongation on the 10^th and 26^th. The gibbous waxing Moon passes close to Saturn on the evening of the 17^th, when at 19h00 the two objects are just over a degree apart in the south. See if you can spot Saturn in the same binocular field as the Moon, in the fading twilight.

Uranus in the constellation of Aries the Ram rises during the early evenings of September and may be seen in binoculars for the rest of the night. It culminates (crosses the meridian) at around 02h00. Uranus lies 8° south of Mesarthim (gamma Arietis), and has a visual magnitude +5.71 Uranus is on the threshold of naked eye visibility, so is best seen through a small telescope. There is a location chart showing the movement of Uranus on the Aries/Pisces on the Remote Planet page accessible from the MENU (MORE) above.

Neptune is at opposition and its nearest to the earth this year on the 7^th. Despite this, the planet is very faint and is below naked eye visibility at magnitude +078. The planet lies 41 minutes of arc to the NW of the visual magnitude +5.43 star 82 Aquarii. It is theoretically visible all night long and there is finder chart on the Remote Planets page, mentioned above.

A minor meteor shower peaks during the early morning of the 9^th, when through the night, you may see about 10 Piscid shooting stars an hour. The radiant, or apparent origin is in the constellation of Pisces, hence the name.

Constellations visible in the south around midnight, mid-month, are as follows: Aquarius, the western fish of Pisces, sometimes known as ‘The Circlet’ because of its shape, and the large autumn square of Pegasus. Almost overhead are the great galaxy in Andromeda and the ‘W’ shaped constellation of Cassiopeia.

All times are GMT     1° is one finger width at arm’s length. 

SEPTEMBER EVENTS SUMMARY (Using Coelix Software).