TONIGHT'S SKY - October 2021

Moon Phases and Distances

PHASE DATE
New Moon Wed 6
First Quarter Mon 13
Full Moon Thu 21
Third Quarter Fri 29

In its 29 day orbit the Moon will be at perigee (closest to Earth) on Sat 9th at a distance of 363,386 km, and at apogee (furthest from Earth) on Mon 25th at 406,615 km.

Planets

Mercury is not visible this month being so close to the Sun.

Venus is bright in the west all this month as the 'evening star'. It is at its furthest from the Sun from our viewpoint. It will be visible in twilight from around 7.45pm early in the month before setting by 11pm. Each night it will become visible a little later and set a little later. By the end of the month it will be setting by midnight.

Mars, like Mercury, is too close to the Sun this month to be seen from our latitude.

Jupiter will be visible high in the north this month. Early in the month it can be seen from around 7.45pm, reach its highest position around 10pm, and then be lost in the west by 4am. Later in October it will be seen from 8pm, be highest at 8.30pm, and set in the west by 2.30am. Being a planet so far from the Sun, it's slow orbit will mean only a small difference or change of position night by night.

Saturn, being further out than Jupiter, moves even slower in our night skies. Early in the month it can be seen from 8pm high in the north-west before it reaches its highest elevation at 9pm then fades in the west by 3am. It will be a little later each night until by the end of October it will be seen from 8.30pm before dropping below the horizon by 2.40am.

Meteors

The Orionids are visible from the 15th-29th but peak on the 21st-22nd with estimates of around 30 meteors per hour. The best time will be from around midnight until early dawn. The shower is centred on Orion the Hunter near the red supergiant star Betelgeuse. These meteors are typically very fast and bright. They enter the atmosphere at about 66 km per second and vaporise 100 km above the surface leaving persistent trails in the sky. The Conversation has a detailed guide by Dr Tanya Hill and Professor Jonti Horner.

Humans have observed and marvelled at meteor showers and comets throughout recorded history but the connection between specific meteor showers and comets is a recent understanding. For some ancient cultures they were omens or messengers. The Orionids shower was first recorded by the Chinese in 288 AD and is associated with Comet Halley.

That famous comet is named after Astronomer Royal Edmund Halley. In 1705, after studying earlier records of certain comets that seemed to re-occur and applying Newton's new laws of motion, Halley calculated its elliptical orbit around the sun and successfully predicted its return in 1758 (which sadly he did not live to see).

Comet Halley orbits the sun every 75 years on a path that takes it out beyond Neptune. Each time it passes through the inner solar system it leaves in its wake a trail of particles for our planet to pass. Twice a year Earth intersects the comet's previous orbits and we enjoy these meteor showers - the Eta Aquarids in April-May and the Orionids in October. In 1986, during it last visit to the inner solar system, Comet Halley was observed intensely from telescopes and a flotilla of space probes.

This photo by the International Halley Watch was taken on 8 March that year and shows a broad dust tail and below it a faint ion tail both emanating from the coma or head of the comet, deep within which lies the nucleus itself. Heat from sun causes the 5.5km rocky-icy nucleus to vent or release streams of material that are then blown away by the solar wind - the constant stream of photons and particles given off in all directions by the sun.

Enjoy a 'close encounter' video compilation by ESA's Giotto FlyBy. While Comet Halley's next appearance is not due until July 2061, twice every year we can enjoy Orionids and Eta Aquarids as reminders of its former passes through the inner solar system.

And Taurids as well...

Another meteor shower that could be enjoyed late this month, although less impressive than the Orionids, is the Taurids linked to Comet Encke. This is a long duration shower in Spring that actually peaks in the first week of November. There are two branches - one near the star cluster Pleiades, and the other close to the red star Aldebaran in Taurus - each giving 10 meteors per hour that can be bright and slow with colourful fireballs.

Stars and Constellations

In the north

As the year progresses we see Aquila (the Eagle) and its principal star Altair (Alpha Aquilae) directly north but Lyra (the Lyre) and its bright star Vega (Alpha Lyrae) have left our northern skies this month.

In the west

Scorpius is spectacular as that constellation has now moved fully down to the west with the red-giant star Antares noticeable as the middle of three stars that form the centre of the arachnid's body. Following behind during the night and higher in the west again this month is our centaur- archer Sagittarius.

In the east

This month we can again easily spot the star Formalhaut - that featured in last month's Skynotes - in the constellation Piscis Austrinus (Southern Fish).

In the south

If away from bright lights you can see in the south-east the isolated Large and Small Clouds of Magellan (our galaxy's nearest neighbours) as well as the vast band of billions of stars and dark dust clouds that make up what we can see of our own galaxy, the Milky Way. In the evenings this month it runs in a broad arc from south to north. From our location about 28,000 light years from the centre of our disc shaped galaxy we have an edge-on view along the plane of the galaxy where the density of stars is greatest. 

As you look away from the Milky Way to either side of the sky you see few stars. In those directions you are peering through only a thousand or so light years out into intergalactic space, to the north or south of the galactic plane. Being inside something in the dark doesn't help much in trying to work out its size or shape! Where we are in the galaxy and its size and shape was not fully understood until quite recently.

Much lower to the horizon this month we find The Pointers, Alpha and Beta Centauri, in the constellation of Centaurus (the mythical centaur). From our latitude of 37.8 degrees south they and nearby Crux (Southern Cross) never disappear below the horizon as they are circumpolar which leads us to a little digression about our home planet and our experience on it...

