October Sky Notes

The Media boys and girls are at it again. After insisting on using the term “supermoon” to describe normal, regularly occurring events, and claiming that the star T Coronae will appear like a second Sun in the sky, when it will, actually, just about reach naked-eye visibility (at the time of writing the nova event is still yet to happen), the current headlines in the science press are that the Earth will soon gain a second Moon.

This news concerns the recently discovered asteroid 2024 PT5 which belongs to a class of asteroids known to make close approaches to the Earth. Typically such “close” approaches are of a mere one million to two million miles, that is 4-8 times farther away than the Moon which lies at an average distance of 250 000 (quarter of a million) miles.  There have been one or two observed passes in recent years that have come closer.

A Moon is a rocky or icy body that moves in a fixed path (orbit) around a larger body under the influence of that bodies gravitational field. What will happen to 2024 PT5 is that it will be influenced by the Earth’s gravitational field into a “horseshoe” shaped trajectory that will fling it back into interplanetary space some 57 days later.  This type of trajectory, usually called a “slingshot” is used by NASA and other space agencies wishing to send probes to the outer planets and was first employed by the Pioneer 11 space probe in 1974 when it used the gravity assist of Jupiter to alter its course toward Saturn.

2924 PT5 will only be under the influence of Earth’s gravitational field for 57 days and will not even complete a single full orbit around the Earth. Therefore it cannot be considered as a “Moon” but is merely a passing asteroid, albeit one of particular interest.

The asteroid is a relatively tiny fragment left over from the formation of the Solar System and is far too small to be visible to the naked-eye, binoculars or small telescopes. Some dedicated amateurs who have equipment set up to track and locate Near Earth Asteroids may be lucky enough to find it as a miniscule track on an image of a star-field because they know what they are looking for but, otherwise it is down to the large telescopes in professional observatories to keep an eye on this for us.

The giant planets, especially Jupiter and Saturn with their much greater and stronger gravitational fields  influence asteroids like this all the time. It is clear that many of the smaller Moons of the giant planets are gravitationally captured asteroids which have been pulled into fixed orbits around the planet.  Whilst the International Astronomical Union has made an (arbitrary) decision on how big something has to be in order to be a planet, it has made no such comparable decision about the size of moons, so the smallest speck of dust in a fixed orbit can be considered a Moon.

Sky Notes

Please note all times given in this article are in GMT so remember to add an hour to get the time in BST.

Sky Schedule

Sun: From the beginning to the end of the month the period of dark sky increases from 8.5 hours to over 10 hours.  The longer nights allow us to view the summer constellations and the Milky Way that runs through them against a dark sky whilst also introducing the Autumn constellations and the interesting objects in them.  

Mercury:  Mercury is not visible this month as it lies in the same direction as the Sun.

Venus:  Venus can be seen low in the South-West as the Sun dips below the horizon. The planet, itself, will set around 6.00pm. 

Mars:  Mars rises around midnight and is seen against the background stars of Gemini.  

Jupiter:  The largest planet rises after 8.30pm in the constellation of Taurus.  The coloured bands and zones on the surface of the planet can be distinguished with small telescopes as can the Great Red Spot, a colossal hurricane, greater in diameter than the planet Earth. The Galilean moons can be seen with a good pair of binoculars or a small (bird-spotting) telescope.  DSLR images of the planet will also pick up the moons.  These moons orbit Jupiter such that we sometimes see them pass between the Earth and the giant planet along our line of sight.  

Saturn: The ringed planet can be found against the stars of Aquarius, visible all night long and at a maximum altitude of about 25 degrees (a little more than the span between your thumb and little finger, at arm’s length). The rings are currently edge-on to us and you would be lucky to see them at all.  Saturn’s largest moon, Titan, can be seen with a good pair of binoculars or a small telescope.  On 14th the Moon will lie beneath Saturn in the sky.

Uranus and Neptune: Uranus is visible all night long throughout October and is seen against the background stars of Taurus close to the Pleiades star cluster.  It is now only a telescope object as it is receding in distance from the Earth.  Look for a tiny greenish disc compared to the pinpoints which are the stars.  Neptune is seen against the background stars of Pisces and close to the tail of Cetus (below the asterism known as the Great Square of Pegasus) Neptune will also require a telescope to find it.  Neptune displays a smaller and truly blue disc compared to that of Uranus although it will appear only as a bluish star to most small instruments.  

Meteor Shower:  There are a number of minor showers that occur during October, none of them expected to produce many bright meteors. However, the Orionid shower can produce a Maximum rate of anything between 20 and 100 meteors per hour.  These meteors are caused by debris from the comet 1P/Halley and peak on October 21st. 

 The New Moon occurs on October 2nd  with First Quarter on 10th, Full Moon then follows on 17th  October and Last Quarter on 24th.