Circumpolar or 'around the pole'

As our planet rotates some stars rise in the east and set in the west during the night - that's diurnal motion, the daily turning of the Earth. It is the same reason why the moon, the planets and the Sun all appear to rise in the east and set in the west. But stars that are far enough south (or far enough north if in the northern hemisphere) trace a half-circle path. During the night they remaining above the local horizon and do not set. These particular stars go on to complete their circular journey during the day when they can't be seen, of course, before rising again next night to do it all over again.

However, there are two extremes of this 'around the pole' motion. Imagine being exactly at the south (or north pole) during a months-long winter night. You are standing on the planet's axis with a clear view around the horizon. The world is rotating beneath you - no stars rise or set, all stay above the horizon, all are circumpolar. And with no daytime in the antarctic (or arctic) winter you can watch the full circling of stars for an entire 24 hours. Contrast that with the quite different experience of being at the equator - all the stars rise and set, none are circumpolar. Which stars you see as circumpolar depends on your latitude, how far south (or north) of the equator you happen to be. Such an interesting intersection of astronomy, geography and living on a planet that spins!

If you use a fixed camera set for a long exposure you can create a beautiful star trails photo of such stars making their paths across the sky during the night. A well known and superb image was taken by David Malin at Siding Spring NSW looking out over the famous and iconic Australian Astronomical Observatory (AAO). It featured as Astronomy Picture of the Day (APOD) in July 2005.

Did you notice in the photo how the star trails are not all white? Some are blue, yellow and even red. Star trails can bring out the colours of the stars because their faint light photons are collected or added together over the long exposure. Colour and temperature are related, so trails like these give you a relative sense of how hot stars are, how much they Vary in surface temperature.

First Peoples night sky

For tens of thousands of years across this ancient continent our first peoples have had traditions of night sky observing and story-telling full of cultural meaning and connection to the land and its people.

Last month the full moon was featured coinciding with the spring equinox so this month we look at a First Peoples' story about our nearest neighbour.

A scarred moon in the sky

In the Boorong tradition of north-west Victoria, the moon is known as Mityan and represents the spotted-tailed Quoll, a small predator that hunts mostly at night by the light of the moon. Its coat bears patterns like the varied phases of the moon. In stories, Mityan attempted to take one of the wives of Unurgunite, an important ancestral being. A fight ensued and Mityan lost. After his defeat he fled beaten and scarred.

As the moon moves across the day and night skies showing us its monthly phases, so too Mityan wanders as an outcast day and night without rest or companion. The dark and light areas of the moon's face are reminders of the scars Mityan suffered for his wrongdoing.

Once widespread in Victoria, quoll are small den-dwelling and tree-climbing predators found today in the south-west of Western Australia and in Tasmania. After being extinct for 50 years in eastern Australia they are now being reintroduced there. 

Two other bright objects

While Mityan the moon is the brightest object in the night sky, the second brightest object is also again visible this month. In Boorong stories that is Ginabongbearp (Jupiter), one of the ancient sprit beings or Nurrumbunguttias who look over the land and its people. And the third brightest object of the night is also there this month - his wife Chargee Gnowee (Venus) whose sister just happens to be Gnowee (the Sun). As you might expect being bright and moving independently of the stars, although not always close together in the sky, this husband and wife couple has an important place in tradition and story.

On This Day

3rd 1942, first object to reach space, the experimental V2 ('Vengeance') rocket, was launched from Peenemünde, Germany in a brief flight over the Baltic.

4th 1957, Sputnik (USSR) was launched to become the first artificial satellite.

4th 2004, SpaceShipOne was launched as the first private spacecraft into space.

5th 1923, Edwin Hubble (USA) established that M31, the Andromeda Galaxy, is separate to our own Milky Way Galaxy.

7th 1959, first photos of the moon's far side are taken by Luna 3 (USSR).

9th 1604, a Type1A supernova 20,000 light years away in constellation Ophiucus is visible from Earth, and on 17th Johannes Kepler observes and publishes his account of the new star. It is the most recent supernova visible to the naked eye in our galaxy.

10th 1967, the United Nations' Outer Space Treaty on the peaceful exploration and use of space was established. By now 109 nations are signatories and several other agreements and conventions have been created to cover space law.

11th 1958, Pioneer 1 (USA), a battery- powered probe aiming for lunar orbit, fails to reach escape velocity and burns up.

11th 1968, first crewed Apollo mission, Apollo 7 (USA), launched into Earth orbit in test of Saturn V rocket and Command and Service Module (CSM).

12th 1964, USSR's Voskhod 1 ('Sunrise') was the first spacecraft with a crew of more than one. In this case, three cosmonauts who orbited for 41 hours.

13th 1773, the Whirlpool Galaxy M51a, 31 million light years away in constellation of Canes Venatica, is discovered by astronomer Charles Messier.

18th 1967, Venera 4 (USSR) is the first probe to analyse the atmosphere of another planet when it does so at Venus.

19th 1910, birth of Subrahmanyan Chandrasekhar who had major insights into stellar evolution and black holes.

24th 1998, Deep Space 1 (USA) probe was launched to test innovative technologies, including an ion engine, while visiting Asteroid Braille and Comet Borrelly.

27th 1994, first sub-stellar object orbiting a star is found, a brown dwarf at Gliese 229.

29th 1991, Galileo probe (USA) is the first to visit an asteroid, Gaspra 951, on its way to Jupiter.

31st 2000, Expedition 1, first resident crew of the International Space Station, arrived by a Russian Soyuz craft for a 136 day stay lasting until March 2001. The three-person crew (one American and two Russians) made the station fully operational, hosted three visiting US Space Shuttles, and received two Russian Progress supply vehicles.